Photochemistry and radiation chemistry of interstellar ices lead to the synthesis of prebiotic molecules which may be delivered to planets by meteorites and/or comets.
In the interstellar medium, UV photolysis of condensed methanol (CH3OH), contained in ice mantles surrounding dust grains, is thought to be the mechanism that drives the formation of "complex" molecules, such as methyl formate (HCOOCH3), dimethyl ether (CH3OCH3), acetic acid (CH3COOH), and glycolaldehyde (HOCH2CHO). The source of this reaction-initiating UV light is assumed to be local because externally sourced UV radiation cannot penetrate the ice-containing dark, dense molecular clouds. Specifically, exceedingly penetrative high-energy cosmic rays generate secondary electrons within the clouds through molecular ionizations. Hydrogen molecules, present within these dense molecular clouds, are excited in collisions with these secondary electrons. It is the UV light, emitted by these electronically excited hydrogen molecules, that is generally thought to photoprocess interstellar icy grain mantles to generate "complex" molecules. In addition to producing UV light, the large numbers of low-energy (< 20 eV) secondary electrons, produced by cosmic rays, can also directly initiate radiolysis reactions in the condensed phase. The goal of our studies is to understand the low-energy, electron-induced processes that occur when high-energy cosmic rays interact with interstellar ices, in which methanol, a precursor of several prebiotic species, is the most abundant organic species. Using post-irradiation temperature-programmed desorption, we have investigated the radiolysis initiated by low-energy (7 eV and 20 eV) electrons in condensed methanol at - 85 K under ultrahigh vacuum (5 x 10(-10) Torr) conditions. We have identified eleven electron-induced methanol radiolysis products, which include many that have been previously identified as being formed by methanol UV photolysis in the interstellar medium. These experimental results suggest that low-energy, electron-induced condensed phase reactions may contribute to the interstellar synthesis of "complex" molecules previously thought to form exclusively via UV photons.
The exposure of multilayers of an adsorbate to low energy (155 eV) electrons under ultrahigh vacuum (UHV) conditions followed by temperature-programmed desorption (TPD) is shown to be an effective method to identify radiolysis products. In conjunction with isothermal experiments, postirradiation TPD experiments were used to identify eight previously known radiolysis products of methanol. The utility of the method is demonstrated by the identification of a previously unknown methanol radiolysis product: methoxymethanol (CHsOCH20H). Moreover, this technique allows study of the dependence on initial electron energy, providing additional insight into the physical processes underlying radiation chemistry.
We have investigated the dynamics of low-energy (1−20 eV) electron-induced reactions in condensed thin films of methanol (CH 3 OH) through both electron-stimulated desorption (ESD) and postirradiation temperature-programmed desorption (TPD) experiments conducted under ultrahigh vacuum conditions. Results of ESD experiments, involving a high-sensitivity time-of-flight mass spectrometer, indicate that anion (H − , CH − , CH 2 − , CH 3 − , O − , OH − , and CH 3 O − ) desorption from the methanol thin film at incident electron energies below about 15 eV is dominated by processes initiated by the dissociation of temporary negative ions of methanol formed via electron capture, a resonant process known as dissociative electron attachment (DEA). However, postirradiation TPD investigation of radicals, especially •CH 2 OH and CH 3 O• remaining in the methanol thin film, demonstrates that electron impact excitation, not DEA, is the primary mechanism by which the radical−radical reaction products methoxymethanol (CH 3 OCH 2 OH) and ethylene glycol (HOCH 2 CH 2 OH) are formed. This apparent dichotomy between the results of ESD and postirradiation experiments is attributed to the low DEA cross section for methanol compared to that of species such as halomethanes. Our results suggest that for molecules such as methanol, low-energy electron-induced electronic excitation, rather than DEA, plays a dominant role in ionizing radiation-induced chemical synthesis in environments such as the interstellar medium. ■ INTRODUCTIONBecause of its simple chemical structure, methanol is a prototypical candidate for radiolysis studies of oxygencontaining biomolecules such as DNA. The radiation chemistry of methanol is of particular interest because methanol is exposed to different types of ionizing radiation in varying environments and phases. For example, liquid methanol is used as a solvent in radiation-induced grafting of copolymer composites. 1 The radiation chemistry of condensed methanol is also of astrochemical interest because methanol is found in relatively high abundance in protostar environments. Methanol is thought to be an important precursor in cosmic ices not only to species such as methyl formate (HCOOCH 3 ), ethylene glycol (HOCH 2 CH 2 OH), and dimethyl ether (CH 3 OCH 3 ) but also to many prebiotic species such as simple sugars and amino acids. 2−4 Because of such applications, the high-energy radiolysis of methanol has been extensively studied over a period spanning seven decades. 5−9 More recent advances, however, have demonstrated that studying the interactions of low-energy electrons with condensed matter is essential to obtaining a fundamental understanding of radiation chemistry because the interactions of high-energy radiation, such as cosmic rays (E max ∼ 10 20 eV), with matter produce large numbers of low-energy (<15 eV) secondary electrons, which are thought to initiate radiolysis reactions in the condensed phase. 10,11 In this publication, we investigate the chemistry induced in condensed methanol by such low-energ...
5 a b s t r a c t 6 Available online xxxx 12 13UV photon-driven condensed phase cosmic ice reactions have been the main focus in understanding the extra-14 terrestrial synthesis of complex organic molecules. Low-energy (≤20 eV) electron-induced reactions, on the 15 other hand, have been largely ignored. In this article, we review studies employing surface science techniques 16 to study low-energy electron-induced condensed phase reactions relevant to astrochemistry. In particular, we 17 show that low-energy electron irradiation of methanol ices leads to the synthesis of many of the same complex 18 molecules formed through UV irradiation. Moreover, our results are qualitatively consistent with the hypothesis 19 that high-energy condensed phase radiolysis is mediated by low-energy electron-induced reactions. In addition, 20 due to the numbers of available low-energy secondary electrons resulting from the interaction of high-energy 21 radiation with matter as well as differences between electron-and photon-induced processes, low-energy 22 electron-induced reactions are perhaps as, or even more, effective than photon-induced reactions in initiating 23 condensed-phase chemical reactions in the interstellar medium. Consequently, we illustrate a need for 24 astrochemical models to include the details of electron-induced reactions in addition to those driven by UV 25 photons. Finally, we show that low-energy electron-induced reactions may lead to the production of unique 26 molecular species that could serve as tracer molecules for electron-induced condensed phase reactions in the 27 interstellar medium. 28 Astrochemistry 31 Low-energy electrons 32 Temperature programmed desorption 33 Infrared reflection absorption spectroscopy 34 Cosmic ices 35 Interstellar medium 36 Synthesis of prebiotic molecules 37 38 39 40 41 45 is filled with complex molecules [2]. In addition to these optical spectral 46 absorption bands, vibrational emission bands have been used to telescop-47 ically identify complex molecules such as polyaromatic hydrocarbons 48 (PAHs Q6 ), fullerenes (C 60 , C 70 ), and diamondoids [3]. Moreover, (sub) mil-49 limeter rotational transitions of molecules have been exploited to identify 50 within interstellar and circumstellar clouds approximately 200 different 51 gas phase molecules including glycolaldehyde (HOCH 2 CHO) [4], a poten-52 tial prebiotic molecule. The synthesis of such complex/prebiotic mole-53 cules in the interstellar medium is thought to occur via three possible 54 mechanisms: (1) gas-phase reactions, (2) surface reactions on bare 55 carbonaceous or silicaceous dust grains, and (3) energetic processing of 56~100 ML (monolayer)-thick ice mantles surrounding micron-sized dust 57 grains [5]. In this review, we will explore the use of surface science tech-58 niques to understand the third mechanism, energetic ice processing, 59 which includes both surface and bulk reactions. Specifically, we will re-60 view the recent work which examines the role of low-energy electrons 61 in the synthesis of prebiot...
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