Collisions moléculaires: théorie, expérience et observation Alexandre Faure Institut de Planétologie et d’Astrophysique de Grenoble CNRS / Université de Grenoble Alpes (France)

Journées de la SF2A 2019–Nice, 14-17 mai 2019 Outline Introduction I. Collisional excitation: theory vs experiment + II. Molecular diagnostics: CN, CH , CH2NH Conclusion Chemistry starts at z ≃1000

CHEMISTRY + + (HeH , H2 , H2, etc.) (April 2019) LETTER https://doi.org/10.1038/s41586-019-1090-x

Astrophysical detection of the helium hydride ion HeH+ Rolf Güsten1*, Helmut Wiesemeyer1, David Neufeld2, Karl M. Menten1, Urs U. Graf3, Karl Jacobs3, Bernd Klein1,4, Oliver Ricken1, Christophe Risacher1,5 & Jürgen Stutzki3

During the dawn of chemistry1RE,2, whenSEAR CHthe temperatureLETTER of the young reaction networks19,20 in local plasmas, and might ultimately invalidate Universe had fallen below some 4,000HeH kelvin,+ @149.1 the ions of the ! mlight towards present modelsNGC of the7027 early Universe. elements produced in Big Bang nucleosynthesis recombined in reverse The deployment of the German Receiver for Astronomy at Terahertz 9 106 order of their ionization potential. WithNGC their 7027 higher ionization Frequencies+ (GREAT) heterodyne spectrometer on board the 2+0.10 + HeH J = 1–0 10 potentials, the helium ions He and He were the first to combine StratosphericCO J = Observatory 11–10 (×0.04) for Infrared Astronomy (SOFIA) has now with free electrons, forming the first neutral atoms; the recombination opened up new opportunities. Although 10the5 HeH+ J = 1–0 transition at of hydrogen followed. In this metal-free and low-density environment, 149.137 µm (2010.183873 GHz; ref. 21) cannot be observed from ground- neutral helium atoms formed the Universe’s first molecular bond in based observatories, skies become transparent104 during high-altitude 0.05 ne

+ (K) the helium hydride ion HeH through radiative association with flights with SOFIA. The latest advances in terahertzTemperature technologies have mb

T + + 22 protons. As recombination progressed, the destruction of HeH enabled the operation of the high-resolution103 spectrometern(He ) upGREAT at + created a path to the formation of molecular hydrogen. Despite its frequencies above 2 THz, allowing the) or temperature (K) HeH J = 1–0n(H) line to be targeted. –3 0 λ10∆6 ×λ n≈(HeH+7) unquestioned importance in the evolution of the early Universe, the The resolving power of this heterodyne instrument,102 / 10 , permits HeH+ ion has so far eluded unequivocal detection in interstellar space. the HeH+ J = 1–0 line to be distinguished unambiguously from other, In the laboratory the ion was discovered3 as long ago as 1925, but only nearby spectral features such as the CH Λ-doublet mentioned previously. + 101 in the late 1970s was the possibility that HeH–50 might exist in0 local During50 three flights100 in May 2016, theDensity (cm telescope was pointed towards 4–7 astrophysical plasmas discussed . In particular, the conditionsVelocity in (km NGCs–1) 7027 (the total on-target integration time was 71 min). Weak planetary nebulae were shown to be suitable for producing potentially emission in the HeH+ J = 1–0 line was clearly100 detected (Fig. 1), as was Fig. 1 | Spectrum of the HeH+ J = 1−0 ground-state rotational 0.014 0.016 0.018 0.0200.022 detectable column densities of HeH+. Here we report observations, emission from the nearby CH doublet. Notably, the lines are well sepa- transition, observed with upGREAT onboard SOFIA pointed towards Distance (pc) based on advances in terahertz spectroscopy8,9 and a high-altitude rated in frequency (Extended Data Fig. 1). The velocity profile of the 10 NGC 7027. ‘Contaminating’ emission+ from the nearby+ but well separated observatory , of the rotational ground-stateΛ transition of HeH at a HeH line matches nicely that of the excited CO J = 11–10 transition, CH -doublet has been removed from the data (see Methods for details 106 wavelength of 149.1 micrometresof data in theprocessing). planetary The nebula spectrum NGC has 7027. been rebinnedwhich was to a observed resolution in parallel. The velocity-integrated line brightness −+1 −1 This confirmation of the existenceof 3.6 of km HeH s (24in nearby MHz). interstellarFor comparison, space the COtemperature, J = 11–10 ∫ lineTmb isd v = 3.6 ± 0.7 K km s , corresponds to a line flux of 5 constrains our understandingsuperimposed of the chemical (at anetworks spectral resolution that control of 0.58 1.63 km s×− 110); −this13 erg transition s−1 cm− was2. Because the 14.310″ half-power beam response of the formation of this molecularobserved ion, in particularin parallel andthe ratesprobes of the radiative dense inner upGREAT edge of the includes molecular most of the NGC 7027 ionized gas sphere, this result + association and dissociative recombination.envelope near the ionization front from whichwill the be HeH close emission to the total is HeH+ flux emitted10 in4 the J = 1–0 line. The flux is − − − The planetary nebula NGCexpected 7027 seems to originate. a natural Tmb candidate, main-beam for brightness a somewhat temperature. higher thanThe greythe upper limit (1.26 × 10 13 erg s 1 cm 2) assigned + + 16 search for HeH : the nebula shadingis very youngshows the(with area a abovekinematic and below age of the zeroto any line residual for each HeH spectral contribution in the ISO103 observations, in the attempt

11 channel. ) or temperature (K)

only 600 years) , and its shell of released stellar material is still rather to separate the line from its blend with –3 the CH doublet (see Extended Data n compact and dense. The central star is one of the hottest known (with Table 1 for the fluxes observed with upGREAT102 during this experiment). e 13 + Temperature an effective temperature, Teff, of some 190,000 K) and is very luminous We have modelled the HeH abundance across NGC 7027. We approx- We confirm the conclusion reached previously that, in the plane- n(He+) (with a luminosity of 1.0 × 104 L , where L is the luminosity of the imated+ the nebula+ as a constant-pressure, spherically symmetric shell, tary⊙ nebula environment,⊙ the reaction He + H → HeH + hν—which 101 n(H) Sun)12. Under these conditions, the Strömgren +spheres that are created and adjusted the pressure to obtain a StrömgrenDensity (cm sphere of angular radius dominates HeH formation in the early Universe—can be neglected, 106 n(HeH+) + + ″ × by the hard intense radiationas field can ofthe the reaction nebula’s H2 central + He hot→ HeH white + H4.6 (h—theν is the geometric photon energy). mean deduced from the0 1.4 GHz radio continuum 23 + 10 4 dwarf are not yet fully developed,Moreover, and the we radiation confirm field that drives the photodissociation ioniza- image of. WeHeH adopted is slow a stellar com- luminosity of 1.0 ×0.0210 10 L⊙, a stellar 0.0215effective 0.0220 0.0225 + + 5 12 11 tion fronts into the molecular envelope. The He Strömgren sphere will temperature of 1.9 × 10 K (ref. ), a source distance of 980 parsecsDistance, (pc) +pared with reactions (2) and (3) in the region in which the HeH emis- extend slightly beyond the H sion zone, arises, and itand is incan this therefore thin overlap also be layer neglected. and Fora He the abundance reactions of (1), 0.12 (2) relative to H. Using the CLOUDY photoion- that HeH+ will be produced. Detailed calculations13 led to predictions ization code24, we calculated profilesFig. 2 of | Temperaturetemperature andand densitydensity (for profiles H, for NGC 7027, as and (3), we critically reviewed the most recent available estimates for the predicted by our astrochemical model for this source. The bottom panel for the intensities of the v = 1–0 R(0) and P(2) rotational–vibrational He+ and electrons) as a function of position across the shell (Fig. 2). The rate coefficients, as detailed in Extended Data Table 2. Published values presents an expanded view of the top panel. The model transitions in the near-infrared; however, these predictions have not mean electron density within the ionized shell is 4.9 × 104 cm−3, and that for the rate14 coefficient,15 k1 for the16 radiative association reaction+ (reaction assumes a dissociative4 −3 recombination rate (reaction (2)) of been confirmed, despite deep searches . Observations with the in the H/He−16 3overlap−1 layer is roughly= 2 ×× 10 − cm10 . 4 −0.47 3 −1 (1)) vary widely; here we adopt a value of 1.4 × 10 cm s , based on k2 3.0 10 (T/10 K) + cm s (see Extended Data Table 2), and 25 26,27 −16 3 −1 Infrared Space Observatory (ISO)’sthe most Long recently Wavelength published Spectrometer cross-section . WeExperimental then computed studies the equilibrium a radiative abundance association of HeHrate (reaction, including (1)) of k1 = 6.0 × 10 cm s , of the pure rotational J = 1–0 ground-state transition at 149.137 µm the three reactions identified asadjusted being important to fit the observedin the layers J = in1–0 which line intensity. T, temperature; n, of the dissociative recombination reaction (reaction+ (2))—involving7,13 (where J is total angular momentum)measurements were impaired of the cross-section by the limited at energies HeH up is tomost 40 abundantelectronvolts: density. resolving power of the spectrometer (∆λ = 0.6 µm), which did not + (eV)—derive values for the thermal rate coefficient (k2) that are plainly Λ ++ −15 −1 −2 allow the HeH transition to beinconsistent separated from with thethe nearby cross-sections -doublet presented in that same study.He + AHH →predictedeH + inh νa 0.87 × 10.3″ slit is 5.9(1 ×) 10 erg s cm , comparable of the methylidyne radical (CH) at 149.0926 µm and 149.39 µm. −10 3 −1 15 −15 −1 −2 reanalysis of those+ measurements yields k2 = 3.0 × 10 cm s (at to the reported upper limit of 5 × 10 erg s cm . Very debatable tentative detections of HeH have been4 reported in a kinetic temperature of 10 K)—a value that is much smaller than that+ − Comparing the observed J = 1–0 flux with the predictions of our the envelope of the supernova SN 1987A17 and in a high-redshift qua- HeHH+ e → eH+ (2) 18 originally inferred from the measurements. To compute the emissivity excitation model in the well constrained physical environment of the sar , but all are unconfirmed and are suggested+ to be considered as of the HeH J = 1–0 transition, we made use of recent estimates for NGC 7027 nebula casts light on the relative importance of the different upper limits. This lack of direct evidence of the very existence of the ++ the rate coefficients for electron-impact excitation28. At the densitiesHeHH of+ mechanisms→ HH2 + e for the formation and(3 destruction) of the helium hydride molecule has called into question our understanding of the underlying relevance to NGC 7027, collisional de-excitation can be neglected, and ion, and in particular constrains the radiative association rate (reac- 1Max-Planck Institut für Radioastronomie, Bonn,= Germany. 2The Johns Hopkins University, Baltimore, MD, USA. 3I. Physikalisches Institut, Universitättion zu(1)) Köln, and Cologne, the dissociative Germany. 4University recombination of rate (reaction (2)). In view the J 1–0 emissivity5 is determined by the total rate of excitation from Applied Sciences Bonn-Rhein-Sieg, SanktJ Augustin, = 0 to Germany.all states Institut with de J Radioastronomie> 0, for which Millimétrique, we obtain Saint-Martin-d’Hères, a value of 2.8 France.× 10− *e-mail:7 of [email protected] the large discrepancies reported in the literature, and because our 4 –0.5 3 −1 model based on the latest cross-sections underpredicts the observed (T/10 K) cm s (where T is the temperature). N A T U R E | www.nature.com/nature Given the rate coefficients discussed above, our model predicts an line fluxes, our findings may stimulate further studies of these reac- integrated main-beam brightness temperature of 0.86 K km s−1, a tions (and of the corresponding radiative association of He and H+ factor of roughly four below the value we observe. The most uncer- that dominates under the conditions of the early Universe). The vali- tain of the rate coefficients is probably k1; if we take the approach dation of these uncertain values by our astronomical measurements is of adjusting its value to fit the observed line intensity, we obtain limited by the relative simplicity of our physical model for the source; −16 3 −1 12 k1 = 6.0 × 10 cm s . Figure 2 shows the results obtained using as in previous modelling efforts , we have approximated an elongated this value. As expected, the production of HeH+ peaks sharply in the nebula as being spherically symmetrical. Future non-spherically sym- He+/H overlap layer, reaching a peak abundance relative to H nuclei of metrical models could refine our estimates, but are beyond the scope 4.0 × 10−8. The column density from the centre to the edge of the neb- of this study. ula is 2.4 × 1012 cm−2; the total line flux emitted by NGC 7027 in the Although HeH+ is of limited importance on Earth today, the chem- J = 1 − 0 transition, based on the model, is 2.1 × 10−13 erg s−1 cm−2. istry of the Universe began with this ion. The lack of definitive evi- For the infrared rovibrational lines, we compute fluxes that are consist- dence for its very existence in interstellar space has been a dilemma ent with previous nondetections. For the v = 1–0 R(0) line, observed for astronomy. The unambiguous detection reported here brings a dec- using a circular aperture of 8″ that did not fully encompass the source14, ades-long search to a happy ending at last—a success that has become the predicted flux of 1.3 × 10−14 erg s−1 cm−2 is roughly three times possible thanks to maturing terahertz technologies (incorporated in the less than the observed upper limit. For the v = 1–0 P(2) line, the flux upGREAT instrument) and the timely availability of the unique SOFIA

N A T U R E | www.nature.com/nature Astrochemistry: 50 years of discoveries

ISM Comets 220 220 70 70

200 200 60 60 1802018 180 2016

160 160 50 50 140 140

4 per year 40 40 120 120

100 100 1.5 per year 30 30 80 80 Total number of molecules 60 60 Total number of molecules 20 20

40 40 CH 10CN 10 CN 20+ 20 C3 CH C2 0000 1940 1960 1980 2000 2020 1860 1880 1900 1920 1940 1960 1980 2000 2020 Year Year

Source: http://www.astrochymist.org/ Non-local-thermodynamic-equilibrium (non-LTE)

• Pressure in the ISM are very CH NH excitation @ T=30K -10 2 low (< 10 mbar) 40

30 non-LTE Tkin 20 regime • Inelastic collisions are 10

0 insufficient to maintain LTE Trad -10

-20 Excitation temperature (K) • Deviations from LTE, e.g. -30 -40 1 2 3 4 5 6 7 8 9 10 masers, are natural in space 10 10 10 10 10 10 10 10 10 10 -3 H2 density (cm ) Collisional data

• Interpreting a spectra requires to solve: – the radiative transfer – the statistical equilibrium

• Molecular data: – energies of levels – radiative rates (s-1) – collisional rate coefficients (cm3s-1) Colliding partners in the ISM particle CO(j) + e- à CO(j’) + e- atom CO(j) + H à CO(j’) + H atom CO(j) + He à CO(j’) + He

molecule CO(j) + H2(j2) à CO(j’) + H2(j2’) Rotational energy transfer

• Cross sections and rate coefficients – heavy particles σ ~ 10 Å2 k ~ 3*10-11 cm3s-1 – electrons σ ~ 3000 Å2 k ~ 10-6 cm3s-1

• Collision timescales (<< chemistry in general) – heavy particles t ~ 1 week (dense ISM) – electrons t ~ 1 year (diffuse ISM) I. COLLISIONAL EXCITATION Methods

• Scattering theory – Electronic Schrödinger equation (CCSD(T)/CBS) – Nuclei dynamics • Quantum close-coupling method • Classical, mixed quantum/classical or statistical methods – Loreau, Lique, Faure ApJL (2018)

• Experiment – Crossed molecular beams (relative cross sections) – Double resonance (absolute rate coefficients) – Raman spectroscopy – Pressure broadening Experimental breakthrough

week ending PRL 109, 023201 (2012) PHYSICAL REVIEW LETTERS 13 JULY 2012

Appearance of Low Energy Resonances in CO– Para-H2 Inelastic Collisions

Simon Chefdeville,1,2 Thierry Stoecklin,1,2 Astrid Bergeat,1,2 Kevin M. Hickson,1,2 Christian Naulin,1,2 and Michel Costes1,2,* 1Universite´ de Bordeaux, Institut des Sciences Mole´culaires, F-33400 Talence, France 2CNRS, UMR 5255, F-33400 Talence, France (Received 24 April 2012; published 11 July 2012) We report on crossed-beam experiments and quantum-mechanical calculations performed on the CO j 0 H j 0 CO j 1 H j 0 system. The experimental cross sections determined ð ¼ Þþ 2ð ¼ Þ ! ð ¼ Þþ 2ð ¼ Þ in the threshold region of the CO j 0 j 1 transition at 3:85 cm 1 show resonance structures in good ð ¼ ! ¼ Þ À qualitative agreement with the theoretical ones. These results suggest that the potential energy surface which describes the CO-H2 van der Waals interaction should be reinvestigated for good quantitative agreement.

DOI: 10.1103/PhysRevLett.109.023201 PACS numbers: 34.50.Ez, 34.10.+x, 37.20.+j

Shape resonances, also called orbiting resonances in The lowest relative translational energies are attained only elastic or inelastic collision processes, are associated when both laboratory frame velocities are approximately with quasibound states trapped behind the centrifugal equal. In this respect, deceleration of a single beam does barrier within the shallow potential well which results not suffice. from the weak van der Waals interaction of the colliding The CO j 0 H2 j 0 CO j 1 H2 j 0 sys- partners as they approach [1]. They correspond to particu- tem is suitableð ¼ forÞþ studyð ¼ byÞ! both theoreticalð ¼ Þþ andð ¼ experi-Þ lar quantum-scattering states, or partial waves, each char- mental means and is also of paramount importance in the acterized by a fixed value of total angular momentum J interstellar medium. Molecular hydrogen is by far the most which is conserved throughout the collision. The presence abundant molecule in space, followed by carbon monox- of resonance structures may be revealed by sharp enhance- ide. The ubiquity of CO in galactic and extragalactic ments of the integral cross section when the collision sources makes it an excellent tracer of physical conditions, energy of the colliding partners matches the energy of provided its rates of excitation and relaxation by inelastic the quasibound states. Whereas resonances of this type collisions with H2 are precisely known. Previous QM have been identified in several elastic collision experiments calculations describing CO rotational cooling in the low performed in the 1970s [2– 6], they have so far eluded temperature conditions of the interstellar medium have experimental observation for inelastic collision events [1]. demonstrated the importance of resonances and their sen- Quantum-mechanical (QM) calculations performed on po- sitivity to the underlying PES that is utilized [7,8]. Here, tential energy surfaces (PESs) generated by ab initio meth- we report on the first high-resolution crossed-beam experi- ods predict rich resonance structures in the threshold regions ments to provide the collision energy dependence of state- of rotational transitions for many inelastic collision pro- to-state integral cross sections (otherwise known as the cesses between stable molecules or radicals and molecular excitation function) in the vicinity of the cold regime, hydrogen or helium [7– 13]. Their observation relies on and the experimental results are compared with those of high-resolution crossed-beam scattering experiments oper- new QM calculations. ating in the vicinity of the cold energy regime where most We performed experiments with Eq. (1) in mind: both of these features appear. Arguably, the most promising beam velocities were decreased to the lowest possible crossed-beam methods for observing such phenomena matched values while reducing the beam-intersection angle would employ slow beams of state-selected molecules pre- as much as physically possible. We obtained low velocity pared by Stark or Zeeman deceleration [13– 18]. However, CO and para-H2 beams from cryogenically cooled Even- state-selected and decelerated beams need to be coupled Lavie fast-pulsed valves [19] (see Table I). The cooling not with cryogenic cooling for H2 or He supersonic beams and only slowed down the beams while enhancing their velocity low beam-intersection angles to obtain the prerequisite low resolution but also quenched most of the internal-state energies [13,16]. Indeed, in a collision experiment between population to the ground rotational state. A minimal energy 1 two beams of colliding species sharing a reduced mass , ET 5:5cmÀ was obtained in a recent study of the 1 ¼ with laboratory frame velocities 1 and 2 and beam- S D 2 H2 j 0; 1 SH H reaction [20] and ET ð Þþ1 ð ¼ Þ ! þ ¼ intersection angle , the collision energy in the center-of- 9:8cmÀ when utilizing a neat para-H2 j 0 beam [21] mass frame or more precisely the relative translational in an apparatus designed with a minimumð intersection¼ Þ angle energy is given by 22:5 . For the present study, the apparatus was up- graded¼ to attain 12:5 , allowing energies below the 1 2 2 ¼  ET 2 1 2 212 cos : (1) threshold of the CO j 0 j 1 rotational transition at ¼ ð þ À Þ ð ¼ ! ¼ Þ

0031-9007=12=109(2)=023201(5) 023201-1 Ó 2012 American Physical Society ‘The PES […] should be reinvestigated for good quantitative agreement’ week ending PRL 109, 023201 (2012) PHYSICAL REVIEW LETTERSChefdeville et al. PRL (2012)13 JULY 2012

2 a ) α β

exp. CO(j=0à 1) γ ) 1 2 ) (nm

0 → 2 V theory Integral cross section ( 04 (Jankowski & Szalewicz 2005) 0 2 Total Partial waves b J=0 J=1 J=2 J=3 )

2 J=4 J=5 J=6 J=7 J=8 J=9

J=10 ( section cross Integral 1 1 Partial waves (nm

0 0 510152025 Relative translational energy (cm-1)

FIG. 2 (color online). (a) Experimental integral cross sections in arbitrary units (open circles with vertical error bars at a 95% confidence interval) and theoretical integral cross sections (solid curve) convoluted over the energy spread. (b) Partial wave cross sections and resulting theoretical integral cross sections from QM calculations performed with the PES of Jankowski and Szalewicz [27]. direct comparison with the experiment. The very sharp strikingly different. Most of the intense peaks have shifted features of Fig. 2(b) are smeared out by the energy spread. by ca. 1 to 1:5 cm 1, which can be accounted for by À À À Nonetheless, three waves persist but with a doubled peak the difference in D0 values between the original and the structure for waves and and all shifted towards higher scaled PES, whereas their shapes and strengths are strongly energies. modified. As a result, the initial doubled peak structure of It is well known that resonance features are strikingly wave has disappeared. The position of the three waves is dependent on the PES used to perform the QM calcula- now in reasonable agreement with experiment. It becomes tions. A main source of the discrepancy highlighted in clear that wave borrows its intensity mainly from partial Fig. 2(a) could arise from the dissociation energy, D , of waves J 1, 2, and 3; wave from J 4, 6, and 8; and 0 ¼ ¼ the CO-H2 complex respective to the ground-state (000) wave from J 5, 7, and 9. Nonetheless, an agreement level. Although the PES is recognized to reach spectro- on the respective¼ magnitudes of the 3 waves has not been scopic accuracy for the infrared spectrum and the energy achieved yet. Interestingly, Jankowski and Szalewicz 1 levels of the complex, within 0:1 cmÀ on average, it themselves noticed that their ab initio PES furnished better 1 yields D0 19:5 cmÀ , whereas infrared spectroscopic results for comparison with the temperature dependence of ¼ 1 results converge towards D0 22 cmÀ [28]. The mani- experimental second-order virial coefficients when re- fold of quasibound states trapped¼ behind the centrifugal scaled with a factor f 1:042 [27]. barrier and screened by the variation of the relative trans- In conclusion, our measurements¼ provide the first sensi- lational energy near the CO j 0 j 1 threshold tive probe of a PES under collision conditions approaching ð ¼ ! ¼ 1Þ could be shifted substantially by this 2:5 cmÀ difference. the cold energy regime and the first experimental observa- In an effort to find an agreement with experiment, we tion of partial wave resonances appearing in the integral applied various scaling factors f>1 to the PES, resulting cross sections for an inelastic collision process. It has been in a lowering of the global minimum and a concomitant necessary to rescale the original ab initio PES in order to increase of the dissociation energy. The closest match is obtain QM results in agreement with the experiment, de- found for f 1:05, which yields D 21 cm 1. The spite this initial PES being capable of reproducing infrared ¼ 0 ¼ À results of the QM calculations are presented in Fig. 3, transitions and energy levels of the CO-H2 complex with 1 and it can be seen that the resonance pattern is indeed an overall accuracy of 0:1 cmÀ . Clearly, the criteria

023201-3 Revised results (2015)

The Astrophysical Journal Letters,799:L9(4pp),2015January20 Chefdeville et al.

(a) CO(j=0à 1)

(b)

Figure 2. Integral cross-sections for the CO rotational transition 0 1 induced Chefdeville et al.→ ApJL (2015) by para-H2 as function of collision energy. The corrected experimental data (Naulin & Costes 2014; same conditions as Figure 1)iscomparedtothe new theoretical ICS calculated with the V12 PES and convoluted with the experimental energy spread.

(a) Figure 1. Collisional energy dependence of the integral cross sections for CO excitation (jCO 0, 1 2) by para-H2.(a)TheoreticalICSscalculatedwith = → V12 PES (Jankowski et al. 2012) for both possible transitions 0 2and1 2, (b) comparison of theoretical ICSs convoluted with the experimental→ collision→ energy spread (solid blue line), with a 10% relative contribution of collisions with CO(jCO 1) with experimental data (open circles) in arbitrary units. The = measurements were performed when crossing a beam of neat para-H2 from a 1 pulsed nozzle at ca. 45 K propagating at velocity v1 943 ms with a beam = − of 1% CO in He from another pulsed nozzle at ca. 85 K propagating at v2 1 = 941 ms− ; crossing angle χ was varied from 17.5 to 37◦.5, resulting in a collision 2 2 energy ET (1/2)µ(v1 + v2 2v1v2 cosχ)whereµ stands for the reduced mass of the= collision partners. − (b)

4. RESULTS

The results obtained for CO (jCO 0, 1 2) excitation by = → collision with H2 are displayed in Figure 1:thetheoreticalICSs, calculated with the V12 PES of Jankowski et al. (2012) are given in panel (a); experimental data are displayed in panel (b), along with the theoretical ICSs convoluted with experimental energy spread to allow for comparison. The theoretical data displayed in Figure 1(b) are the sum of the contributions of excitation from the jCO 0 and 1 states. The populations of rotational = level with jCO ! 2arebelowthanourdetectionlimit:therefore Figure 3. Collisional energy dependence of the integral cross sections for CO excitation (jCO 0 1) by normal-H2.(a)TheoreticalICSscalculatedwith the de-excitation contributions from these levels need not be = → taken into account. The 1 2ICSwasincludedasa10% V12 PES (Jankowski et al. 2012)forpara-andortho-H2; (b) theoretical ICSs contribution to the total ICS→ based on average population of convoluted with the experimental collision energy spread (solid blue line), with a 25% relative population of para-H2 (jH2 0) and 75% of ortho-H2 (jH2 1); the jCO 1stateintheexperiment.Notethatthiscontribution experimental data (open circles) in arbitrary= units (for more details, see= the = does not correspond to the jCO 1populationdeterminedwhen caption of Figure 1). probing the central part of the beam= (ca. 1%, corresponding to a rotational temperature close to 1 K); in this collision experiment, performed using the former V04 PES (Jankowski & Szalewicz half the CO beam (from its rising edge to the middle of the 2005), rescaled by a factor of 1.05 to obtain a fair agreement pulse) contributes to the signal; since jCO 1populations (i.e., reproducing the position of the peaks). These former exper- are higher on the edges (up to 15% at the= maximum of the imental data were corrected, similarly as we did for the 0, 1 2 ∼ → signal for jCO 1), the overall contribution due to collisions transitions. The comparison of uncorrected and corrected exper- = of CO(jCO 1) with H2 also is higher (10%, corresponding to imental ICSs, with calculations using the scaled V04 PES can be an average temperature= within the whole beam of about 1.6 K). found in Figure 9 of Naulin & Costes (2014). These experimen- Such a contribution describes well the behavior of the total ICS tal results, along with new theoretical calculations performed below the 0 2threshold. on the V12 PES, are displayed in Figure 2. The experimental→ data on the 0 1transition,aspublished To allow comparison with ortho-H ,experimentalmeasure- → 2 by Chefdeville et al. (2012), were not corrected for the varia- ments on the 0 1rotationalexcitationofCObynormal-H2 tion of the mean interaction time with the beam crossing an- were performed→ (see Figure 3): the theoretical ICSs for para- gle. Furthermore, the calculations presented in this Letter were and ortho-H2,calculatedwiththeV12 PES of Jankowski et al.

3 REPORTS resolved peaks (a, b, and c), with no observable description of this critical parameter (fig. S1) The agreement between experiment and the- difference for the collision partners para-H2 or (13). We estimated that 5% of the interaction en- ory in Fig. 1 is very good. The convoluted theoret- normal-H2.Theexcitationfunctionalsoappears ergy was missing in the potential well. Therefore, ical curve reproduces well the position and width unusual for a molecular collision process. The O2 we scaled the global PES by a factor of 1.05. A of the three experimental peaks. The experimen- (N =1,j =0→ N =1,j =1)transitionviolates previous theoretical study on O2–H2 inelastic scat- tal excitation function falls almost to zero at ET = the propensity rules in rotationally inelastic tering neglecting the fine structure showed that 20 cm–1,whichindicatesanegligiblecontribution 3 collisions of diatomic molecules in S electronic rotation of H2 has almost no influence on the of collision energy transfer in the final observed states (10). Thus, normal scattering cannot occur. magnitude of the cross sections, including the state N =1,j =1fromN =1,j =2residualpop- Scattering can only arise from resonances, and resonances (16). We thus restricted the calcu- ulation in the O2 beam (see also fig. S2) (13). the three observed peaks are proof of this reso- lations to the j =0levelofH2 (i.e., the PES was Furthermore, Fig. 2 demonstrates that inelastic nance behavior. averaged over H2 rotations), considering that scattering with H2 ( j =1)behavesthesameas To gain insight into the resonance structure, our results should also be valid for H2 ( j =1).For with H2 ( j =0),whichjustifiesthetheoretical we performed full quantum close-coupling scat- the scattering calculations, we solved the quantal assumption made (vide supra). tering calculations by using a four-dimensional coupled equations in the intermediate coupling Acomparisonofexperimentalandtheoretical ab initio PES treating O2 and H2 as rigid rotors, scheme by using the MOLSCAT code (17)mod- data shows that each peak in the experimental which had been recently obtained by the coupled ified to take into account the fine structure of the excitation function corresponds to an almost pure cluster method while using single and double O2 energy levels (18, 19). Cross sections were partial wave: peak a to J =2,peakbtoJ =3,and –1 –1 excitation with perturbative contributions from obtained up to ET =50cm on a 0.05-cm grid. peak c to J =4.Thecontributionsfrompartialwaves connected triple excitations and large sets of The results are presented in Fig. 1B for partial J =1andJ >4andtheoverlapsbetweenJ =2,3, atomic basis orbitals (16). Because the well depth waves J = 1 to 7, which contribute at the colli- and 4 remain marginal. To gain insightREPORTS into the ofresolved the PES peaks strongly (a, b, affects and c), the with energy, no width,observable and siondescription energies of sampled this critical in the parameter experiment. (fig. The S1) natureThe of agreement the peaks, webetween calculated experiment adiabatic-bender and the- intensitydifference of for the the scattering collision resonances, partners para we-H per-2 or theoretical(13). We estimated ICSs convoluted that 5% of the with interaction the experi- en- potentialsory in Fig. ( 120 is)andsearchedforallvanderWaals very good. The convoluted theoret- formednormal-H additional2.Theexcitationfunctionalsoappears electronic structure calcula- mentalergy was collision missing energy in the potential spread are well. also Therefore, reported stretchical curve levels reproduces supported well by the these position curves and (21 width). The tionsunusual at a for higher a molecular level of collision theory to process. obtain a The better O2 inwe Fig. scaled 1A. the global PES by a factor of 1.05. A levelsof the three are labeled experimental as N, j peaks.,andl The,where experimen-l is the (N =1,j =0→ N =1,j =1)transitionviolates previous theoretical study on O2–H2 inelastic scat- tal excitation function falls almost to zero at ET = the propensity rules in rotationally inelastic tering neglecting the fine structure showed that orbital20 cm–1 angular,whichindicatesanegligiblecontribution momentum. Figure 3 shows the 3 collisions of diatomic molecules in S electronic rotation of H2 has almost no influence on the Jof= collision 2 potential energy curve, transfer which in correlates the final observed with O2 states (10). Thus, normal scattering cannot occur. magnitude of the cross sections, including the (stateN =1,N j=1,=1)+Hj =1from2( j =0).ThiscurvesupportsoneN =1,j =2residualpop- Scattering can only arise from resonances, and resonances (16). We thus restricted the calcu- ulation in the O beam (see also fig.–1 S2) (13). on September 5, 2013 quasi-bound state2 at E1,1,2 =4.36cm ,slightly the three observed peaks are proof of this reso- lations to the j =0levelofH2 (i.e., the PES was aboveFurthermore, the asymptotic Fig. 2 value demonstrates at E =3.96cm that inelastic–1 but nance behavior. averaged over H rotations), considering that scattering with H ( j =1)behavesthesameas1,1 2 trapped below the2 centrifugal barrier. Tunneling To gain insight into the resonance structure, our results should also be valid for H2 ( j =1).For with H2 ( j =0),whichjustifiesthetheoretical we performed full quantumOther close-coupling scat- theresults scattering calculations, we solved: theoxygen quantal throughassumption the made barrier (vide thus supra). givesand rise to a shape (orwater tering calculations by using a four-dimensional coupled equations in the intermediate coupling orbiting)Acomparisonofexperimentalandtheoretical resonance (5, 22). Another J =2curve, ab initio PES treating O2 and H2 as rigid rotors, scheme by using the MOLSCAT code (17)mod- whichdata shows correlates that eachwith thepeak asymptotically in the experimental closed which had been recently obtained by the coupled ified to take into account the fine structure of the excitation function corresponds to an almost pure channel O2 (N =3,j =4)+H2( j =0)atE3,4 = cluster method while using single and double O2 energy levels (18, 19). Cross sections were partial wave:–1 peak a to J =2,peakbtoJ =3,and –1 –1 16.39 cm , furnishes a different scenario. The excitation with perturbative contributions from obtained up to ET =50cm on a 0.05-cm grid. peak c to J =4.Thecontributionsfrompartialwaves connected triple excitations and large sets of The results are presented in Fig. 1B for partial OJ =1and2–H2 complexesJ >4andtheoverlapsbetween are temporally trappedJ =2,3, in the

–1 www.sciencemag.org atomic basis orbitals (16). Because the well depth waves J = 1 to 7, which contribute at the colli- boundand 4 remain state at marginal.E3,4,4 = 6.19 To gain cm insightbefore into disso- the of the PES strongly affects the energy, width, and sion energies sampled in the experiment. The nature of the peaks, we calculated adiabatic-bender ciating to O2 (N =1,j =1)+H2 ( j = 0), yielding intensity of the scattering resonances, we per- theoretical ICSs convoluted with the experi- aFeshbachresonance(potentials (D20)andsearchedforallvanderWaalsO (023). Therefore→ peak 2 a ) by pH formed additional electronicO2 structure(10 calcula-→ 1mental1) collisionby energypH spread2 are also reported stretch levels supported2 by these00 curves (21). The 02 2 tions at a higher level of theory to obtain a better in Fig. 1A. canlevels be are regarded labeledD2 asO as the N(0, juxtapositionj,and00àl,where202 of) al byis shape the pH2 orbital angular momentum. Figure 3 shows the J = 2 potential curve, which correlates with O2 (N =1,j =1)+H2( j =0).Thiscurvesupportsone –1 on September 5, 2013 quasi-bound state at E1,1,2 =4.36cm ,slightly –1 above the asymptotic value at E1,1 =3.96cm but Downloaded from trapped below the centrifugal barrier. Tunneling through the barrier thus gives rise to a shape (or orbiting) resonance (5, 22). Another J =2curve, which correlates with the asymptotically closed channel O2 (N =3,j =4)+H2( j =0)atE3,4 = 16.39 cm–1, furnishes a different scenario. The O2–H2 complexes are temporally trapped in the

–1 www.sciencemag.org bound state at E3,4,4 = 6.19 cm before disso- ciating to O2 (N =1,j =1)+H2 ( j = 0), yielding aFeshbachresonance(23). Therefore peak a can be regarded as the juxtaposition of a shape Downloaded from Fig. 1. Collisional energy dependence of the integral cross sections for O2 excitation (N =1, j = 0) → (N =1,j =1).(A)Experimentaldatawithpara-H2 (open circles, with error bars representing the statistical uncertainties at 95% of the confidence interval). Each point corresponds to 40 consecutive Fig. 2. CollisionalBergeat energy dependenceet ofal., the in prep. scans of the beamChefdeville intersection angle acquiredet al., between Science 30° and 12.5° with(2013)–0.5° decrement and 100 integral cross sections for O2 excitation (N =1, laser shots per angle; theoretical ICSs were convoluted with the experimental collision energy spread j =0)→ (N =1,j = 1). Experimental data with (solid line). (Inset) Energy-level diagram and excitation scheme of O2 in the N =1state.(B)Theoretical para-H2 (open circles, data of Fig. 1A) and normal- results: partial waves J =2,3,and4(solidlines);partialwavesJ =1andJ =5to7(dashedlines);integral H2 (open triangles). Error bars and scan parameters cross section (dashed-dotted line). Positions of the bound and quasi-bound states (see Fig. 3) labeled with for normal-H2 as defined in Fig. 1A but with 29 their quantum numbers {N, j, l}(seetext)areshownbyverticaldashedlines.a.u.,arbitraryunits. consecutive scans of the beam intersection angle.

Fig. 1. Collisional energy dependencewww.sciencemag.org of the integral crossSCIENCE sections for OVOL2 excitation 341 6 ( SEPTEMBERN =1, 2013 1095 j = 0) → (N =1,j =1).(A)Experimentaldatawithpara-H2 (open circles, with error bars representing the statistical uncertainties at 95% of the confidence interval). Each point corresponds to 40 consecutive Fig. 2. Collisional energy dependence of the scans of the beam intersection angle acquired between 30° and 12.5° with –0.5° decrement and 100 integral cross sections for O2 excitation (N =1, laser shots per angle; theoretical ICSs were convoluted with the experimental collision energy spread j =0)→ (N =1,j = 1). Experimental data with (solid line). (Inset) Energy-level diagram and excitation scheme of O2 in the N =1state.(B)Theoretical para-H2 (open circles, data of Fig. 1A) and normal- results: partial waves J =2,3,and4(solidlines);partialwavesJ =1andJ =5to7(dashedlines);integral H2 (open triangles). Error bars and scan parameters cross section (dashed-dotted line). Positions of the bound and quasi-bound states (see Fig. 3) labeled with for normal-H2 as defined in Fig. 1A but with 29 their quantum numbers {N, j, l}(seetext)areshownbyverticaldashedlines.a.u.,arbitraryunits. consecutive scans of the beam intersection angle.

www.sciencemag.org SCIENCE VOL 341 6 SEPTEMBER 2013 1095 CO-H2 @ CRESU (5-20 K)

Labiad et al., in preparation II. MOLECULAR DIAGNOSTICS Molecules as probes

cosmic microwave [email protected] K (!'()

source continuum (!&)

molecular cloud (!"#, τ)

./ radiotelescope !' = (!"# − !'( − !&)(1 − - ) The Astrophysical Journal,728:36(24pp),2011February10 Ritchey, Federman, & Lambert

CN absorption in diffuse clouds

Figure 12. Profile synthesis fits to the B X (0, 0) band of CN toward HD 73882. See Figure 3 for a description of the plotting symbols. The same range in velocity is shown for each panel. The positions of− the R(0) and R(1) lines of 13CN are indicated by tick marks. When both isotopologues are present, the two profiles are synthesized simultaneously.

MWO, McKellar (1940)

‘From the intensity of the lines [of CN] a rotational temperature of 2.3K follows, which has of course only a restricted meaning.’

Figure 13. Same as Figure 12 except for the CN B X (0, 0) band toward HD 152236. Two line-of-sight components are clearly identified in the R(0) and R(1) lines of 12CN and are also detected in the P(1) line. The− position of the 13CN R(0) line corresponding to the stronger of the two componentsG. is indicatedHerzberg by a tick mark. (1950) Since the R(2) and P(2) lines are below the detection limit, no fits were attempted for these features.

Figure 14. Same as Figure 12 except for the CN B X (0, 0) band toward HD 154368. The positions of the R(0) and R(1) lines of 13CN are indicated by tick marks. The weak absorption feature blueward of the strong−12CN R(0) line is due to a second very weak component along the line of sight, which is positively identified in the spectra of CH+ and CH. VLT, Ritchey et al. (2011)

14 CN excitation and electron densities in diffuse molecular clouds 5

CN excitation and electron densities in diffuse molecular clouds-2 5 -2 1e-09 1e-09 N(CN)=3e12cm N(CN)=3e13 cm

-2 3 -2 1e-09 1e-09 N(CN)=3e12cm N(CN)=3e13-3 cm -3 n=100 cm n=100 cm 2, 2.5,3 3.5 - 1, 0.5, 1.5 1e-10 -3 2.9 -3 n=100 cm n=100 cm 2, 2.5, 2,3.5 2.5, - 1, 3.5 0.5, - 1, 0.5 0.5, 1.5 1e-10 1e-10 2.9 /s)

3 2.8 2, 2.5, 3.5 - 1, 0.5, 0.5 1e-10 /s) 3 1e-11 2.8CN excitation and electron densities in diffuse molecular clouds 5 2.7 1e-11 -2 -2 1e-11 N(CN)=3e12cm N(CN)=3e13 cm 2.7 3 -2 -2 1e-09 1e-09 3 N(CN)=3e12cm N(CN)=3e13 cm 1e-11 3 -3 -3 1e-12 -3 3 n=300 cm-3 n=300 cm n=100 cm -3 n=100 cm-3 2.9 -3 -3 1e-12 2.9 n=300 cm n=300n=100 cm cm n=100 cm 2.9 2, 2.5, 3.5 - 1, 0.5, 1.5 (K) Rate co-efficient (cm

1e-10 01 2.9 (K)

1e-12 T Rate co-efficient (cm 1e-10 2.8 2, 2.5, 3.5 - 1, 0.5, 0.5 2.801 T 1e-13 1e-12 /s) 2.8 CN as a probe of electron density 3 2.8 1e-13 1e-11 2.7 2.72.7 32.7 1e-11 33 3 1e-14 1e-13 -3 -3 -3 -3 1e-14 01e-13 20 40 60 80 100 0 20 40 60 80-3 100 n=1000 cm-3 n=1000-3 cm 0 20 40 60 80 100 01e-12 20 40 60 80 100 n=300n=1000 cm cm 2.9 nn=300=1000n=300 cm cmcm n=300 cm Temperature (K) 2.9Temperature (K) 2.9 Temperature (K) Temperature (K) 2.9 (K) (K) Rate co-efficient (cm 01 01 T

T 2.8 1e-09 1e-09 1e-05 1e-05 1e-122.82.8 2.8 1e-13 Tloc=29 ± 3 mK T =2.725 K 2.72.7 CMB 2.72.7 1e-06 1e-06 0.01 0.1 1 0.01 0.1 1 2, 2.5, 3.5 - 1, 1.5, 1.5 2, 2.5, 3.5 - 1, 1.5, 1.5 0.01 0.1 1 3 0.01 0.1 1 3 -3 -3-3 -3 1e-14 1e-13 Electron density (cm ) ElectronElectron densitydensity-3 (cm (cm ) ) Electron-3 density (cm ) 1e-10 1e-10 0 20 40 60 80 100 0 20 40 60-3 80 100 n=1000 cm-3 n=1000 cm 1e-07 2, 2.5, 3.5 - 1, 1.5, 2.5 1e-07 2, 2.5, 3.5n -=1000 1, 1.5, 2.5cm 2.9 n=1000 cm /s) /s) Temperature (K) Temperature (K) 3 3 2.9 Figure 7. Excitation temperature T01(CN) as a function of elec- 1e-09 1e-05 Figure2.8 7. Excitation temperature T01(CN) as a function of elec- 1e-08 1e-08 tron density for different densities (n = n + n ) and CN col- 2.8 tron densityH for diHff2erent densities (n = nH + nH2 ) and CN col- 1e-11 2.7 1e-11 1e-06umn densities, at a single kineticumn temperature densities, at of a 20 single K. Here kinetic the temperature of 20 K. Here the 1e-09 2, 2.5, 3.5 - 1, 1.5, 1.5 0.01 0.1 1 0.01 0.1 1 dashed2.7 line represents the CMB at 2.725 K while the dotted-3 blue -3 1e-09 0.01 0.1 1 0.01Electron density 0.1 (cm ) 1 Electron density (cm ) 1e-10 dashed line represents the CMB at 2.725 K while the dotted blue 1e-07line gives2, the 2.5, 3.5 measured - 1, 1.5, 2.5 average-3 excitation temperature at 2-3.754 K /s) Electron density (cm ) Electron density (cm ) 3 Rate co-efficient (cm 1e-10 line gives the measured average excitation temperature at 2.754 K

Rate co-efficient (cm (Ritchey et al. 2011). 1e-12 1e-10 Figure 7. Excitation temperature T01(CN) as a function of elec- 1e-12 (Ritchey et al. 2011). 1e-08 tron density for different densities (n = n + n ) and CN col- 1e-11 H H2 1e-11 1e-11 umn densities, at a single kinetic temperature of 20 K. Here the 1e-09 dashed line represents the CMB at 2.725 K while the dotted blue 1e-13 1e-12 which T01 was computed by summing over hyperfine sub- 0 20 40 60 801e-13 100 0 20 40 60 801e-12 100 levels. The dashed horizontalwhich lineline gives givesT01 thewas the measured CMB computed at average 2.725 by excitation K summing temperature over hyperfine at 2.754 K sub- Temperature (K) Rate co-efficient (cm Temperature (K) 1e-10 0 201e-12 40 60 80 100 0 20 40 60 80 100 levels.(Ritchey The et dashed al. 2011). horizontal line gives the CMB at 2.725 K Temperature (K) whileTemperature the horizontal (K) dotted line gives the measured excita- 1e-11tion temperature T01 at 2.754while K. the We horizontal first observe dotted that the line gives the measured excita- Figure 6. A comparison of the hyperfine structure rate from the excess temperature of 29 mKtion cannot temperature be reproducedT01 at at 2.754 very K. We first observe that the N,J,F , . , . which T01 was computed by summing over hyperfine sub- ( )=(2 2 5 3 5) initialFigure level 6. betweenA comparison1e-13 this work of (solid the hyperfine line) structure1e-12low electron rate from density, the indicatingexcess that temperature neutral collisions of 29 mK alone cannot be reproduced at very and the rate of Kalugina et al. (2012) (dashed0 line). 20 40 60 80 100 0 20 40 60 80 100 levels. The dashed horizontal line gives the CMB at 2.725 K (N,J,F)=(2, 2.5, 3.5) initialTemperature level (K) between this workTemperature (solid line) (K) cannot explain the local excitationlowwhile electron the of horizontal CN. density, This dotted indicating confirms line gives that the neutral measured collisions excita- alone and the rate of Kalugina et al. (2012) (dashedthe conclusionsline). of past investigators (Thaddeus 1972; Black cannottion temperature explain theT01 localat 2.754 excitation K. We first of observe CN. This that confirms the Figure 6. for hydrogen atoms1. This kind of analysis hasA comparison been previ- of the hyperfine& van structure Dishoeck rate 1991). from the Second,theexcess conclusions it can temperature be noticed of past of 29 that investigators mK the cannot be (Thaddeus reproduced 1972; at very Black (N,J,F)=(2, 2.5, 3.5) initial level betweenlocal excitation this work (solid is reproduced line) for a rather restricted range 1 & vanlow electron Dishoeck density, 1991). indicating Second, that it neutral can be collisions noticed alone that the ously performed by Blackfor & hydrogen van Dishoeckand atoms the rate (1991). of This Kalugina with kind old et of al. analysis (2012) (dashed has been line). previ- −3 −3 of electron densities: from 0.01cannot cm explainat n=1000 the local cm excitationto of CN. This confirms collision data. −3 −3 local excitation is reproduced for a rather restricted range ously performed by Black & van Dishoeck0.06 (1991) cm at withn=100 old cm with a weak dependence on the −3 −3 Radiative transfer calculations were performed with the ofthe electron conclusions densities: of past from investigators 0.01 cm (Thaddeusat n=1000 1972; Black cm to collision data. 1 CN column density. Assuming& hydrogen van− Dishoeck is entirely 1991). Second,− molec- it can be noticed that the RADEX code (van der Tak et al. 2007),for using hydrogen the Large atoms Veloc-. This kind of analysis has been previ- 0.06 cm 3 at n=100 cm 3 with a weak dependence on the Radiative transfer calculations wereular, performed these with electron the densitylocal correspond excitation to is electron reproduced frac- for a rather restricted range ity Gradient (LVG) approximation forously an expandingperformed by sphere. Black & van Dishoeck (1991) with old CN column density.−5 Assuming−4 hydrogen−3 is entirely−3 molec- RADEX code (van der Tak et al. 2007), usingtions the (x Largee = n Veloc-(e)/n(H2)) inof the electron range densities: 10 − 6 from× 10 0.01. cm at n=1000 cm to The kinetic temperature was fixed atcollisionT =20 K, data. as in the cal- − − ular,0.06 these cm 3 electronat n=100 cm density3 with+ correspond a weak dependence to electron on the frac- ity Gradient (LVG)Radiative approximation transfer calculations for anThis expanding were is performed consistent sphere. with with the the abundance of interstellar C − − culations of Ritchey et al. (2011). The line width (FWHM) + −4 5 − × 4 −1 RADEX (n(C )/n(H2) ∼ 3 × 10 )tions whichCN ( is columnxe the= main density.n(e) source/n(H Assuming2)) of elec- in the hydrogen range is entirely10 molec-6 10 . was taken to be 1.0 kmsThe, corresponding kinetic temperaturecode to a (van Doppler was der Takfixed line et at al.T 2007),=20 K, using as the in the Large cal- Veloc- + −ity1 Gradient (LVG) approximationtrons for an in expanding the diffuse sphere. interstellarThisular, medium. is these consistent electron with density the correspond abundance to of electron interstellar frac- C broadening parameter bculationsof 0.6 kms of Ritchey. The column et al. (2011). density The line width (FWHM) + −4 −5 − × −4 −1 12 In fact, more accurate(n determination(Ctions)/n (x(He =2) n∼( ofe3)/n× the(H102 electron))) in which the range is the 10 main source6 10 of. elec- was taken at two typicalwas values takenN toThe(CN) be kinetic 1.0 = kms 3 temperature× 10, correspondingand was fixed at toT =20 a Doppler K, as in line the cal- + 13 −2 − This is consistent with the abundance of interstellar C × culations of Ritchey et al. (2011).1 density The line can width be achieved (FWHM) fortrons clouds in+ where the diff theuse physical interstellar−4 con- medium. 3 10 cm . The densitybroadening of neutral parameter collisionb of partners 0.6 kms−1 . The column density (n(C )/n(H2) ∼ 3 × 10 ) which is the main source of elec- was taken to be 1.0 kms , correspondingditions are to a reasonably Doppler12 line well known.In fact, For more instance, accurate the ki- determination of the electron (n = n(H) + n(H2)) waswas fixed taken at three at two representative typical values val- N(CN)− =1 3 × 10 and trons in the diffuse interstellar medium. −3 13 broadening−2 parameter b of 0.6 kmsnetic. temperature The column density and the collisiondensity density can be were achieved determined for clouds where the physical con- ues: 100, 300 and 10003 cm× 10. Finallycm . the The electron density abun- of neutral collision partners× 12 In fact, more accurate determination of the electron × −3 was taken−3 at two typical valuesforN(CN) the source = 3 HD10 154368and from the analysis of C2 excita- dance was varied from 2 10 to 1 cm 13, corresponding−2 ditionsdensity are can reasonably be achieved well for clouds known. where For the instance, physical con- the ki- (n = n(H)3 +×n(H10 2))cm was− . fixed The density− at three of neutral representative collision partnersval- × 6 − 2 tion by Sonnentrucker et al. (2007). These authors found to electron fractions n(e)/n in the range 2 10 to 103 . neticditions+50 temperature are reasonably and the well collision known.−3 density For instance, were thedetermined ki- ues: 100, 300(n = andn(H) 1000 + n(H cm2)) was. Finally fixed at the three electron representative± abun- val- − − T −= 20 5 K and n = 150−25,withn(H)=60 cm Results are presented in Fig. 7. In each panel, the excitation× 3 3 3 −3 fornetic the temperature source HD and 154368 the collision from the density analysis were determined of C2 excita- dance was variedues: 100, from 300 2 and10 1000 cmto 1. cm Finallyand ,n corresponding(H the2)=90 electron cm abun-. The CN column density towards temperature T01(CN) is plotted as a function of the elec- −3 −−3 − for the source HD 154368 from the analysis of C2 excita- dance was varied from 2 × 10 to 1× cm 6, corresponding2 ×tion13 by Sonnentrucker−2 et al. (2007). These authors found to electron fractions n(e)/n in the rangethe 2 star10 HDto 154368− 10 . is− 2.7 10 cm and the line width +50 −3 tron abundance. It should be noted that our excitation cal- − 6 2 tion by± Sonnentrucker et al. (2007). These authors found to electron fractions n(e)/n in the range 2 × 101 to 10 . T = 20 5 K and n = 150−25,withn(H)=60− cm Results are presented in Fig. 7. In each panel,is 1.2 the kms excitation(Ritchey et al. 2011).± Interestingly,− this+50 3 culations provide the populations of hyperfine levels, from T = 20 5 K and3 n = 150−25,withn(H)=60 cm Results are presented in Fig. 7. In each panel, the excitation and n(H2)=90 cm−3 . The CN column density towards temperature T01(CN) is plotted as a functionsource also of shows the elec- the second highestand n(H excitation2)=90 cm temperature. The CN13 column−2 density towards temperature T01(CN) is plotted as a function of the elec- × − T01(CN)=2.911±0.004 K, whichthe star is significantly HD 154368 larger is 2.7× than1013 cm2 and the line width tron abundance. It should be noted that our excitation cal- the star HD−1 154368 is 2.7 10 cm and the line width tron abundance. It should be noted that our excitation cal- −1 1 the weighted mean value ofis 2.754 1.2 K. kms Using the(Ritchey physical et con- al. 2011). Interestingly, this We note that the rate coeculationsfficients for provide ortho-H the2(j populations= 1) exceed of hyperfine levels, from is 1.2 kms (Ritchey et al. 2011). Interestingly, this culations provide the populations of hyperfine levels, from source also shows the second highest excitation temperature those for para-H2(j =0)byuptoafactorof10(Lique,private ditions determined by Sonnentruckersource also et shows al. (2007), the second we have highest excitation temperature −3±± communication). However ortho-H2(j =1)canbeneglectedhere found that an electron densityT01T(CN)=2.91101 of(CN)=2.911 0.3 cm is00..004 necessary004 K, K, which which to is is significantly significantly larger larger than than 1 01 the weighted mean value of 2.754 K. Using the physical con- since its abundance in cold1 (T<30 K) diWeffuse note clouds that is the expected rate coefficientsreproduce for ortho-H the2(j measured= 1) exceedT thetowards weighted HD 154368. mean value This of corre- 2.754 K. Using the physical con- We note that the rate coefficients for ortho-H2(j = 1) exceed ∼ × −3 to be at least 30 times lower than thatthose of para-H for para-H2(j =0).2(j =0)byuptoafactorof10(Lique,privatesponds to an electron fractionditionsditionsn(e)/n determined(H determined2) 3 by by10 Sonnentrucker Sonnentrucker,which et et al. al. (2007), (2007), we have we have those for para-H2(j =0)byuptoafactorof10(Lique,private −3 −3 communication). However ortho-H2(j =1)canbeneglectedhere found that an electron density of 0.3 cm is necessary to communication). However ortho-H2(j =1)canbeneglectedhere found that an electron density of 0.3 cm is necessary to ⃝c 0000 RAS, MNRAS 000,000–000 since its abundance in cold (T<30 K) diffuse clouds is expected reproduce the measured T01 towards HD 154368. This corre- since its abundance in cold (T<30 K) diffuse clouds is expected reproduce the measured T01 towards HD∼ 154368.× −3 This corre- to be at least 30 times lower than that of para-H2(j =0). sponds to an electron fraction n(e)/n(H2) 3 10 ,which−3 to be at least 30 times lower than that of para-H2(j =0). sponds to an electron fraction n(e)/n(H2) ∼ 3×10 ,which ⃝c 0000 RAS, MNRAS 000,000–000 ⃝c 0000 RAS, MNRAS 000,000–000 CN excitation and electron densities in diffuse molecular clouds 5

CN excitation and electron densities in diffuse molecular clouds-2 5 -2 1e-09 1e-09 N(CN)=3e12cm N(CN)=3e13 cm

-2 3 -2 1e-09 1e-09 N(CN)=3e12cm N(CN)=3e13-3 cm -3 n=100 cm n=100 cm 2, 2.5,3 3.5 - 1, 0.5, 1.5 1e-10 -3 2.9 -3 n=100 cm n=100 cm 2, 2.5, 2,3.5 2.5, - 1, 3.5 0.5, - 1, 0.5 0.5, 1.5 1e-10 1e-10 2.9 /s)

3 2.8 2, 2.5, 3.5 - 1, 0.5, 0.5 1e-10 CN excitation and electron densities in diffuse molecular clouds 5 /s) 3 1e-11 2.8CN excitation and electron densities in diffuse molecular clouds 5 2.7 1e-091e-11 1e-09 -2 -2 1e-11 N(CN)=3e12cm N(CN)=3e13 cm 2.7 3 -2 -2 1e-09 1e-09 3 N(CN)=3e12cm N(CN)=3e13 cm 1e-11 3 -3 -3 1e-12 -3 3 n=300 cm-3 n=300 cm n=100 cm -3 n=100 cm-3 2, 2.5, 3.5 - 1, 0.5, 1.5 2.9 -3 -3 1e-101e-12 2.9 n=300 cm n=300n=100 cm cm n=100 cm 2.9 2, 2.5, 3.5 - 1, 0.5, 1.5 (K) Rate co-efficient (cm

1e-10 1e-10 01 2.9

2, 2.5, 3.5 - 1, 0.5, 0.5 (K)

1e-12 T Rate co-efficient (cm 1e-10 2.8 /s) 2, 2.5, 3.5 - 1, 0.5, 0.5 01 3 2.8 T 1e-13 1e-12 /s) 2.8 CN as a probe of electron density 3 2.8 1e-111e-13 1e-11 2.7 2.72.7 32.7 1e-11 1e-11 3 1e-14 1e-13 3 3 -3 -3 1e-14 01e-13 20 40 60 80 100 0 20 40 60 80-3 -3 100 n n-=1000 cm-3-3-3 n + n=1000-3 cm 1e-12 0 20 40 60 80 100 01e-12 20 40 60 80 100 nn=300=1000 cm cm 2.9 (enn=300=1000)n ~=300 0.03 cm cmcm cm ~ (C ) n=300 cm Temperature (K) 2.9Temperature (K) 2.9 Temperature (K) Temperature (K) 2.9 (K) (K) Rate co-efficient (cm Rate co-efficient (cm 01 01 T

T 2.8 1e-09 1e-091e-121e-05 1e-05 1e-122.82.8 2.8 1e-13 1e-13 Tloc=29 ± 3 mK T =2.725 K 2.72.7 CMB 2.72.7 1e-06 1e-06 0.01 0.1 1 0.01 0.1 1 2, 2.5, 3.5 - 1, 1.5, 1.5 2, 2.5, 3.5 - 1, 1.5, 1.5 0.01 0.1 1 3 0.01 0.1 1 3 -3 -3-3 -3 1e-14 1e-13 Electron density (cm ) ElectronElectron densitydensity-3 (cm (cm ) ) Electron-3 density (cm ) 1e-141e-10 1e-101e-13 0 20 40 60 80 100 0 20 40 60-3 80 100 n=1000 cm-3 n=1000 cm 0 20 40 60 80 100 1e-070 202, 2.5, 40 3.5 - 60 1, 1.5, 802.5 1e-07 100 2, 2.5, 3.5n -=1000 1, 1.5, 2.5cm 2.9 n=1000 cm /s) /s) Temperature (K) Temperature (K) Harrison, Faure & Tennyson MNRAS (2013) 3 Temperature (K) 3 Temperature (K) 2.9 Figure 7. Excitation temperature T01(CN) as a function of elec- 1e-09 1e-05 Figure2.8 7. Excitation temperature T01(CN) as a function of elec- 1e-08 1e-08 tron density for different densities (n = n + n ) and CN col- 1e-09 1e-05 2.8 tron densityH for diHff2erent densities (n = nH + nH2 ) and CN col- 1e-11 2.7 1e-11 1e-06umn densities, at a single kineticumn temperature densities, at of a 20 single K. Here kinetic the temperature of 20 K. Here the 1e-09 2, 2.5, 3.5 - 1, 1.5, 1.5 0.01 0.1 1 0.01 0.1 1 dashed2.7 line represents the CMB at 2.725 K while the dotted-3 blue -3 1e-06 1e-09 0.01 0.1 1 0.01Electron density 0.1 (cm ) 1 Electron density (cm ) 2, 2.5, 3.5 - 1, 1.5, 1.5 1e-10 dashed line represents the CMB at 2.725 K while the dotted blue 1e-07line gives2, the 2.5, 3.5 measured - 1, 1.5, 2.5 average-3 excitation temperature at 2-3.754 K /s) Electron density (cm ) Electron density (cm ) 3

Rate co-efficient (cm line gives the measured average excitation temperature at 2.754 K 1e-10 1e-10 2, 2.5, 3.5 - 1, 1.5, 2.5 1e-12 Rate co-efficient (cm 1e-07 1e-10 (Ritchey et al. 2011). Figure 7. T /s) (Ritchey et al.Excitation 2011). temperature 01(CN) as a function of elec- 3 1e-12 1e-08 tron density for different densities (n = n + n ) and CN col- 1e-11 Figure 7. Excitation temperature T01(CN) as a function of elec- H H2 1e-08 1e-11 1e-11 umn densities, at a single kinetic temperature of 20 K. Here the tron density for different densities (n = nH + nH2 ) and CN col- 1e-09 dashed line represents the CMB at 2.725 K while the dotted blue 1e-111e-13 1e-12 umnwhich densities,T01 was at computed a single kinetic by summing temperature over of hyperfine 20 K. Here sub- the 0 20 40 60 801e-131e-09 100 0 20 40 60 801e-12 100 levels. The dashed horizontalwhich lineline gives givesT01 thewas the measured CMB computed at average 2.725 by excitation K summing temperature over hyperfine at 2.754 K sub- Temperature (K) Rate co-efficient (cm Temperature (K) dashed1e-10 line represents the CMB at 2.725 K while the dotted blue 0 201e-12 40 60 80 100 0 20 40 60 80 100 levels.(Ritchey The et dashed al. 2011). horizontal line gives the CMB at 2.725 K Temperature (K) linewhile givesTemperature the the horizontal measured (K) average dotted excitationline gives temperature the measured at 2 excita-.754 K Rate co-efficient (cm 1e-10 (Ritchey1e-11tion temperature et al. 2011).T01 at 2.754while K. the We horizontal first observe dotted that the line gives the measured excita- Figure1e-12 6. A comparison of the hyperfine structure rate from the excess temperature of 29 mKtion cannot temperature be reproducedT01 at at 2.754 very K. We first observe that the N,J,F , . , . which T01 was computed by summing over hyperfine sub- ( )=(2 2 5 3 5) initialFigure level1e-11 6. betweenA comparison1e-13 this work of (solid the hyperfine line) structure1e-12low electron rate from density, the indicatingexcess that temperature neutral collisions of 29 mK alone cannot be reproduced at very and the rate of Kalugina et(N,J,F al. (2012))=(2 (dashed, 2.5, 30 line)..5) 20 initial 40 level 60 between 80 100 this0 work 20 (solid 40 60 line) 80 100 levels. The dashed horizontal line gives the CMB at 2.725 K Temperature (K) cannot explainTemperature the (K) local excitation of CN. This confirms which T01 was computed bylow summingwhile electron the horizontal over density, hyperfine dotted indicating sub- line gives that the neutral measured collisions excita- alone 1e-13 and the1e-12 rate of Kalugina et al. (2012) (dashedthe conclusionsline). of past investigators (Thaddeus 1972; Black 0 20 40 60 80 100 0 20 40 60 80 100 levels. The dashed horizontalcannot linetion gives temperature explain the CMB theT01 local atat 2.725 2.754 excitation K.K We first of observe CN. This that confirms the Temperature (K) FigureTemperature 6. (K) for hydrogen atoms1. This kind of analysis hasA comparison been previ- of the hyperfinewhile& van the structure Dishoeck horizontal rate 1991). from dotted the Second,the lineexcess conclusions gives it can temperature the be measured noticed of past of 29 that investigators excita- mK the cannot be (Thaddeus reproduced 1972; at very Black (N,J,F)=(2, 2.5, 3.5) initial level betweenlocal excitation this work (solid is reproduced line) for a rather restricted range 1 & vanlow electron Dishoeck density, 1991). indicating Second, that it neutral can be collisions noticed alone that the ously performed by Blackfor & hydrogen van Dishoeckand atoms the rate (1991). of This Kalugina with kind old et of al. analysis (2012)tion (dashed temperature has been line). previ-T01 at 2.754 K. We−3 first observe that−3 the Figure 6. A comparison of the hyperfine structure rate from the of electron densities: from 0.01cannot cm explainat n=1000 the local cm excitationto of CN. This confirms collision data. excess temperature−3 of 29− mK3 local cannot excitation be reproduced is reproduced at very for a rather restricted range N,J,F , . , . ously performed by Black & van Dishoeck0.06 (1991) cm at withn=100 old cm with a weak dependence on the −3 −3 ( )=(2Radiative2 5 transfer3 5) initial calculations level between were this performed work (solid with line) the low electron density, indicatingofthe electron that conclusions neutral densities: of collisions past from investigators alone 0.01 cm (Thaddeusat n=1000 1972; Black cm to and the rate of Kalugina et al.collision (2012) data.(dashed line). 1 CN column density. Assuming& hydrogen van− Dishoeck is entirely 1991). Second,− molec- it can be noticed that the RADEX code (van der Tak et al. 2007),for using hydrogen the Large atoms Veloc-. This kindcannot of analysis explain has been the previ- local excitation0.06 cm 3 ofat CN.n=100 This cm confirms3 with a weak dependence on the Radiative transfer calculations wereular, performed these with electron the densitylocal correspond excitation to is electron reproduced frac- for a rather restricted range ity Gradient (LVG) approximation forously an expandingperformed by sphere. Black & vanthe Dishoeck conclusions (1991) of with past old investigatorsCN column (Thaddeus density.−5 1972; Assuming Black−4 hydrogen−3 is entirely−3 molec- RADEX code (van der Tak et al. 2007), usingtions the (x Largee = n Veloc-(e)/n(H2)) inof the electron range densities: 10 − 6 from× 10 0.01. cm at n=1000 cm to The kinetic temperature was fixed atcollisionT =20 K, data. as in the cal- − − 1 & van Dishoeck 1991). Second,ular,0.06 it these cm can3 be electronat n noticed=100 cm density that3 with the+ correspond a weak dependence to electron on the frac- for hydrogen atoms . Thisity kind Gradient of analysis (LVG) has approximation been previ- for anThis expanding is consistent sphere. with the abundance of interstellar C − − culations of Ritchey et al. (2011). The lineRadiative width transfer (FWHM) calculations were+ performed with the−4 5 4 − local excitation∼ is reproduced× tionsCN for ( column axe rather= density.n( restrictede)/n(H Assuming2)) range in the hydrogen range is entirely10 − molec-6 × 10 . ously performed by BlackThe &1 van kinetic DishoeckRADEX temperaturecode (1991) (van with was der Takfixed old et at al.T 2007),=20(n(C K, using)/n as(H the in2 the Large) cal-3 Veloc-10 ) which is− the main source of− elec- was taken to be 1.0 kms , corresponding to a Doppler line of electron densities: from 0.01 cm 3 at n=1000 cm 3 to + collision data. −ity1 Gradient (LVG) approximationtrons for an in expanding the diffuse sphere. interstellarThisular, medium. is these consistent electron with density the correspond abundance to of electron interstellar frac- C broadening parameter bculationsof 0.6 kms of Ritchey. The column et al. (2011). density The line width−3 (FWHM) −3 + −4 −5 − × −4 −1 12 0.06 cmIn fact,at n more=100 accurate cm (withn determination(Ctions a) weak/n (x(He =2 dependence) n∼( ofe3)/n× the(H102 electron)) on) in which the the range is the 10 main source6 10 of. elec- wasRadiative taken at transfer two typical calculationswas values taken wereN toThe(CN) be performed kinetic 1.0 = kms 3 temperature× with10, corresponding theand was fixed at toT =20 a Doppler K, as in line the cal- + 13 −2 −1 CN column density. AssumingThis hydrogen is consistent is entirely with the molec- abundance of interstellar C RADEX×code (van der Tak et al. 2007), usingculations the of Large Ritchey Veloc- et al. (2011).density The line can width be achieved (FWHM) fortrons clouds in+ where the diff theuse physical interstellar−4 con- medium. 3 10 cm . The densitybroadening of neutral parameter collisionb of partners 0.6 kms−1 . The column density (n(C )/n(H2) ∼ 3 × 10 ) which is the main source of elec- ity Gradient (LVG) approximation forwas an expanding taken to be sphere.1.0 kms , correspondingular,ditions these are× to electron a reasonably Doppler12 density line well known. correspondIn fact, For more to instance, electron accurate the frac- ki- determination of the electron (n = n(H) + n(H2)) waswas fixed taken at three at two representative typical values val- N(CN)− =1 3 10 and trons in the diff−use5 − interstellar× −4 medium. −3 13 broadening−2 parameter b of 0.6tions kmsnetic. ( temperaturex Thee = columnn(e)/n density and(H2 the)) collision indensity the range density can be 10 were achieved determined6 for10 clouds. where the physical con- Theues: kinetic 100, temperature 300 and 1000 was3 cm× fixed10. Finally atcmT =20. the K, The electron as density in the abun- cal- of neutral collision partners× 12 In fact, more accurate determination+ of the electron × −3 was taken−3 at two typical valuesThisforN(CN) the is consistent source = 3 HD10 with 154368and the from abundance the analysis of interstellar of C2 excita- C culationsdance was of Ritchey varied from et al. 2 (2011).10 Theto 1 line cm width13, corresponding− (FWHM)2 + −4 ditionsdensity are can reasonably be achieved well for clouds known. where For the instance, physical con- the ki- − (n = n(H)3 +×n(H10 2))cm was− . fixed The density− at three of neutral representative collision∼ partnersval-× 1 × 6 − 2 (ntion(C by)/n Sonnentrucker(H2) 3 10 et) al.which (2007). is the These main authors source of found elec- wasto taken electron to be fractions 1.0 kmsn(e,)/n correspondingin the range to 2 a Doppler10 to 10 line3 . neticditions+50 temperature are reasonably and the well collision known.−3 density For instance, were thedetermined ki- ues: 100,− 300(n = andn(H) 1000 + n(H cm2)) was. Finally fixed at the three electron representative± abun- val- 1 − − tronsT −= in 20 the di5ff Kuse and interstellarn = 150 medium.−25,withn(H)=60 cm broadeningResults are parameter presentedb inof Fig. 0.6 7. kms In each. The panel, column the excitation density× 3 3 3 −3 fornetic the temperature source HD and 154368 the collision from the density analysis were determined of C2 excita- dance was variedues: 100, from 300 2 and12 10 1000 cmto 1. cm Finallyand ,n corresponding(H the2)=90 electron cm abun-. The CN column density towards temperature T01(CN) is plotted as a function× of the elec- −3 In fact,−−3 more− accurate determinationfor the source HD of the 154368 electron from the analysis of C2 excita- was taken at two typical values N(CN)dance = was 3 varied10 fromand 2 × 10 to 1× cm 6, corresponding2 ×tion13 by Sonnentrucker−2 et al. (2007). These authors found 13 −2 to electron fractions n(e)/n in the rangedensitythe 2 star10 can HD beto 154368− achieved 10 . is− 2.7 for clouds10 cm whereand the the physical line width con- +50 −3 ×tron abundance. It should be noted that our excitation cal- − 6 2 tion by± Sonnentrucker et al. (2007). These authors found 3 10 cm . The density of neutralto electron collision fractions partnersn(e)/n in the range 2 × 101 to 10 . T = 20 5 K and n = 150−25,withn(H)=60− cm Results are presented in Fig. 7. In each panel,is 1.2 the kms excitation(Ritchey et al. 2011).± Interestingly,− this+50 3 culations provide the populations of hyperfine levels, from ditions are reasonably well known.T = 20 For5 instance, K and3 n the= 150 ki-−25,withn(H)=60 cm (n = n(H) + n(H2)) was fixed at threeResults representative are presented val- in Fig. 7. In each panel, the excitation and n(H2)=90 cm−3 . The CN column density towards temperature−3 T01(CN) is plotted as anetic functionsource temperature also of shows the elec- and the second the collision highestand n density(H excitation2)=90 were cm temperature determined. The CN13 column−2 density towards ues: 100, 300 and 1000 cm . Finallytemperature the electronT01(CN) abun- is plotted as a function of the elec- × − T01(CN)=2.911±0.004 K, whichthe star is significantly HD 154368 larger is 2.7× than1013 cm2 and the line width tron− abundance.3 −3 It should be noted thatfor our the excitation source HD cal- 154368 fromthe the star analysis HD−1 154368 of C is2 2.7excita-10 cm and the line width dance was varied from 2 × 10 to 1tron cm abundance., corresponding It should be noted that our excitation cal- −1 1 the weighted mean value ofis 2.754is 1.2 1.2 K. kms Using kms the(Ritchey(Ritchey physical et et con- al. al. 2011). 2011). Interestingly, Interestingly, this this We note that the rate coeculationsfficients for provideculations ortho-H× the2 provide(−j populations6= 1) the exceed− populations2 of hyperfinetion of by hyperfine Sonnentrucker levels, levels, from from et al. (2007). These authors found to electron fractions n(e)/n in the range 2 10 to 10 . source+50 also shows the second highest−3 excitation temperature those for para-H2(j =0)byuptoafactorof10(Lique,private ditions determined± by Sonnentruckersource also et shows al. (2007), the second we have highest excitation temperature T = 20 5 K and n = 150−25,with−3±n(H)=60 cm Results are presented in Fig. 7. In eachj panel, the excitation found that an electron−3 densityT01T(CN)=2.91101 of(CN)=2.911 0.3 cm ±is00..004 necessary004 K, K, which which to is is significantly significantly larger larger than than communication). However ortho-H2( =1)canbeneglectedhere and n(H2)=90 cm . The CN column density towards temperature T01(CN) is plotted as a1 function of the elec- reproduce the measured T01 towardsthe13 weighted HD−2 154368. mean value This of corre- 2.754 K. Using the physical con- since its abundance in cold1 (T<30 K) diWeffuse note clouds that is the expected rate coefficients for ortho-H2(j = 1) exceed ×the weighted mean value of 2.754 K. Using the physical con- We note that the rate coefficients for ortho-Hthe star2(j HD= 1) 154368 exceed is 2.7 10 cm and∼ the× line−3 width tronto abundance. be at least 30 It times should lower be than noted that thatthose of para-H our for excitation para-H2(j =0).2(j =0)byuptoafactorof10(Lique,private cal- sponds to an−1 electron fractionditionsditionsn(e)/n determined(H determined2) 3 by by10 Sonnentrucker Sonnentrucker,which et et al. al. (2007), (2007), we have we have those for para-H2(j =0)byuptoafactorof10(Lique,private −3 is 1.2 kms (Ritchey et al. 2011). Interestingly, this −3 culations provide the populations of hyperfinecommunication). levels, However from ortho-H2(j =1)canbeneglectedhere found that an electron density of 0.3 cm is necessary to communication). However ortho-H2(j =1)canbeneglectedheresource also shows the secondfound highest that excitation an electron temperature density of 0.3 cm is necessary to ⃝c 0000 RAS, MNRAS 000,000–000 since its abundance in cold (T<30 K) diffuse clouds is expected reproduce the measured T01 towards HD 154368. This corre- since its abundance in cold (T<30 K) diffuse clouds is expected± reproduce the measured T01 towards HD∼ 154368.× −3 This corre- to be at least 30 times lower thanT that01(CN)=2.911 of para-H2(j =0).0.004 K, whichsponds is significantly to an electron larger fraction thann(e)/n(H2) 3 10 ,which−3 to be at least 30 times lower than that of para-H2(j =0). sponds to an electron fraction n(e)/n(H2) ∼ 3×10 ,which 1 the weighted mean value of 2.754 K. Using the physical con- We note that the rate coefficients for ortho-H2(j = 1) exceed ⃝c 0000 RAS, MNRAS 000,000–000 those for para-H2(j =0)byuptoafactorof10(Lique,private ditions determined by Sonnentrucker et al. (2007), we have ⃝c 0000 RAS, MNRAS 000,000–000 −3 communication). However ortho-H2(j =1)canbeneglectedhere found that an electron density of 0.3 cm is necessary to since its abundance in cold (T<30 K) diffuse clouds is expected reproduce the measured T01 towards HD 154368. This corre- −3 to be at least 30 times lower than that of para-H2(j =0). sponds to an electron fraction n(e)/n(H2) ∼ 3×10 ,which

⃝c 0000 RAS, MNRAS 000,000–000 CH+ emission in Photon Dominated Regions

L66 CERNICHARO ET AL. Vol. 483 • CH+ ubiquity is an enigma: + – C + H2 endothermic (4620 K) + - – CH is destroyed by H, H2, e

• In PDR, the UV field can provide a reservoir of rovibrationally excited H2

1 FIG. 1.—CH lines observed in NGC 7027 (the continuum has been subtracted from the data). The solid lines are Gaussian line profile fits to the observed features (for blends, contributions from individual components are shown as dashed lines). The J 5 2–1 line at 179.62 mm was previously assigned to H2O. For the J 5 4–3 line, spectra from two different detectors—LW1 (bottom panel; the sharp rise at short wavelengths is due to the strong [O III] 88.36 mm line) and SW5 (upper panel)—are shown. The J 5 4–3 line is blended with the J 5 29–28 line of CO at 90.16 mm, which could contribute with an intensity less than 10219 W cm22—the J 5 27–26 and J 5 26–25 lines of CO can be seen in the bottom left-hand panel of Fig. 2 with intensities of 110219 W cm22. The lines at 59.5 and 60.7 mm in the top left-hand panel are unknown. Cernicharo et al. ApJ (1997)

three lines we derive the following rotational constants for the five scans with two 0.5 s ramps at each grating position, carrier: B 5 13.95 H 0.03 cm21 and D 5 0.0017 H 0.0005[here cm21 towardsampled at 1y15NGC of a resolution 7027] element. The total on-target (1 s errors). The observed frequencies and inferred rotational integration time was 23,930 s. The LWS has a circular aperture constants indicate that the carrier must be diatomic and should of diameter 850, significantly larger than the ionized or neutral contain a hydrogen atom. The rotational constants are com- regions of NGC 7027. The analysis presented in L96 was based parable to those of CH (B 5 14.19 cm21, D 5 0.0014 cm21) on the three observations carried out in revolution 39, pro- and show an excellent agreement with those of the CH1 ion cessed with Version 5 of the OLP. Although the current data (B 5 13.9302 cm21 and D 5 0.0014 cm21; Carrington & have a total on-target time that is only 65% larger than that Ramsay 1982). analyzed by L96, the final co-added spectrum has a much Prompted by the observation of three lines in harmonic better SyN, by more than a factor of 2 in some wavelength relation that are consistent with CH1, we have processed all regions, thanks to the much-improved instrumental filter the ISO LWS01 grating spectra of NGC 7027 so far obtained profiles derived from additional observations of Uranus and up to ISO revolution 377, in an effort to detect the J 5 5–4 and the implementation of an algorithm that corrects for detector 6–5 transitions of CH1, which are predicted at wavelengths of sensitivity drifts. The fluxes derived from the individual inde- 72.15 and 60.26 mm and were not detected by L96. In total, pendent observations are in good agreement, and we estimate there are nine separate observations, each covering a wave- an accuracy of better than 20% for the absolute flux calibra- length range from 43 to 195 mm, obtained during revolutions tion. 27, 39, 342, 349, 356, 363, and 377. All the data were calibrated Figure 1 shows the five CH1 lines detected in our new using Version 6 of the Off-Line Processing package (OLP). spectra, including the two lines at 72.15 and 60.26 mm, which The observation from revolution 27 consists of three scans, are clearly detected in the present data. These five lines with two 0.5 s integration ramps at each grating position, definitely establish the presence of CH1 in NGC 7027. This is sampled at 1y4.5 of a spectral resolution element, the latter the first time that CH1 is seen through its pure rotational being 0.3 mm for the SW1–SW5 detectors (l , 93 mm) and spectrum. CH1, a common molecule in the diffuse interstellar 0.6 mm for the LW1–LW5 detectors (l . 84 mm). Each of the medium, has been detected in the optical spectrum of only two five observations obtained between revolutions 342 and 377 evolved carbon-rich stars: in emission in the Red Rectangle, consists of six scans with one 0.5 s ramp at each grating and in absorption in HD 213985 (Balm & Jura 1992; Hall et al. position, sampled at 1y9 of a resolution element. Three 1992; Waelkens et al. 1992; Bakker et al. 1997). Given the observations were obtained in revolution 39. Each consists of carbon-rich nature of NGC 7027 and the strong radiation field Need for state-to-state chemistry

0 10

-1 10 • « Chemical » pumping is nascent distribution -2 crucial for short-lived i.e. 10

-3 reactive species 10 LTE @ 500K -4 10

-5 • State-resolved data from 10 full model Occupation probability

quantum and quasi-classical -6 10 calculations -7 10 inelastic model

-8 10 0 250 500 750 1000 1250 1500 1750 2000 -1 Rotational energy (cm ) Faure et al. MNRAS (2017) + CH as a tracer of H2(v>0)

Inelastic model Reactive model State-to-state chemistry and excitation of CH+ 619 3 typical sources: the Orion Bar and the planetary nebula NGC 7027. We were able to reproduce, within error bars, the ISO and Herschel 4à3 measurements by adjusting the CH+ column density as a single free 2.5 parameter. 3à2 Obviously, there is no unique solution and this work can be further improved e.g. by computing self-consistently the CH+ abundance,

) 2 5à4 6à5 instead of fixing its column density. The impact of the H2 rota- -2 tional distribution within the different vibrational manifolds should à W.cm 2 1 be also investigated. This will require to extend the calculations on 2 -19 1.5 C+( P) H2(νH2 ,jH2 ) to higher rotational levels jH2 > 1, of partic- ular importance+ in the ground vibrational state ν 0. Collisions NGC 7027 H2 = between CH+ and H2 should be also considered. We have assumed Flux (10 1 + -2 N(CH )=2e15 cm in this work that hydrogen is in atomic form in the region of maxi- mum CH+ abundance. However, a non-negligible molecular hydro- Observations genfractionisnecessarytoformCH+ from C+ H2.Theknowledge 0.5 à + 1 0 of state-to-state rate coefficients for the CH+ H collisions might + 2 even provide constraints to the H2 fraction. Calculations of the PES for the electronic ground state of CH+ are in progress in Bordeaux 0 3 0 250 500 750 1000 (Halvick and co-workers). Finally, the generalization of this work Upper level energy (K) to the excitation of other reactive species such as OH+ and SH+,

3 for which state-resolved formation rates are becoming available Figure 8. Same as in Fig. 6 except that the electron fraction is xe 10 = − (Gomez-Carrasco´ et al. 2014;Zanchet,Roncero&Bulut2016), and the CH+ column densityFaure is adjusted et to al. best MNRAS reproduce the (2017) observations of Cernicharo et al. (1997)andWessonal.(2010)towardsNGC7027. will be presented in future works.

Godard & Cernicharo (2013). Fig. 8 also reports our results when ACKNOWLEDGEMENTS the formation rates are those for H2(νH2 2,jH2 0). The agree- ment with ISO observations is now within= error bars.= This suggests This work has been supported by the Agence Nationale de la Recherche (ANR-HYDRIDES), contract ANR-12-BS05-0011-01 that the relative population of the state νH2 2 could be large in the PDR of NGC 7027. The observations of higher= frequency tran- and by the CNRS national programme ‘Physico-Chimie du Mi- sitions of CH+ (j 7 6, etc.) might help to confirm this result. lieu Interstellaire’. IFS acknowledges generous financial support Finally, we note that= the→ contribution of electron collisions is sub- from Region´ Haute-Normandie, the GRR Electronique, Energie et 3 3 stantial here at an electron fraction xe 10− cm− :A75percent Materiaux,´ the ‘Fed´ eration´ de Recherche Energie, Propulsion, Envi- increase in the intensity of the ground-state= j 1 0 transition was ronnement’, the fund Bioengine and the LabEx EMC, via the project observed. This effect decreases for higher transitions= → but electrons PicoLIBS. OR and AZ acknowledge the financial support of Min- still contribute above 10 per cent for the j 4 3 transition. As isterio de Economia e Innovacion´ under grant FIS2014-52172-C2 a result, electrons compete with hydrogen atoms= → for the excitation and the European Research Council under ERC Grant Agreement n. and destruction of CH+ in this carbon-rich circumstellar envelope. 610256 (NANOCOSMOS). Franc¸ois Lique and Pierre Hily-Blant are acknowledged for useful discussions.

4CONCLUSION REFERENCES We have presented a detailed theoretical study of the rotational excitation of CH+ due to chemical pumping, excitation and destruc- Agundez´ M., Goicoechea J. R., Cernicharo J., Faure A., Roueff E., 2010, tion. The investigated colliders were hydrogen atoms and free elec- ApJ, 713, 662 trons. State-to-state and initial-state-specific rate coefficients were Amitay Z. et al., 1996, Phys. Rev. A, 54, 4032 computed for the inelastic, exchange, abstraction and dissociative Bakker E. J., van Dishoeck E. F., Waters L. B. F. M., Schoenmaker T., 1997, A&A, 323, 469 recombination processes using the best available PESs and scatter- Bernard Salas J., Pottasch S. R., Beintema D. A., Wesselius P. R., 2001, ing methods. State-to-state rate coefficients were also computed for A&A, 367, 949 2 the formation reaction, C+( P) H (ν ,j ) CH+ H, by ex- + 2 H2 H2 → + Black J. H., 1998, Faraday Discussions, 109, 257 tending the calculations of Zanchet et al. (2013) to the vibrationally Bonnet L., 2013, Int. Rev. Phys. Chem., 32, 171 excited state (νH 2,jH 0). Good agreement with available Bovino S., Grassi T., Gianturco F. A., 2015, J. Phys. Chem. A, 119, 11973 2 = 2 = experimental thermal rate coefficients was observed, except for the Bulut N., Lique F., Roncero O., 2015, J. Phys. Chem. A, 119, 12082 abstraction reaction CH+ H at low temperature (<50 K). The full Carata L., Orel A. E., Raoult M., Schneider I. F., Suzor-Weiner A., 2000, set of collisional and chemical+ data were then implemented in a Phys. Rev. A, 62, 052711 radiative transfer model based on the escape probability formal- Castor J. I., 1970, MNRAS, 149, 111 ism. These non-LTE calculations have confirmed previous studies Cernicharo J., Liu X.-W., Gonzalez-Alfonso´ E., Cox P., Barlow M. J., Lim T., Swinyard B. M., 1997, ApJ, 483, L65 that suggested that chemical pumping has a substantial effect on Cox P., Huggins P. J., Maillard J.-P., Habart E., Morisset C., Bachiller R., the excitation of CH+ in PDRs (Godard & Cernicharo 2013; Nagy Forveille T., 2002, A&A, 384, 603 et al. 2013; Zanchet et al. 2013). However, in contrast to these pre- Douglas A. E., Herzberg G., 1941, ApJ, 94, 381 vious works, we have employed for the first time a comprehensive Epee´ Epee´ M. D., Mezei J. Z., Motapon O., Pop N., Schneider I. F., 2016, theoretical set of fully state-to-state data. Our non-LTE model was MNRAS, 455, 276 applied to typical PDR conditions and, in particular, to two proto- Falgarone E. et al., 2010, A&A, 518, L118

MNRAS 469, 612–620 (2017) 1973ApL....13..119G

Methanimine (CH2NH) in the galactic center 1973ApL....13..119G

Laboratory

Observation 25 of the doublet, as in the case of methyl formate (HCOOCH3). The strong emission of the

4 intrinsically weak 110 111 line, as first detected by Godfrey et al., was thus attributed to ! apopulationinversionratherthanananomalouslyhighabundance.26 The lack of collisional data, however, has precluded so far any definite conclusion. Using our rate coefficients, the

3 5 3 critical density of the upper level 110 can be estimated as 1.5 10 cm which means that ⇠ ⇥ 3 7 3 non-LTE effects are expected at densities in the range 10 10 cm .However,onlynon- CH NH selection / propensity rules LTE radiative2 transfer calculations can predict the actual rotational populations of CH2NH for a given set of physical conditions, as illustrated below.

-9 15 10 100

_ 3 03 0 →2 _ 2 00 02 11 _ ) 0 →1 2 00 01 12 -1 ) 10 0 →1 s 2 00 01 3 0 →2 ) -10 00 02

-1 10 62.8 61.2 10

1 168 _ 10 1 1 0 →1 _ 11 00 11 202 56.5 → 000 111 5 -11

Cross section (Å 17.5 10 93.1 Level energy (cm 0.1 → Rate coefficient (cm 000 110 _ 0 →1 101 00 10 1.82_ 0 000 0.01 -12 0 10 20 30 40 50 60 10 5 10 15 20 25 30 k =0 k =1 -1 a Collision energya (cm ) Temperature (K)

22 Figure 2: Lowest 8 levels ofFigure CH2NH, 3: Crosswith energies sections taken (left from panel) the and CDMS rate catalog. coefficientsFor (right each panel) for excitation out of level, the strongest radiativethe transitions000 level to are levels indicated101, 202 by, 1 arrows11 and and110 due the numbers to para-H give2(j2 the=0)collisions. 6 1 corresponding Einstein coefficient (in units of 10 s ) taken from Ref. 22.Faure The et spectrum al. J. Phys. Chem. Lett. (2018) consists of a-type (ka =0)andb-type (ka = 1) transitions with corresponding dipole ± moments µa =1.340 DandµbThe=1 star.446 formingD. region Sgr B2 is an exceptionally massive cloud complex in the Galactic

1 Center, where shocks and supersonic turbulence are ubiquitous. Towards the north core of the level 220 at 28.3 cm .Ratecoefficients were obtained in the temperature range 5–30 K Sgr B2 (Sgr B2(N)), the continuum emission is produced by a complex region of ionized by integrating the cross sections over Maxwell-Boltzmann distributions of relative velocities. gas (so-called HII region). A detailed description of this source can be found in Ref. 27. Cross sections and rate coefficients for the lowest 4 transitions out of the ground state 000

We focus below on the 110 111 transition of CH2NH at 5.29 GHz. This line was first are presented in Fig. 3 as function of collisional energy and! temperature, respectively. Many 4 resonances are observed inobserved the cross in sections, 1973 by as Godfrey expectedet from al. thetowards deep potential Sgr B2 withwell. theWe 3.80 beam (at 5.29 GHz) of the Parkes 64-meter telescope. New observations towards Sgr B2(N) were performed with can also notice that the favored transitions are 000 101 and 000 202,correspondingto ! ! 3The critical density defines the density at which the collisional rate out of the upper state of a transition the usual propensity rule j1 =1, 2 and ka =0.Inaddition,fortheka =1doublet, equals the spontaneous radiative rate. the lower level is found to be favored. This propensity was also observed for higher ka =1 8 doublets and this corresponds to j1 being preferentially oriented along the direction of the greatest moment of inertia (the c axis). This result is observed in other molecules and is 23 responsible, in particular, of the anti-inversion of the doublet 110 111 in interstellar H2CO.

Here, however, because both collisional and radiative transitions between the ka =1and ka =0ladders are allowed (in contrast to H2CO), the above propensity is in competition with inter-ladder transitions. In particular the spontaneous decay rate from the 111 to 000 level is about 1.6 times that from the 110 to the 101 level (see Fig. 2), which favors inversion

7 New GBT observations

0.4 observation 0.3

0.2

0.1

0

-1 -0.1 +64 km.s -1 -0.2 +82 km.s Radiation temperature (K) 5.288 5.289 5.290 5.291 5.292 Frequency (GHz)

Faure et al. J. Phys. Chem. Lett. (2018) CH2NH probes a cold dilute gas (T ~ 30K, n ~ 104 cm-3)

0.4 observation non-LTE model 0.3 LTE model(×10) 0.2

0.1

0

-1 -0.1 +64 km.s -1 -0.2 +82 km.s Radiation temperature (K) 5.288 5.289 5.290 5.291 5.292 Frequency (GHz)

Figure 4: Observational and model spectra of methanimine 110 111 transition at 5.29 GHz ! towards Sgr B2(N). Relative intensities of the (partially resolved) hyperfine structure in the 1 optically thin limit are shown at the bottom. The nominal source velocity is +64 km.s . Faure et1 al. J. Phys. Chem. Lett. (2018) Asecondvelocitycomponentisresolvedat+82km.s . The non-LTE model predicts a population inversion with a negative excitation temperature Tex = 0.48 K. The LTE model spectrum has been multiplied by a factor of 10 to show it more clearly.

Computational details

The analytical expansion for an asymmetric-top-linear molecular system can be written as:12

V (R, ✓1, 1, ✓2, 2)= vl1m1l2l(R)tl1m1l2l(✓1, 1, ✓2, 2), (2) X

where the functions tl1m1l2l(✓1, 1, ✓2, ) are products of spherical harmonics, as given ex- plicitly in Eqs. (5-6) of Ref. 33. The indices l1, m1 and l2 refer to the (✓1, 1) and (✓2, 2) dependence of the PES, whereas l runs from 0 to the sum of l1 and l2. The expansion

coefficients vl1m1l2l(R) were obtained through a least-squares fit on the random grid of 3000 angular geometries at each intermolecular separation (22 radial grid points in the range

R =4 15 a0). All anisotropies up to l1 =14, l2 =6and l =20were included, resulting in 3054 expansion functions. We then selected only significant terms using a Monte Carlo error estimator (defined in Ref. 34), resulting in a final set of 251 expansion functions with anisotropies up to l1 =13, l2 =6and l =16. The root mean square was found to be lower

11 CONCLUSIONS Conclusions

• Interaction potentials have now reached a high level of refinement

• Collision data are no longer a limiting factor in the modelling of non-LTE spectra

• Strongly non-LTE situations offer new diagnostics, e.g. masers à SKA Current / future works

(April 2019) Article

Cite This:ACS Earth Space Chem. XXXX, XXX, XXX−XXX http://pubs.acs.org/journal/aesccq

Interaction of Chiral Propylene Oxide (CH3CHCH2O) with Helium: Potential Energy Surface and Scattering Calculations Alexandre Faure,*,† Paul J. Dagdigian,‡ Claire Rist,† Richard Dawes,§ Ernesto Quintas-Sanchez,́ § Francoiş Lique,∥ and Majdi Hochlaf⊥ †Université Grenoble Alpes, Centre National de la Recherche Scientifique (CNRS), Institut de Planetologié et d’Astrophysique de Grenoble (IPAG), F-38000 Grenoble, France ‡Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218-2685, United States §Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, United States ∥Laboratoire Ondes et Milieux Complexes (LOMC), UMR 6294, Centre National de la Recherche Scientifique (CNRS)− Université du Havre, 25 Rue Philippe Lebon, BP 1123, 76 063 Le Havre Cedex, France ⊥ Université Paris-Est, Laboratoire Modelisatioń et Simulation Multi Echelle (MSME), UMR 8208, Centre National de la Recherche Scientifique (CNRS), 5 Boulevard Descartes, 77454 Marne-la-Vallee,́ France *S Supporting Information

ABSTRACT: The first chiral interstellar organic molecule, propylene oxide (CH3CHCH2O), was detected recently toward the galactic center. Accurate determination of its abundance relies on the knowledge of collisional cross sections. We investigate here the rotational excitation of propylene oxide induced by collisions with helium. The calculations are based on a three-dimensional CH3CHCH2O− He potential energy surface computed using the explicitly correlated coupled-cluster theory extended to the complete basis set limit [CCSD(T)-F12b/CBS]. The interaction energies are fitted using an interpolating moving least squares method, and this potential is refitted using a partial wave expansion based on spherical harmonics. Rotational cross sections are obtained at the quantum close-coupling level for a collision energy of 10 cm− 1. Convergence issues and collisional propensity rules are discussed. KEYWORDS: astrochemistry, organic molecules, chirality, energy transfer, quantum scattering

1. INTRODUCTION The first chiral interstellar organic molecule, propylene oxide

Downloaded via Alexandre Faure on April 23, 2019 at 20:21:59 (UTC). (CH CHCH O), was detected recently in the cold gas toward A chiral molecule is one that cannot be superimposed onto its 3 2 the galactic center molecular cloud Sgr B2(N).2 Current mirror image. The two distinct forms of such molecules are observations cannot determine if an enantiomeric enrichment called enantiomers, and they have opposite optical activity: one of propylene oxide is present in Sgr B2(N), but, in principle, is dextrorotatory (D), and the other is levorotatory (L). Many high-precision polarization measurements of the absorption See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. chemical reactions produce L and D molecules in equal signal could provide the answer. Bergantini et al.3 have shown amounts (racemic mixture). In all forms of life we know on that propylene oxide can form within interstellar ice via a Earth, however, only one of the two enantiomers of chiral cosmic-ray-driven non-equilibrium chemistry involving reac- amino acids (L-amino acids) and chiral sugars (D-sugars) is tions of suprathermal atomic oxygen with propylene involved in the assembly of proteins and nucleic acids, which (CH3CHCH2). These authors also suggested that this process are also chiral. This property, called homochirality, is a should produce racemic mixtures of propylene oxide, of which one enantiomer could be preferentially photolyzed4 or fundamental characteristic of the chemistry of life. 5 The origin of biomolecular homochirality is unknown, and it radiolized. Once produced on the grains, propylene oxide is often considered as closely related to the origin of life itself. can desorb into the gas phase via thermal and non-thermal Many theories exist, and they can be broadly classified as terrestrial or extraterrestrial. The common assumption of Special Issue: Complex Organic Molecules (COMs) in Star- abiotic theories is that a primordial small enantiomeric excess Forming Regions fi was ampli ed (and preserved) by autocatalytic processes on Received: March 22, 2019 Earth. The evidence of small enantiomeric excesses of L-amino Revised: April 15, 2019 1 acids in the Murchison meteorite has provided support for the Accepted: April 16, 2019 extraterrestrial origin. Published: April 16, 2019

© XXXX American Chemical Society A DOI: 10.1021/acsearthspacechem.9b00069 ACS Earth Space Chem. XXXX, XXX, XXX− XXX Current / future works

• Organics

– Nitriles, HC2NC, HC5N, C6H5CN – Ro-vibration (HCN, HC3N) à JWST

• Reactive ions + + + – OH , SH , H2O (ERC COLLEXISM PI Lique)

• H2O-molecule – H2O-CO, see Loreau et al. J Chem Phys (2018) Acknowledgements

• A. Bergeat, M. Costes, C. Naulin Bordeaux • P. Halvick, T. Stoecklin Bordeaux • A. Remijan Charlottesville hYdRiDeS • A. Bacmann, P. Hily-Blant Grenoble hYdRiDeS!

• F. Lique, I. Schneider Le Havre hYdRideS! • J. Tennyson London ! • O. Roncero, A. Zanchet Madrid hYdRiDeS! • K. Szalewicz Newark • L. Wiesenfeld Orsay • H. Labiad, S. Le Picard, I. Sims Rennes • P. Jankowski Torun