REVIEWS

Laboratory spectroscopy techniques to enable observations of interstellar ion chemistry

Brett A. McGuire 1,2,3 ✉ , Oskar Asvany 4, Sandra Brünken 5 and Stephan Schlemmer 4 ✉ Abstract | Molecular ions have long been considered key intermediates in the evolution of molecular complexity in the interstellar medium. However, owing to their reactivity and transient nature, ions have historically proved challenging to study in terrestrial laboratory experiments. In turn, their detection and characterization in space is often contingent upon advances in the laboratory spectroscopic techniques used to measure their spectra. In this Review, we discuss the advances over the past 50 years in laboratory methodologies for producing molecular ions and probing their rotational, vibrational and electronic spectra. + We largely focus this discussion around the widespread H3 cation and the ionic products originating from its reaction with carbon atoms. Finally, we discuss the current frontiers in this research and the technical advances required to address the spectroscopic challenges that they represent.

More than 200 molecular species are now known to The primary initiator in this network is the simple + + be present in the interstellar medium (ISM), but only polyatomic cation H3 . The importance of H3 in initia­ slightly more than 15% of these are ionic: either posi- ting ion–molecule chemistry has long been recognized, 1 + tively or negatively charged . Yet these charged species and H3 has been invoked as a key intermediate in have a substantial role in driving chemical evolution in reaction networks for decades2,4,5. Although laboratory space, where low densities and temperatures present knowledge of its spectrum dates back to 1980 (ref.6), considerable obstacles to reactivity. In a relatively diffuse it was not detected in the ISM, where it is now widely (ref.7) + gas, the time between collisions for two species in space observed, until 1996 . The reaction of H3 with can often be measured in days or months, and even then, hetero­atoms (X = O, C or halogens, among others) is

only if one of them is hydrogen (H or H2). When these generally exothermic and barrierless, resulting in a species do find each other, they typically only react if protonated species XH+: 1Department of Chemistry, there is no energetic barrier to do so. Ions help to solve ++ Massachusetts Institute both of these problems. X+HX3 → H+H(2 1) of Technology, Cambridge, Because ions are charged, their long-​range inter­ MA, USA. actions induce dipole moments in nearby neutral species. A cascade of reactions can proceed from the for- 2National Radio Astronomy Thus, an attractive potential forms between the charged mation of XH+, building up larger and more complex Observatory, Charlottesville, VA, USA. and uncharged species, substantially increasing the fre- species, often through subsequent reaction with H2. quency of collisions. Reactions with ions are also often Alternative initial reaction steps can occur, depending 3Center for Astrophysics, Harvard and Smithsonian, barrierless and exothermic, requiring no input of energy on the astrophysical environment (Fig. 1). There is sub- Cambridge, MA, USA. to proceed. As a result, much of the initial build-​up of stantial discussion in the literature as to the extent of 4I. Physikalisches Institut, chemistry in the ISM is initiated through ion–molecule complexity that can be built solely through ion–molecule Universität zu Köln, Köln, reactions, which are often faster by orders of magnitude reactions and the competition of these reactions with Germany. than those between neutral collision partners2. Figure 1 grain-​surface and neutral–neutral reactions8–10. There 5Radboud University, Institute shows a portion of this early reaction chemistry, demon­ is little debate, however, about the importance of for Molecules and Materials, strating the role of ion–molecule reactions in generat- molecular ions in kick-​starting chemical complexity FELIX Laboratory, Nijmegen, Netherlands. ing small hydrocarbons, water and more complex species in the ISM. + ✉e-mail:​ [email protected]; such as protonated methanol (CH3OH2 ). The latter is In this Review, we examine many of the laboratory [email protected]​koeln.de thought to be an important intermediate in the forma- techniques that have enabled the observation of mol­ 3 https://doi.org/10.1038/ tion of still more complex species but has not yet been ecular ion spectra, the current status of the field and s42254-020-0198-0 detected in the ISM1. the impact of recent advances in the laboratory on the

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Key points (and thus the sensitivity) is directly proportional to the path length. A common technique for increasing the • Molecular ions have a key role in driving chemistry in the interstellar medium because path length, without increasing the physical size of their reactions are usually exothermic and the attractive potentials that they induce the cell beyond what can reasonably be housed in a lab- increase reaction rates. oratory, is to use a system of mirrors to ensure that the • The laboratory spectra needed to study ions in space are challenging to collect, light reflects through the absorbing material many times as ions are both difficult to produce in large quantities and highly reactive. before it is detected. Indeed, in Oka’s set-up,​ the effective • A boom in ion studies was aided by the combination of spectroscopy techniques that path length was 32 m from a 2-​m-​long cell. There are greatly increased the optical path length with others that modulated the signal at several variations of this optical set-​up; the most com- known frequencies while leaving noise unmodulated. mon are White-type​ 12 and Herriott-type​ 13 arrangements, • Action spectroscopy in cryogenic ion traps indirectly measures absorption by observing as well as simpler double-​pass set-ups​ that use a polariz- changes in counts of the mass-selected,​ stored ions. ing grid and a polarization-​rotating rooftop reflector14. • Action spectroscopy is by far the most sensitive technique and has developed into a Oka also implemented a phase-​locked detection scheme versatile tool in the full spectral range of molecular spectroscopy, covering rotational, vibrational and electronic motion. using frequency modulation, which is widely used in spectroscopy. Two more recent techniques, cavity ring-down​ spec- future observation of ionic species in the ISM. The dis- troscopy (CRDS) and noise-​immune cavity-​enhanced cussion centres on a few selected ions that we believe heterodyne velocity-​modulation spectroscopy (NICE-​ exemplify not only the main spectroscopic techniques OHVMS), further exploit multi-​pass geometries and and advances, but also the close connections between noise-​rejection methods. CRDS uses a pair of high- laboratory spectroscopy and observational astronomy: ​reflectivity (R ≥ 99.99%) supermirrors to create an + + + + + 15 H3 , CH3 , CH , C3H and C60 . The selection of ions optical cavity with path lengths of many kilometres . also determines the techniques that we highlight herein. With each pass within the cavity, a fraction of the light As a consequence, many important experimental meth- leaks out of the back of one mirror and is detected as ods, such as cavity-​enhanced Fourier-transform​ micro- a decay signal. A molecular absorber within the cavity wave (FTMW) spectroscopy and velocity-​modulation increases this decay rate. Because the change in rate is infrared absorption spectroscopy, are outside our scope directly proportional to the molecular absorption, here and are only referenced or discussed briefly. an absorption spectrum can thus be determined. In the + case of H3 , CRDS techniques have been used to measure, + + H3 as the cornerstone of ion chemistry for example, the electron recombination rate of H3 + + The infrared (IR) spectrum of H3 was measured by by monitoring the decay rate of the H3 signal in the Oka6 in 1980 using a sophisticated direct-​absorption cavity16,17. spectrometer outfitted with a multi-​pass set-​up and a In addition to cavity enhancement, NICE-​OHVMS frequency-​modulated detection system. The discharge (Fig. 2) incorporates two modes of modulation18. Its cell was liquid-​nitrogen cooled in a manner similar to predecessor, NICE-​OHMS (noise-​immune cavity-​ earlier work on HCO+ (ref.11). In direct-absorption tech- enhanced optical heterodyne molecular spectroscopy), niques, the strength of the observed absorption signal uses a laser locked to the cavity resonant frequency19. A pair of frequency-​modulated sidebands, 180° out of + + + phase with one another, is then added to the laser at a H5O2 CH3OHH CH5 + + spacing equal to an integer of the cavity free spectral N2H HCO XH+ H2O H2 range, allowing the sidebands and laser to be simul-

+ + taneously coupled into or out of the cavity. When no H3O CH3 H2 H2 absorber is present, the beat signals of the two side-

+ X + bands exactly cancel each other out, eliminating all H O H H CH2 2 2 2 noise from the system and creating a background-​free OH+ CH+ scenario. An absorption signal is detected when one of the sidebands is absorbed by a molecular carrier. H O H + C 2 3 Compared with NICE-OHMS,​ NICE-OHVMS​ adds an H2 extra modulation component by varying the velocity of H2 molecular ions within the sample. This further increases O+ H + C+ 2 the sensitivity of the experi­ment, as was pioneered by cr cr Saykally and co-workers,​ who applied velocity modula-

O H2 C tion in standard absorption experiments to a large class of molecular ions20–22. NICE-​OHVMS has been used to | Fig. 1 Astrochemical ion–molecule reactions. An astro- measure transitions of many molecular cations23–25, in chemical reaction tree depicting several pathways in which particular H + (refs26–30), to sub-​Doppler accuracy, ena- ion–molecule reactions build molecular complexity in the 3 109 + bling measurements made in the IR region to approach interstellar medium . The H3 cation has a fundamental + microwave-​level accuracies on rotationally resolved role in initiating this network. H3 forms upon ionization transitions. The sensitivity of NICE-​OHVMS is rou- of molecular hydrogen (H2) by cosmic rays (cr) and then

consecutive collisions with H2. The colours and thicknesses tinely more than two orders of magnitude higher than of the arrows depict different reaction mechanisms that that of state-​of-​the-​art multi-​pass direct-​absorption occur depending on the astrophysical environment. spectrometers26,31.

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a

Wavemeter

CaF2 window OPO PZT

PZT Det. 1 f B.R. Det. FM PDH Det. 2 DM DPSS EOM Lock-in 1 Lock-in 2 PPLN AOM Mixer 1 Mixer 2 LP

Det. 3 YDFL f het 90° Locking Fast electronics Slow (<100 Hz) (100 Hz–15 kHz)

b Mixer 1 Mixer 2

4

2

0

Signal (arb. units) –2

–4

–1,000 –500 0 500 1,000 –1,000 –500 0 500 1,000 Detuning frequency (MHz) Detuning frequency (MHz)

Fig. 2 | NICE-OHVMS. a | Schematic of an instrument for noise-immune​ cavity-enhanced​ heterodyne velocity-modulation​ spectroscopy (NICE-OHVMS).​ An ytterbium-​doped fibre laser (YDFL) drives an optical parameteric oscillator (OPO) and is phase modulated by an electro-​optic modulator (EOM) that produces the required sidebands for locking and heterodyne 26 + –1 detection . b | Example NICE-​OHVMS spectra of the R(1,0) transition of H3 at 2,725.8984 cm from the two mixers, showing the in-​phase (red) and quadrature (blue) components of the velocity-modulation​ signal. The transition frequency is determined to megahertz precision by a least-squares​ fit to the Lamb dip at the centre of each spectrum, as described in ref.27. AOM, acousto-​optic modulator; B.R. Det., back reflection detector; Det. 1, mid-infrared​ transmission detector; Det. 2, mid-​IR transmission detector; Det. 3, silicon detector; DM, dichroic mirror; DPSS: diode-pumped​ solid-state​ laser;

fhet, heterodyne frequency; FM, flipper mirror; fPDH, Pound–Drever–Hall frequency; LP, longpass filter; PPLN, periodically poled lithium niobate crystal; PZT, piezoelectric transducer. Adapted with permission from ref.26, OSA Publishing.

Laser-induced​ reactions stored ion ensemble that is detected with near-100% + H3 isotopologues. Many of the ions that were first efficiency in an external mass analyser. As the pro­duct detected using Oka-​type discharge absorption experi- ions are produced only upon excitation of the parent ments have been subsequently investigated in cryogenic ion, the method is intrinsically background-​free, offer- ion traps using action spectroscopy, owing to the tech- ing unprecedented gains in sensitivity. In the most nique’s high selectivity and unprecedented sensitivity. popular version of this action spectroscopy, the ion In these experiments, ions of interest are mass-selected​ of interest dissociates upon absorption of one32–34 or and isolated in an ion trap, where they can be investi- more photons35–37, and thus unimolecular decay is used gated over long time periods. This approach therefore to detect the photon absorption. Many variations of offers notable advantages over discharge techniques this method have been added to the toolbox of action and the (largely uncontrolled) production of many dif- spectroscopy, and several associated with ions in the ferent short-​lived ions. Moreover, buffer-​gas cooling astrophysical context are highlighted below. of the ions in a cryogenic trap prepares the ensemble Laser-​induced reaction (LIR) is an action spectro­ in only a few quantum states and thus simplifies the scopic technique pioneered by Schlemmer and Gerlich38,39, spectra. Within the trap, the interaction of the ions with and uses the products of a bimolecular chemical reac- light leads to a change in the mass composition of the tion to record high-resolution​ spectra of stable molecular­

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a 400 vibrations40. For the spectroscopy of the fundamental LIR R(1,0) R(1,1)u R(2,2)l vibrational bands, proton transfer to molecular oxygen 300 was more suitable, owing to its smaller endothermicity ) + –1 41

H of 78 cm (ref. ): 2 (O

i 200

N + + H+3 OO22→ H+H(2 3) 100 + The rovibrational lines of H3 are measured by count- + b ing the number of product O2H ions as a function of LIICG the laser frequency, which results in only three lines for these low-​temperature experiments (Fig. 3a). The cryo­ 300 genic operation in combination with a highly accu- – H e )

+ 42 3 rate frequency-​comb system enabled measurements (H

i +

N of transition frequencies for H3 to sub-​megahertz 200 accuracy43. Together with measurements obtained by NICE-​OVHMS28,29, among other techniques44, a very accurate picture of this fundamental molecular ion is 5.88 5.90 6.20 6.22 42.06 42.08 now available. –1 + Frequency–2,720 (cm ) As H3 is a key molecule for understanding astro- + chemistry, so too are its deuterated variants H2D , Fig. 3 | Action spectroscopy of H +. a | Rovibrational transitions of H + measured with + + 3 3 D2H and D3 , which are heavily enriched in the ISM the laser-induced​ reaction (LIR) method. The frequency position for each data point has owing to isotopic fractionation effects45,46. Differences been accurately determined with a frequency comb42. The trap temperature was 4 K, in the zero-point​ energies of hydrogen species and their except for the R(2, 2)l line, for which the trap was heated to ~45 K. This line is partly power- deuterated counterparts make exchange reactions that saturated.​ b | The same rovibrational transitions of H + as shown in panel a, measured 3 favour deuterated ionic products slightly exothermic: with the laser-​induced inhibition of complex growth (LIICG) method. Ni, number of ions ref.43 detected. Adapted with permission from , Elsevier. + + H+3 HD → HD2 +H2 (4)

+ ions. The first application of this technique to H3 Owing to the low cosmic [HD]/[H2] ratio, this reaction + involved proton transfer to an Ar atom: (Eq. 4) produces considerable amounts of H2D only at + + very low temperatures (below ~20 K). A high [H2D ]/[H3 ] ++ H+3 Ar → ArH+H(2 2) ratio can thus be invoked as an observational tracer of extremely cold conditions, that is, of times in cold mol­ This reaction is endothermic by 0.57 eV (~4,600 cm–1), ecular clouds shortly before a new star forms, which then + + which was overcome by exciting overtones of H3 destroys the H2D witness of this event. The endothermicity of the reverse reaction,

+ + ν ν2 ν3 4 HD2 +H23→ H+HD (5) + CH3 can then be exploited in LIR experiments, in which the + ν5 appearance of H3 products indicates excitation of ν2 ν1 + 43,47 + the parent H2D ions. In this way, the rovibrational , ν3 CH D ν4 2 48,49 + + ν6 and even rotational , spectra of H2D and D2H were determined. The knowledge gained from these experi­ ments, together with that from other pioneering lab- ν ν ν ν 5 1 50–53 4 6 + oratory investigations over the past four decades , ν3 ν CD H 2 2 enabled the detection of these isotopologues in the ISM54–57 and, in particular, the measurement of the ortho- to-para​ ratio of their spin isomers. Owing to its sensiti­ ν4 ν3 + CD3 vity to the thermal history of an interstellar region, this

ν2 ratio is an important probe of the age of interstellar sources undergoing star formation57,58. 1,000 1,500 2,000 2,500 3,000 + Frequency (cm–1) CH3 isotopologues. Deuterium exchange reactions similar to those mentioned above can be applied for the Fig. 4 | LIR measurements of CH + isotopologues. Measured and simulated low- 3 high-resolution​ spectroscopy of isotopologues of CH +, resolution vibrational spectra of isotopologues of CH + (ref.60). The measured spectra (red) 3 3 + (ref.59) were recorded by using the laser-induced​ reaction (LIR) method with the free-electron as first demonstrated for CH2D : 110 laser FELIX . The spectroscopic simulations (black) were run by using the PGOPHER pro- + + 111 + 61,62 CH2D+HC23→ H+HD (6) gramme and, in the case of CH3 , also drawing on prior work . The infrared-active fun- −1 damental vibrational modes are numbered (νn, n = 1–6). All features in the 3-µm (3,000-cm ) region have also been recorded in high resolution49,59,61,62. As an example, the high-resolution By using such LIR schemes, all IR-​active vibrational + 60 + + + spectrum of the ν1 mode of CD2H is shown in Fig. 5. Data (measured) from ref. . modes of CH2D , CD2H and CD3 were recorded in low

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the millimetre-wave​ frequency of a connected pure rota- J tional transition (blue arrow), the signal was modulated 2 20 and the rotational transition thus detected with a high v = 1 signal-​to-​noise ratio. In this way, 25 rotational transi- 1 11 tions of CD H+ were detected in the range up to 1.1 THz 0 2 00 (blue sticks in Fig. 5). + Similar to H3 , the deuterium exchange processes of + CH3 and its isotopologues are thought to be responsible for the deuteration observed in the ISM. Owing to the v = 0 relatively high exothermicity of the exchange reactions 6 111 (for example, the reverse of Eq. has an exothermicity 67 000 of >400 K) , deuteration can occur even at elevated ISM temperatures (20 K < T < 80 K). Although a complete and high-​resolution data set now exists for the rotational + transitions of the CH3 family of molecules, the hypoth­ + esized interstellar deuteration via CH3 , unfortunately, rests on weak grounds, as attempts to detect the cationic species involved have so far failed67,68.

0 200 400 600 800 1,000 0 50 100 150 Laser-​induced inhibition of complex growth –1 ν (GHz) ν –3,000 (cm ) One limitation of the LIR method is the need for an endothermic chemical reaction with a neutral reaction | + Fig. 5 High-resolution spectra of CD2H . Stick spectra summarizing the 108 rovibra- partner that does not condense at the temperature of the + tional (red, right) and 25 pure rotational (blue, left) lines recorded for CD2H (top right) cryogenic trap (normally H ). An alternative, and more 49 –1 2 in an ion-trap experiment . Note the different frequency (ν) units (cm versus GHz) on generally applicable, action technique is laser-​induced the abscissa. The rovibrational lines belong to the mode whose envelope is depicted ν1 inhibition of complex growth (LIICG). This technique in Fig. 4. The insets (where J and v are the rotational angular momentum and vibrational quantum numbers, respectively) illustrate the applied spectroscopic methods: the emerged with the advent of ion traps cooled to 4 K and rovibrational lines were measured using laser-induced​ reaction (LIR) (right), whereas exploits the fact that excitation of a stored cation inhibits the rotational lines were detected with a double-resonance​ scheme based on LIR (left). attachment of He in a cryogenic He bath, as pioneered 69 Adapted with permission from ref.49, Elsevier. by Maier, Gerlich and co-workers​ . The main challenge of this technique lies in the cooling of the parent ions, such that a considerable fraction forms weakly bound resolution60 and are shown in Fig. 4, together with a sim- complexes with He. As an example, the transitions of the + + Fig. 3b ulated CH3 spectrum, based on the seminal work of the H3 ion measured using LIICG are shown in . Oka group61,62. Owing to the limited resolution in this The LIICG lines are measured by counting the number + experiment, only the contours of the vibrational bands of H3 –He complexes as a function of the excitation fre- could be resolved, but the availability of high-resolution​ quency. When the excitation frequency is on-resonance​ + lasers in the 3-​µm region allows for the investigation with a H3 transition, the molecules do not undergo 49,59,61–63 + of all C–H stretches in rovibrational detail . For complexation, and the number of H3 –He complexes in + example, the ν1 stretch of CD2H was measured by using the trap is reduced. The LIICG signals then appear as 49 (Fig. 5) + this approach . In this case, high resolution was narrow Gaussian dips in the number of H3 –He counts. obtained by applying an optical parametric oscillator as As this is not a background-​free method, the LIICG the light source and by cooling the investigated ions in lines exhibit a signal-​to-​noise ratio inferior to those of a cryogenic ion trap. the LIR method. Nevertheless, LIICG is an indispensable More than 80% of known interstellar molecules were alternative for molecular ions for which no suitable LIR + (refs43,70) detected for the first time in the ISM by their pure rota- scheme has been found. Beyond H3 , LIICG tional spectra1, making their laboratory measurement has been applied to measure the rovibrational spectrum + 71,72 a high priority. When a rovibrational action scheme such of the enigmatic CH5 molecule , and has since been as LIR is feasible for a molecule, this can be extended applied to many additional cations49,73–75. by rotational excitation to a rotational–rovibrational Among these additional species is CH+ (ref.73), which double-​resonance scheme to record pure rotational was among the first molecules, and the first molecular spectra. Such a double-​resonance scheme was first ion, detected in the ISM (through its electronic transi- + (ref.59) – (ref.64) 76,77 demonstrated for H2D and OH . Later, tions) . Since then, it has been recognized to be wide- guided by the pioneering work of Amano in measur- spread in the ISM and has also been observed by its + (ref.65) 78–80 + ing the first rotational transitions for CH2D , the rotational fingerprints . Only recently has CH been double-​resonance method was exploited to obtain a detected by its vibrational transitions in the IR region, + complete set of high-​resolution transitions for CH2D and indeed in the same source in which the first high-J​ (ref.66) + (ref.49) + and, subsequently, CD2H . For CD2H , rotational emission (where J is the rotational angular the double-​resonance measurement scheme can be momentum quantum number) was detected78. By ser- explained using the example given in Fig. 5. As shown endipity, CH+ was observed through its IR emission lines in the inset, a rovibrational transition (red arrow) was in the hot expanding shell of the planetary nebula NGC used to create a spectroscopic LIR signal. By scanning 7027 in a search that was originally designed to target

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the IR signatures of the HeH+ ion81,82. The detected R(0) et al.90, was used. These latter sources provide rota- line of CH+ is shown in Fig. 6, and compared with the tionally cold (a few Kelvin) molecules by adiabatically laboratory LIICG measurement of this same transition, expanding a short pulse of high-pressure​ gas through a which is the only IR laboratory detection as yet. pinhole nozzle (1 mm in diameter) into a high-​vacuum chamber. By attaching a narrow nozzle immediately after Rotational-state-​ dependent​ complex formation the pinhole, and coupling with high-voltage​ electrodes, In 2012, a series of regularly spaced, unidentified rota- a plasma discharge strikes during the pulse that ion- tional emission lines was detected in astronomical izes the sample, generating ions directly from neutral observations of the Horsehead Nebula photodisso- gases and also permitting the generation of new species, ciation region with the IRAM 30-​m telescope83. The both ions and neutral molecules, by dissociation in the spectral pattern could be well described by a linear discharge and subsequent reactions in the supersonic molecule with a rotational constant of B = 11,244 MHz. expansion. Rather than a direct-​absorption technique, On the basis of the chemical and physical conditions the rotationally cold product species were then probed of the region, and comparison with quantum chemi- using cavity-​enhanced FTMW spectroscopy91. This cal calculations, it was speculated that the carrier was well-​established experimental method has enabled the + C3H . Subsequent astronomical and quantum chemical rotational spectroscopic characterization of a multitude studies have either supported the assignment through of protonated92,93 and anionic species94,95. chemical arguments84–86 or suggested as an alternative Another variant of action spectroscopy was used – (refs87,88) + the molecular anion C3H . to definitively identify C3H as the carrier of the astro- Laboratory efforts to precisely measure the frequen- nomical lines. This method relies on the rotational-​ cies of the rotational emission lines and confirm the state-dependent​ attachment of rare gas atoms (ROSAA) carrier were challenging in terms of producing the mol­ to cold molecular ions, as outlined in detail in refs96,97. ecule and in detecting its rotational transitions, which Briefly, hydrocarbon ions were produced by electron

proved to be extremely weak. The first tentative labora- impact ionization of allene (C3H4) in a storage-ion + tory detection and assignment (to C3H ) was reported source, then mass-selected, injected into the crygogenic as a personal communication (K. N. Crabtree, 2013) in ion trap, and cooled to the ambient temperature of 4.1 K ref.85 and published in full 2 years later89. Rather than through collisions with He gas. Under these conditions, producing the molecule in a discharge cell, as Oka did the ions form weakly bound complexes with the He + for H3 , a type of high-​voltage discharge coupled to a atoms through ternary collision processes. These com- supersonic expansion source, as pioneered by McCarthy plexes are also destroyed in binary collisions with He, quickly reaching a steady-state​ concentration of parent and cluster ions. Unlike in LIICG, in which cluster formation is inhib- NGC 7027 ited for the vibrationally excited ions, in ROSAA the He attachment rate, and thus the steady-​state concentration, depends on the rotational state of the ion. As a con­ sequence, the number of cluster ions is modified when Laboratory the parent ion is rotationally excited by irradiation with microwave or terahertz radiation on-​resonance with a 2,766.544 2,766.548 2,766.552 rotational transition. The spectrum is then read out as a fractional change in the number of ion–He com- plexes versus frequency (see, for example, the spectra + Fig. 7 for C3H in ). The relative change in the number of ion–He complexes formed depends not only on the photon absorption cross section, but also on the rela- tive population of a specific targeted rotational level, the achievable population change between rotational levels 2,763 2,764 2,765 2,766 2,767 2,768 and the change in the attachment rate for these levels. In the case of C H+, the observed relative change was Frequency (cm–1) 3 only a few per cent (Fig. 7). Fig. 6 | Laboratory and space detection of CH+ in the infrared. Spectra showing Despite these challenges and limitations, ROSAA the laboratory (inset) and space detection of the R(0) transition of CH+ (refs73,81). has proved to be a versatile tool for rotational spectros­ For the laboratory measurement, CH+–He complexes were counted as a function of copy of molecular ions and, in particular, for reactive the irradiating frequency, using the laser-induced​ inhibition of complex growth (LIICG) ions that are extremely difficult to produce in high method. The interstellar emission spectrum was acquired with the iSHELL spectrograph enough number densities for conventional absorption at the NASA Infrared Telescope Facility towards the NGC 7027 planetary nebula. The laboratory spectral profile is a very narrow Gaussian owing to the 4-K​ cold trapped or FTMW spectroscopy. Several more recent examples 98,99 ion ensemble, whereas the astronomical signal is broadened (note the difference in include the deuterated ammonium and methylium 49 + 73,97 + 97,100 wavenumber scale) because of the Doppler shift caused by the fast motion of the CH+ ions , isotopes of CH (refs ), HCO (refs ) and ions in the hot expanding shell of NGC 7027. The rest frequency as measured in the CN+ (ref.101), as well as SiH+ (ref.102), CO+ (ref.97) and CF+ laboratory is shown as a vertical black dashed line in both traces. Although the detail (ref.98). The technique has also recently been further of a single transition is shown here, the identification of CH+ and other molecules in developed by implementing a double-resonance​ scheme interstellar spectra is typically supported by many such lines. Data (space) from ref.81. for the rotational (and rovibrational) characterization of

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100 2 3 200 3 200 1 2 2 50 Laboratory Laboratory Laboratory 0 150 150 1 J=2–1 1 J=4–3 J=5–4 –1 0 0 ROSAA signal (%) 100 100 (K) (K) 0 –2 ROSAA signal (%) ROSAA signal (%) –1 mb mb * (mK) Horsehead PDR

A –1 T T

T Horsehead PDR IRAM Obs. 50 IRAM Obs. 50 –50 Sgr B2(N) GBT Obs. 0 0

–100 –50 –50 44,976.5 44,979.5 44,982.5 89,955.6 89,957.6 89,959.6 112,442.6 112,445.6 112,448.6 Frequency (MHz) Frequency (MHz) Frequency (MHz)

+ 96 Fig. 7 | Laboratory and space detection of C3H . Green Bank Telescope rare gas atoms (ROSAA) technique are overplotted in red . The ROSAA (GBT) observations of the high-mass​ star-forming​ region Sagittarius (Sgr) signals are vertically offset and inverted for clarity. The laboratory B2(N) (left, black) from the PRIMOS Project112, showing the absorption linewidths are dominated by power-​broadening effects. By contrast, the signal attributed to the J = 2–1 transition (where J is the rotational angular observed line profile towards Sgr B2(N) is due to absorbing foreground + 84 momentum quantum number) of C3H (ref. ). Observations of the gas with different velocity components, and the narrow linewidths in the Horsehead photodissociation region (PDR) using the IRAM telescope83 Horsehead PDR are caused mainly by the turbulent motion of the emitting

(centre and right, black) attributed to the J = 4–3 and J = 5–4 transitions gas. TA*, intensity as antenna temperature; Tmb, intensity as main beam + 83,84 96 (in emission). The laboratory measurements of the corresponding C3H brightness temperature. Data from refs (observational) and ref. transitions made using the rotational-​state-​dependent attachment of (laboratory).

+ 75 the CH2NH2 ion , allowing its application to an even the equivalence of the five protons in this very floppy wider class of molecular ions. molecule, the spectrum contains contributions from three nuclear spin isomers, which have begun to be Future perspectives untangled72. However, even at low temperatures, the The development of ion spectroscopy and associated spectrum remains complex, in part owing to the inter- laboratory techniques has led to the detection of numer- play between the end-​over-​end rotation of the almost ous molecular ions in the ISM. Indeed, several species spherical molecule and the fluxional nature of its inter- that have long been speculated to be key reaction inter- nal rotation. Despite these difficulties, the methods mediates (Fig. 1) have since been identified on the basis of described here, in particular the double-​resonance laboratory spectra acquired through techniques outlined method in action spectroscopy, may finally enable the in this Review. These identifications now allow not only determination of the lowest energy transitions, and thus the detection of these species, but also the determination perhaps the best candidates for interstellar detection, in of their abundances and physical conditions. This infor- this elusive molecule. mation can then be compared to astrochemical models Perhaps more crucially, the end products in Fig. 1 are designed to simulate our understanding of the reaction thought to be precursors to a host of yet more complex pathways in the ISM. The agreement, or lack thereof, molecules. For example, the protonation of possibly the

with the models in turn reveals which reaction pathways most widespread complex species, methanol (CH3OH), + warrant further study. Ions are often one of the most cru- to form CH3OH2 is thought to be central to under- cial intermediates, making their chemistry particularly standing how molecules of even greater complexity, impactful on the wider chemical network. such as precursors of amino acids, could be formed in Figure 1 shows a bottom-​up version of interstellar abundances that would enable their detection9. In the chemistry, in which species are formed from the most laboratory, chemical reactions to establish a LIR scheme + abundant, primary ions, such as H3 . Despite the tre- would involve condensable gases and thus would require mendous success of this approach and the recent detec- modifications of the trap instrument to allow overlap tion of many molecular ions, numerous important ions of the trap volume with a molecular beam carrying remain to be detected. Among these, are the deuterated the neutral reaction partner to prevent its freezing to the + isotopologues of CH3 , for which detailed laboratory cold trap wall. Such a development would provide access spectra are now available but are hard to find in obser- to the spectroscopy of many more molecular ions, even vations owing to detection limits, even in the age of the beyond the scope of astrochemistry. Indeed, preliminary + ALMA and NOEMA observatories. results in recording low-​resolution spectra of CH3OH2 + The spectra of many more ions have yet to be recor­ with the approach used for CH3 and its isotopologues ded in the laboratory. The end products of chemical appear promising103. Therefore, the next important step evolution are key species in the reaction scheme in is to record the high-resolution​ rovibrational spectrum Fig. 1 + and form when, for example, an ion is completely of CH3OH2 . + Fig. 1 hydrogenated, as in the case of CH5 . Through action It should also be noted that is only one way spectroscopy, considerable progress has been made of looking at astrochemistry. Although bottom-​up + in deciphering the rich spectrum of CH5 . Owing to pathways are certain to be efficient for the formation of

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+ 108 relatively small molecules from more abundant elements, complex of C60 with He atoms , employing yet another a top-​down route to molecular complexity involves method of action spectroscopy in ion traps. the destruction of large molecules or (very) small In addition to its being the first definitively identified (ref.104) + grains. After the detection of interstellar C60 , DIB, the discovery of C60 in sizable amounts raises ques- + + the logical next step was to search for the C60 ion. C60 tions regarding the chemical routes for its production and has long been considered as a possible carrier of the whether such a large species may be the top-down​ source diffuse interstellar bands (DIBs), a widespread forest for chemically enriching the ISM. Regardless, this detec- of unidentified absorption features first discovered tion and many other recent successes highlight the con- by Mary Lea Heger in 1919 and seen in diffuse gas tinuing need to develop more sensitive and more accurate within the galaxy105. Nearly eight decades later, two of laboratory methods for the spectroscopic characterization the strongest DIB features were assigned106 to an elec- of molecular ions. Although challenging to study both + tronic absorption in C60 on the basis of early matrix in the laboratory and observationally, they undoubtedly measurements107. Almost a century after the discovery have key roles in the chemical evolution of the Universe, of the DIBs, this assignment was confirmed through the and are thus crucial species to observe and understand. + measurement of the electronic spectrum of C60 in an ion-​trap experiment involving the pre-dissociation​ of a Published online 7 July 2020

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Phys. pulsed nozzle particle source. Rev. Sci. Instrum. 52, NASA through a Hubble Fellowship (grant no. HST-​HF2- Chem. Lett. 4, 4051–4054 (2013). 33–45 (1981). 51396) awarded by the Space Telescope Science Institute, 70. Savić, I., Gerlich, D., Asvany, O., Jusko, P. & 92. Ohshima, Y. & Endo, Y. Rotational spectroscopy which is operated by the Association of Universities for Schlemmer, S. Controlled synthesis and analysis of jet-cooled​ molecular ions and ion complexes. Research in Astronomy for NASA, under contract NAS5- + of HeH3 in a 3.7 K ion trap. Mol. Phys. 113, Chem. Phys. Lett. 256, 635–640 (1996). 26555. The National Radio Astronomy Observatory is a 2320–2332 (2015). 93. Lattanzi, V., Thorwirth, S., Gottlieb, C. A. & facility of the US National Science Foundation operated under 71. Asvany, O., Brünken, S., Kluge, L. & Schlemmer, S. McCarthy, M. C. Two isomers of protonated isocyanic cooperative agreement by Associated Universities, Inc. S.S., COLTRAP: a 22-pole ion trapping machine for acid: evidence for an ion–molecule pathway for O.A. and S.B. thank the Deutsche Forschungsgemeinschaft spectroscopy at 4 K. Appl. Phys. B 114, 203–211 HNCO ↔ HOCN isomerization. J. Phys. Chem. Lett. 3, for long-​term support of the development of the action spec- (2014). 3420–3424 (2012). troscopy techniques in the Cologne laboratory through This paper reports vibrational LIICG, demonstrated 94. McCarthy, M. C., Gottlieb, C. A., Gupta, H. & numerous projects. for protonated methane. Thaddeus, P. Laboratory and astronomical – 72. Asvany, O., Yamada, K. M. T., Brünken, S., Potapov, A. identification of the negative molecular ion C6H . Author contributions & Schlemmer, S. Experimental ground-​state Astrophys. J. 652, L141–L144 (2006). The authors contributed equally to all aspects of the article. + combination differences of CH5 . Science 347, 95. Gupta, H. et al. Rotational spectra of the carbon – – 1346–1349 (2015). chain negative ions C4H and C8H . Astrophys. J. 655, Competing interests 73. Doménech, J. L., Jusko, P., Schlemmer, S. & L57–L60 (2007). The authors declare no competing interests. Asvany, O. The first laboratory detection of vibration– 96. Brünken, S., Kluge, L., Stoffels, A., Asvany, O. & rotation transitions of 12CH+ and 13CH+ and improved Schlemmer, S. Laboratory rotational spectrum of Peer review information + measurement of their rotational transition frequencies. l-C​ 3H and confirmation of its astronomical detection. Nature Reviews Physics thanks Satoshi Yamamoto and the Astrophys. J. 857, 61 (2018). Astrophys. J. 783, L4 (2014). other, anonymous, reviewer(s) for their contribution to 74. Kohguchi, H., Jusko, P., Yamada, K. M. T., Application of the ROSAA method, confirming the the peer review of this work. Schlemmer, S. & Asvany, O. High-​resolution infrared presence of a key ionic intermediate in the ISM. + spectroscopy of O2H in a cryogenic ion trap. J. Chem. 97. Brünken, S., Kluge, L., Stoffels, A., Pérez-​Ríos, J. Publisher’s note Phys. 148, 144303 (2018). & Schlemmer, S. Rotational state-​dependent Springer Nature remains neutral with regard to jurisdictional 75. Markus, C. R., Thorwirth, S., Asvany, O. & attachment of He atoms to cold molecular ions: claims in published maps and institutional affiliations. Schlemmer, S. High-​resolution double resonance an action spectroscopic scheme for rotational action spectroscopy in ion traps: vibrational and spectroscopy. J. Mol. Spectrosc. 332, 67–78 (2017). © Springer Nature Limited 2020

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