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Dissociative Photoionization of Polycyclic Aromatic Hydrocarbon

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Dissociative Photoionization of Polycyclic Aromatic Hydrocarbon Dissociative Photoionization of Polycyclic Aromatic Hydrocarbon Molecules Carrying an Ethynyl Group G. Rouill´e, S. A. Krasnokutski, D. Fulvio, and C. J¨ager Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany [email protected] Th. Henning Max Planck Institute for Astronomy, K¨onigstuhl 17, D-69117 Heidelberg, Germany and G. A. Garcia, X.-F. Tang, and L. Nahon Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin BP 48, F-91192 Gif-sur-Yvette Cedex, France ABSTRACT The life cycle of the population of interstellar polycyclic aromatic hydrocarbon (PAH) molecules depends partly on the photostability of the individual species. We have studied the dis- sociative photoionization of two ethynyl-substituted PAH species, namely, 9-ethynylphenanthrene and 1-ethynylpyrene. Their adiabatic ionization energy and the appearance energy of frag- ment ions have been measured with the photoelectron photoion coincidence (PEPICO) spec- troscopy technique. The adiabatic ionization energy has been found at 7.84 ± 0.02 eV for 9- ethynylphenanthrene and at 7.41 ± 0.02 eV for 1-ethynylpyrene. These values are similar to those determined for the corresponding non-substituted PAH molecules phenanthrene and pyrene. The appearance energy of the fragment ion indicative of the loss of a H atom following photoionization is also similar for either ethynyl-substituted PAH molecule and its non-substituted counterpart. The measurements are used to estimate the critical energy for the loss of a H atom by the PAH cations and the stability of ethynyl-substituted PAH molecules upon photoionization. We con- clude that these PAH derivatives are as photostable as the non-substituted species in H I regions. If present in the interstellar medium, they may play an important role in the growth of interstellar arXiv:1508.00259v1 [astro-ph.GA] 2 Aug 2015 PAH molecules. Subject headings: astrochemistry — ISM: molecules — molecular data — molecular processes 1. Introduction 1700 K through the hydrogen-abstraction-C2H2- addition (HACA) mechanism (Frenklach et al. The existence of interstellar polycyclic aromatic 1985; Frenklach & Feigelson 1989; Cherchneff et al. hydrocarbon (PAH) molecules has been inferred 1992; Wang & Frenklach 1994; Cherchneff & Cau from the observation of infrared emission bands 1999). Experiments with the pyrolysis of small hy- (Leger & Puget 1984; Allamandola et al. 1985). drocarbons in the gas phase have actually shown It has been proposed that PAH molecules, be- that temperatures lower than 1800 K are required fore being blown into the interstellar medium for an efficient condensation of the pyrolysis prod- (ISM), are formed in the envelope of carbon- ucts into PAH molecules, presumably through rich stars at temperatures in the range 900– 1 HACA (J¨ager et al. 2007, 2009). In the various with that of regular ones. The latter comprise regions of the ISM, where temperatures are lower exclusively fused, six-membered aromatic carbon than 100 K, free-flying PAH molecules are thought cycles. They found that while a regular PAH to be formed through the destruction of carbona- would survive in H I regions, its substituted coun- ceous grains in shocks (see Chiar et al. 2013, and terparts would undergo fragmentation in the same references therein). Additionally, despite the low UV radiation field. This could suggest that the temperatures, PAH species could grow through ethynyl derivatives would not survive in H I re- a series of chemical reactions that require little gions, in contrast with our expectation that they or no activation energy. Accordingly, Mebel et al. are photostable (Rouill´eet al. 2012, 2013). (2008) have introduced the ethynyl addition mech- The possible presence of ethynyl-substituted anism (EAM), which is barrierless with regard PAH molecules in the ISM and the lack of to activation energy, as a means to form PAH data concerning their photostability called for molecules in cold gas-phase environments, includ- an experimental study. We present here lab- ing the conditions of the ISM. oratory measurements on the dissociative pho- Ethynyl-substituted PAH species appear as in- toionization of two ethynyl-substituted PAH termediates in the HACA mechanism and the species, 9-ethynylphenanthrene (C16H10) and 1- EAM as well. Thus they may be present in ethynylpyrene (C18H10). The photostability in- the ISM, either as species ejected from stellar dex R as defined by Jochims et al. (1999) can be envelopes before their transformation into reg- derived from few experimental data, namely, the ular PAH molecules or as intermediates formed adiabatic ionization energy, the appearance en- locally. The HACA mechanism has been stud- ergy of the singly dehydrogenated fragment ion, ied as a formation process of PAH species by and the internal thermal energy of the parent taking into account the conditions that prevail ion. Their values have been determined in pho- in stellar envelopes (Frenklach & Feigelson 1989; toelectron photoion coincidence (PEPICO) spec- Cherchneff et al. 1992; Cherchneff & Cau 1999). troscopy experiments and, after deriving the value The EAM, however, has yet to be evaluated as of R for both ethynyl-substituted PAH species, we an interstellar growth process, i.e., with regard conclude that they are as photostable as the non- to the abundance of the ethynyl radical (C2H) substituted molecules. and considering the competition with destruction mechanisms, especially photofragmentation. Rel- 2. Experimental details atively high amounts of C2H have been found in the ISM (Tucker et al. 1974), in diffuse clouds The experiments were carried out at the beam- 1 (Lucas & Liszt 2000; Gerin et al. 2011; Liszt et al. line DESIRS (Nahon et al. 2012) of the syn- 2 2012), in translucent clouds (Turner et al. 2000), chrotron SOLEIL . This beamline delivers pho- in certain dark clouds (Wootten et al. 1980; tons with an energy in the range 5–40 eV, Ohishi et al. 1992), in circumstellar envelopes thus covering the VUV wavelength domain. (Huggins et al. 1984), in protoplanetary disks Photons at chosen energies were used to pho- (Henning et al. 2010), and in massive star-forming toionize PAH species in a molecular beam pro- 3 regions (Beuther et al. 2008). On the other hand, duced with the vacuum apparatus SAPHIRS the photostability of ethynyl-substituted PAH (Richard-Viard et al. 1996). The photoelectrons molecules has yet to be experimentally charac- and the photoions – parents and fragments – were terized. analyzed using the double-imaging PEPICO spec- trometer DELICIOUS III (Garcia et al. 2013). Jochims et al. (1999) carried out laboratory Briefly, it consists of a velocity-map-imaging measurements on the photostability of several PAH molecules. While the set of substances in- 1Dichro¨ısme Et Spectroscopie par Interaction avec le Ray- cluded methyl-, vinyl-, and phenyl-substituted onnement Synchrotron species, it did not contain ethynyl derivatives. 2Source Optimis´ee de Lumi`ere d’Energie Interm´ediaire du Nevertheless, they defined a structure-dependent LURE index of photostability that allowed them to com- 3Spectroscopie d’Agr´egats PHotoIonis´es par le Rayon- pare the behavior of substituted PAH molecules nement Synchrotron 2 (VMI) electron analyzer and a modified Wiley- ber. This chamber is the first of the three differ- McLaren ion imaging spectrometer operated in entially pumped vacuum chambers of SAPHIRS, coincidence. The coincidence treatment provides which communicate by means of skimmers 1 mm photoelectron images that can be tagged by their in diameter. The jet and the skimmers separat- corresponding ion mass and ion translational en- ing the chambers were aligned so as to produce a ergy. From these images, photoelectron spectra molecular beam of He atoms and PAH molecules. can be obtained from all the masses in the molecu- In the third chamber, the molecular beam crossed lar beam simultaneously. The energy of the VUV the synchrotron radiation beam in the ionization radiation was calibrated using the photoioniza- volume of the PEPICO spectrometer. 1 + + 2 + tion of N2(X Σg ) into N2 (B Σu ) as a reference Adiabatic ionization energies were obtained (Innocenti et al. 2013). from the mass-selected photoelectron images We used samples of phenanthrene (C14H10, recorded at a fixed photon energy of 9 eV. The Aldrich, purity ≥99.5%), 9-ethynylphenanthrene proximity of this energy to the ionization thresh- (C16H10, Aldrich, purity 97%), pyrene (C16H10, old of the molecules led to the production of slow Aldrich, purity 99%), and 1-ethynylpyrene (C18H10, electrons and the possibility to lower the extrac- abcr, purity 96%), as received, without undertak- tion field to 90 V cm−1 with the consequent gain in ing a further purification. kinetic energy resolution (Garcia et al. 2013). The In each experiment, the sample powder was corresponding mass-selected photoelectron spec- heated in an oven placed in the source chamber tra were obtained from the images applying the of SAPHIRS, so as to increase its vapor pressure pBasex algorithm for Abel inversion (Garcia et al. up to a useful level. The oven temperature was 2004). A gas filter upstream the monochromator 383 K for phenanthrene, 493 K for pyrene, up was filled with Kr to effectively cut off the high to 443 K for 9-ethynylphenanthrene (413 K for harmonics of the undulator (Mercier et al. 2000). energy-dependent breakdown measurements and For these measurements, the photoelectrons and 443 K for photoelectron spectroscopy measure- ions were counted during 6483 s for phenanthrene, ments; see below in this Section), and 403 K for 3627 s for 9-ethynylphenanthrene, and 18034 s for 1-ethynylpyrene. In order to prevent its obstruc- 1-ethynylpyrene. tion by condensing sample vapor, the nozzle was The appearance energy of the ion fragments in- heated to a temperature 10 K higher than that dicative of dissociative photoionization was mea- of the oven. The phenanthrene and pyrene pow- sured by varying the photon energy from 15 to ders were poured in a stainless-steel boat that was 20 eV with a step of 0.1 eV.
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