Synthesis of complex organic molecules in simulated methane rich astrophysical ices

Considerable research is presently focused on the origins of the molecular diversity observed in the interstellar medium (ISM)^1,2 and in objects such as comets and meteorites.^2–4 This interest is at least partly related to the implications of this diversity in the formation of pre-biotic molecular species, and hence, the origin of life on our and other planets. More than 280 different molecular species have been identified in the ISM of our galaxy,⁵ ranging from simple (and abundant) diatomics such as H₂ and CO to rarer, complex organic molecules such as C₂H₅OCHO, H₂NCH₂COOH, and HC₁₁N.^5,6

It has been suggested that complex, prebiotic molecular species may be produced by radiation and/or thermal processing of various astrophysical ices, such as the mantles that condense around dust grains within dense and cold (10-20 K) molecular clouds of the ISM,¹ or, more relevant here, on the surfaces of icy bodies within the solar system (comets, planets, and their satellites).⁷ Such ices are exposed to multiple radiation fields, often in the presence of intense magnetic fields (e.g., Io) which tend to accelerate charged particles from the solar wind toward the satellite surfaces. Dust grains in dense molecular clouds are exposed to high intensities of ultraviolet (UV) irradiation from the young and hot stars that are formed in these stellar interstellar nurseries and also to (a) X-rays, electrons, and ions from either coronal mass ejections or just part of the stellar wind, and (b) galactic cosmic radiation.⁸ More importantly, for the present study, planetary ices, like those observed for some of the satellites of Jupiter and Saturn,⁷ are exposed to energetic photons and charged particles of solar and galactic origin, the intensity of which can be enhanced via their interactions with planetary magnetic fields.⁹ 

When high-energy radiation (e.g., γ-rays, X-rays, electrons, and energetic ions) interacts with matter, it produces copious numbers (∼4 × 10⁴ electrons per MeV of energy deposited) of non-thermal secondary low-energy electrons (LEEs). Calculations indicate that these latter are produced with energy distributions that exhibit modal energies of ∼10 eV and mean energies of a few tens of eV. Almost all secondary electrons have energies less than 100 eV¹⁰ which for the purposes of this article represents the upper energy limit for LEEs. Studying the interactions of LEEs with condensed molecules at low temperatures can thus help us to identify novel chemistry¹¹ and to better understand the radiation chemistry that follows the primary action of high-energy radiation fields.¹² While many laboratory studies that have attempted to investigate the chemistry occurring in astrophysical ices have used high-energy particle irradiation or UV,¹¹ there is increasing interest in the possible role of secondary LEEs in such chemistry.¹³ Our previous work using LEE irradiation¹⁴ has reported the synthesis and prompt desorption of new chemical species from simple molecular ices of several monolayer (ML) thickness. The ices consisted of small molecules such as ¹⁶O₂, ¹⁸O₂, N₂, CD₄, and C₂D₂, which were condensed at 20 K under ultra-high vacuum conditions and were irradiated by low-energy (<60 eV) electrons. Measurements at very low fluence showed the prompt desorption of large cationic reaction/scattering products [e.g., C₂H₅⁺ from pure CH₄, C_nD_m⁺ (n,m = 2-6) from C₂D₂ films, and H₃O⁺ and H₃CO⁺ from films containing both CH₄/O₂].¹⁴ The formation of such comparatively complex molecular species, including benzene, by LEE-initiated cation reactions in simple molecular films, supports the notion that such mechanisms may lead to the synthesis of similarly complex molecules in astrophysical ices subjected to space radiation and indicates that LEE irradiation experiments can provide useful information about such processes. However, this latter study focused mainly on prompt desorption of new molecular cationic species, formed during LEE irradiation, and did not investigate (1) processes that lead to the formation and desorption of new negative ion species during extended times of LEE impact and (2) what new molecules are formed by LEE, and which remain in the films, that can be studied by various post-irradiation analysis techniques such as X-ray photoelectron spectroscopy (XPS) or temperature programed desorption (TPD) mass spectroscopy of molecules.

Here we use multiple in situ surface science techniques to study such LEE-induced chemistry in films containing CH₄ and O₂. Methane is a commonly encountered species in the ISM¹⁵ and on bodies throughout the solar system.¹⁶ Molecular oxygen forms in the eject of Pop III progenitor supernovae,¹⁷ and there is spectroscopic evidence for condensed O₂ on satellites such as Ganymede¹⁸ and Europa and Callisto,¹⁹ on which condensed methane is also reported.²⁰ Our measurements presented here show that prolonged LEE irradiation of simple molecular ices can lead to the formation of potential biotic precursors that reveal their presence in slowly growing anion desorption yields, as well as in the XPS and TPD spectra from LEE and X-ray irradiated icy solids. Comparisons between the TPD mass spectra of LEE irradiated and unirradiated films allow us, with the National Institute of Standard and Technology (NIST) database as a reference for electron impact mass spectra, to tentatively identify some of the products formed in the films by LEEs. Such assignments are supported by TPD measurements of pure and mixture films composed of potential products. New species observed in the TPD of irradiated pure films include, among others, C₃H₆, C₂H₆, and C₂H₅OH for mixture films. XPS furthermore confirms the formation of C—O, C—C, C=O, and COO— bonds in the newly formed irradiation products that remain in the molecular ices after irradiation.

All experiments were performed in an Ultra-High Vacuum (UHV) system maintained at a base pressure of ∼6 × 10⁻¹¹ Torr by an ion pump. For the present work, sample films were formed by vapor deposition onto a Pt-foil substrate, the temperature of which can be controlled between 22 K and 300 K with a recently added closed-cycle He cryostat and resistive heating elements. Film cryogenic temperatures were estimated from the measurements with a chromel/gold thermocouple attached to the cryostat. When the cryostat is in thermal equilibrium with a cold target, the chromel/Au thermocouple temperature is estimated to be equal or slightly higher than that of the Pt-substrate foil at the end of the cold finger. This Pt substrate was cleaned between experiments by resistive heating of the foil to 900 C as monitored with an optical pyrometer. It is noted that during resistive heating, the thin Pt-substrate foil slightly expands and temporarily loses thermal contact with the underlying ceramic insulator (itself in close contact with the cryo head) such that the thermal load on the cryohead is minimized, as verified by the chromel/Au thermocouple, which indicates only a slight increase of ∼3 K or so during resistive heating. The thickness of sample films condensed onto the substrate is expressed in monolayers (MLs) and is determined by a volumetric dosing procedure with an absolute accuracy of ∼30%^21,22 and a repeatability of about 0.2 ML.²³ Films of pure ¹⁸O₂ (97.4% isotopic purity, MSD Isotopes), CH₄ (99.99% Matheson), and films containing mixtures of the two gases were deposited at 22 K. The composition of mixture films was characterized by the ratio of the partial pressures in the gas-handling manifold, prior to film deposition [e.g., ¹⁸O₂/CH₄ (1:1)]. In most experiments (and unless indicated otherwise), films of 25 ML were used. We chose the ¹⁸O₂ isotope to avoid parasitic signals from ¹⁶O containing vacuum contaminants in mass spectrometric measurements. Sample films were irradiated isothermally at 22 K with 70 eV electrons generated from an electron gun (Kimball Physics ELG-2). The energy resolution of this beam is approximately 0.5 eV. Typical transmitted electron beam currents, measured on the clean Pt foil, were of the order of 130 nA. When focused into a spot of ∼50 mm² area, as determined by measurements with a fluorescent phosphor plate, this current corresponds to a flux of ∼15 × 10¹¹ electrons s⁻¹ cm⁻².

The LEE-induced chemistry occurring during irradiation was analyzed in situ by Electron Stimulated Desorption (ESD). Post-irradiation analysis was performed by X-ray Photoelectron Spectroscopy (XPS), as previously been described,²⁴ and by Temperature Programmed Desorption (TPD).¹² With the latter two techniques, it was necessary to electron-irradiate entire samples, which required oscillatory voltages to be applied to the deflector plates at the exit of the electron gun, to raster the electron beam across a larger area of ∼1 cm². The transmitted current in the raster mode was ∼80 nA giving an electron flux of ∼5 × 10¹¹ electrons s⁻¹ cm⁻².

To identify, or otherwise characterize, new electron-induced products seen in TPD and XPS measurements, sample films containing candidate products were also analyzed by these two techniques. A list of candidate molecules, their stated purity, and supplier are given in Table I.

The term ESD describes the desorption of charged or neutral atomic and molecular species from materials during electron irradiation. “Prompt” desorption refers to the ejection into vacuum of charged and neutral species via processes such as molecular dissociation or reactive-ion molecule scattering^11,12,25,26 from “fresh” sample films. After extended periods of electron irradiation, the ESD signals may change as the original molecular constituents of the film are destroyed and new molecules are synthesised.^11,12,27,28 In the present experiments, a quadrupole mass spectrometer (QMS) is used to monitor the yields of desorbed cations and anions.

In a typical ESD experiment, the Kimball Physics ELG-2 electron source was focused into a spot of ∼50 mm². Desorbed ions were detected with an Extrel QMS mounted normal to the sample surface at a distance of ∼1.5 cm. Custom made ion optics positioned at the entrance of the QMS increase the ion detection efficiency.

XPS is a standard surface science technique in which the yields and energies of x-ray photoelectrons ejected from the core levels of surface atoms are measured to provide information on their chemical states.²⁹ The kinetic energy of photoelectrons, ejected under irradiation with X-rays of known wavelength, is measured to determine their initial atomic binding energy. This latter is modulated by the number and nature of the chemical bonds shared by the original atom. The short mean-free path of photoelectrons limits the depth of sample films probed by XPS to a few nanometers.³⁰ 

In this study, XPS measurements were performed following the periods of electron irradiation to assess the changes in the chemical state of O and C atoms within the film. Samples were electron irradiated using the ELG-2 gun in the raster mode. XPS measurements were performed with a standard non-monochromatized Al kα source (1486.6 eV) operated at a power of 350 W and incident on the sample at an angle 45° with respect to the surface normal. This X-ray source, operating under identical conditions, was also used as an alternative radiation field in other experiments to assess sample degradation during XPS measurements and to simulate the effects of higher energy primary radiation. The photoelectron energy analyzer was positioned along the sample normal. The spectra were analyzed using the XPSpeak data analysis freeware³¹ that provides background subtraction and peak-fitting with Gaussian-Lorentzian line shapes.

TPD uses mass spectrometry to measure the yields of neutral molecular species desorbed from sample films as the substrate temperature is increased.³² Molecular species desorbing from the film exhibit distinctive sublimation temperatures which are characteristic of the type and strength of their interactions within the film and with the surface. This property together with the measurement via the quadrupole mass spectrometer of characteristic mass spectra (or “cracking patterns”) permits the identification of reaction products in irradiated molecular solids.^12,33,34

Here, TPD measurements were performed using the same QMS employed for the ESD experiments, which includes a low profile, high transmission electron impact ionizer that permits the detection of neutral species as previously described.³⁵ It is noted that during ESD experiments, the electron filament in this low profile ionizer is turned off. TPD experiments proceeded by irradiating sample films at 22 K using the electron gun in the raster mode. Subsequently, the cryostat was stopped and the substrate temperature was raised to 220 K by resistive heating at 25 W. During this time, the thermal desorption of molecular species from the film was monitored with the QMS with the ionizer turned on. A bias voltage was applied to the sample to repel the 70 eV electrons used in the ionizer of the QMS. The TPD data were analyzed to identify plausible irradiation products. Whenever possible, the TPD measurements were performed on the unirradiated films containing specific candidate products to assist our identification of products generated in our films. Some TPD measurements were obtained with a modified system. In brief, these modifications included (i) temperature measurement with a chromel-alumel thermocouple spot-welded directly to the edge of the Pt foil; (ii) the direct resistive heating of the substrate with DC currents of up to 7A delivered from a floatable power supply (Kepco ATE 25-10M); and (iii) the control of the heating process to achieve a linear heating rate of 10 K/min via a custom LabVIEW program.

In this section, we present the ESD measurements showing the evolution of new anion desorption signals, which demonstrate that in the combined presence of O₂ and CH₄, electron irradiation leads to the synthesis and gradual accumulation of new molecular species. Figure 1 shows two regions from the mass spectra of anions desorbed under electron impact from films of (a) CH₄, (b) O₂, and (c) a 1:3 mixture of ¹⁸O₂/CH₄, respectively. Here, the ESD spectra were measured at 70 eV incident electron energy after the films were pre-irradiated with a fluence of ∼1.05 × 10¹⁵ electrons cm⁻² (also at 70 eV). At such energies, the anion signals represent molecular fragments generated by (1) primary 70 eV electrons, through the dipolar dissociation (DD) process,³⁶ which simultaneously produces both an anionic fragment and a cationic fragment or (2) lower energy secondary electrons, again via DD or by dissociative electron attachment (DEA), which leads to the formation and desorption of an anionic and neutral fragment(s).³⁶ The chosen incident energy is comparable to that of 60 eV used in our earlier study of the ESD of cations via LEE-induced ion-molecule reactions;¹⁴ no additional LEE-driven processes are expected at the slightly higher energy since these must proceed via identical dissociation mechanisms and/or by the creation of secondary electrons of similar lower energies. However desorption yields were observed to be slightly higher at 70 eV than at 60 eV (i.e., by ∼10%).

In LEE irradiated films of pure CH₄ [Fig. 1(a)], the only anion signals observed below 30 amu are C⁻, CH⁻, and CH₂⁻. These signals are observed to increase only slightly with increasing fluence. The yields of anions from ¹⁸O₂ films [Fig. 1(b)] are dominated by ¹⁸O⁻ (not shown); though at high magnification, weak signals associated with other isotopes of oxygen are observed (i.e., ¹⁶O) as well as ¹⁸O₂⁻ ions at 36 amu and ¹⁸O¹⁶O⁻ at 34 amu, which are likely the result of reactive scattering.³⁷ Some C⁻ signal from vacuum contamination, possibly CO, is also apparent [Fig. 1(b)]. Interestingly, during extended electron irradiation of the ¹⁸O₂/CH₄ mixture [Fig. 1(c)] the yields of C⁻, CH⁻, and CH₂⁻ increase steadily with fluence and new peaks gradually appear at higher mass. These new anion species are absent entirely from pure CH₄ and pure ¹⁸O₂ films and are assigned as C₂⁻, C₂H⁻, and C₂H₂⁻.

Figure 2 shows in greater detail how the desorption yields of such anions (i.e., CH⁻, CH₂⁻, ¹⁸O⁻, C₂⁻, and C₂H⁻) from a 20 ML thick ¹⁸O₂:CH₄ (1:3) ice film vary with electron fluence (corresponding to increasing irradiation time at a constant flux of 5 × 10¹¹ electrons s⁻¹ cm⁻²). In all cases, complex behaviors are observed during extended irradiation. For most fragments, electron impact induces an immediate, transient decrease in the anion desorption signal. This is seen more clearly in signals that can be linked to unreacted O₂ (i.e., ¹⁸O⁻) and CH₄ (i.e., CH⁻ and CH₂⁻). The anion signals do not decrease in a simple exponential fashion that would correspond to the simple removal or destruction of the source molecules from the film.³⁸ Rather, the fluence-response curves display distinctive structure.

For example, maxima are observed in the fluence-response curves for CH₂⁻ at fluence close to ∼0.6 × 10¹⁵ electrons cm⁻² and in that of ¹⁸O⁻, at 0.4 × 10¹⁵ electrons cm⁻². No maximum is present in the yield of CH⁻_, though subtle structure can be discerned. The yields of C₂-containing species, specifically C₂H⁻ and C₂⁻, reach maxima at longer fluence; i.e., at ∼1 × 10¹⁵ electrons cm⁻² for C₂H⁻ and at ∼2.5 × 10¹⁵ electrons cm⁻² for C₂⁻. We also observed a similar behavior for the desorption of C₂H₂⁻, which as shown in Fig. 1 is clearly a desorption product from electron-irradiated ¹⁸O₂/CH₄ films. Unfortunately this signal was superimposed on a background from CN-containing contaminants, generated in other experiments (performed between the measurements presented in Figs. 1 and 2) and so these data are not presented here.

Collectively, the data of Fig. 2 show increases in the ESD signal for the C₂⁻, C₂H⁻, ¹⁸O⁻, and CH⁻ anions at elevated fluence. This suggests that these yields contain contributions from the dissociation of “new” larger molecular species that are synthesised and gradually accumulate at the film surface by electron-induced reactions between CH₄ and ¹⁸O₂ (or their fragments), i.e., reactions which, according to Fig. 1(a), do not occur when oxygen is absent from the target film. Three regions of distinct chemistry can be identified in Fig. 2. At lowest fluence <∼0.6 × 10¹⁵ electrons cm⁻², the fluence-response curves are dominated by the loss of unreacted ¹⁸O₂ and CH₄, as evidenced by the rapid decrease in the yields of ¹⁸O⁻, CH⁻, and CH₂⁻. Furthermore, initial reactions between ¹⁸O₂ and CH₄ fragment species produce new species for which the electron impact cross sections for desorption of the latter fragments can be larger giving rise to the maxima between ∼0.5 × 10¹⁵ and ∼2.5 × 10¹⁵ electrons cm⁻². Assuming that desorbed anions are the result of molecular fragmentation, kinetic energy considerations indicate that the desorption of anions containing two C atoms requires that the newly synthesised parent species in the film contain at least 2 C atoms plus an additional C and/or O atom(s). This is because to desorb from a film, an anion must have sufficient kinetic energy to overcome surface induced polarisation,³⁹ which typically is of the order of 0.5–1 eV. When a molecule undergoes binary dissociation, available kinetic energy is distributed in inverse proportion to the reduced mass of the fragment species;⁴⁰ the lighter the fragment relative to the molecule’s initial mass, the greater the kinetic energy it receives. Thus, the observation of fragments containing two C-atoms having sufficient KE to desorb, is strong evidence that more massive species have been created in the film. Unsurprisingly, such fragments begin to appear at slightly higher fluence after the creation of the first C—C and C—O containing species.

Since the ESD signals of C₂⁻ and C₂H⁻ are only observed from samples containing both CH₄ and O₂, it is reasonable to assume that the LEE-synthesized molecules from which they originate contain O atoms. Why then is there no increase (or modulation) in the yields of O containing fragments, other than O⁻, observed? The reason is almost certainly linked to the negative electron affinities (EA) of CO (−1.37 eV)⁴¹ and CO₂ (vertical EA of −3.8 eV and adiabatic EA of −0.6 eV),⁴² which mitigate against the formation of such fragments during molecular dissociation via DEA or DD. Generally, the inclusion of high electron affinity O atoms within an organic molecule increases the possibility that a negatively charged fragment may be formed upon dissociation. For example, while DEA to hydrocarbon molecules producing H⁻ and C_xH_y⁻ anions is observed to occur exclusively via high-lying Feshbach resonances,^43,44 the addition of an O atom, to form an alcohol or ether, modifies the electronic structure and permits DEA at lower electron energies via both shape and Feshbach resonances.^45,46 While the dominant anionic fragments from such resonances usually contain the O atom, stabilization of the excess electron on other fragment species should be possible if they possess a positive electron affinity (i.e., the case of H⁻ from DEA to various alcohols⁴⁵). Our previous measurements on the prompt desorption of positively charged species from O₂/CH₄ mixtures unambiguously demonstrated the formation of molecular products containing C and O.¹⁴ 

Additional ESD experiments, similar to those of Fig. 2, were performed on 20 ML thick films at ¹⁸O₂ concentrations of 0%, 15%, 25%, 50%, and 100% (Fig. 3). The maximum ESD yields and the fluences at which they are observed depend strongly on the percentage concentration of O₂. The variation of the maximum ESD yield with O₂ concentration is shown explicitly in Fig. 3. The largest ESD yields were observed at O₂ concentrations between 25% and 50% corresponding to ¹⁸O₂:CH₄ composition ratios of between 1:3 and 1:1, the former value being used throughout our experiments.

Our anion ESD results indicate that new, more complex molecules are synthesized in the films by electron impact and accumulate and remain in the films, at least at their surfaces. However, the molecular anions we see desorbing at high fluence are clearly electron impact induced fragments of even larger molecules, which are formed by electron-induced ion/radical reactions with surrounding molecules. Thus, further information on the new chemical species that accumulate in the films, or at their surfaces, can be obtained via changes in the kinetic energy spectra of photoelectrons that are ejected during soft X-ray irradiation. In the present section, we present results of such XPS measurements, which clearly show the formation of new chemical bonds involving either C—O or C=O bonds.

The evolution of the O1s XPS signal from a 25 ML thick film of an ¹⁸O₂/CH₄ (1:3) mixture film held at 22 K during film exposure to 70 eV electrons (∼5 × 10¹¹ electrons s⁻¹ cm⁻²) is shown in Fig. 4. The indicated t_ir refers to the electron irradiation time for a fresh film prior to the start of the 15 min XPS data acquisition of the O1S spectrum. The latter can be resolved into two peaks, P1 (539.55 eV) and P2 (538.40 eV), consistent with unreacted O₂ which, due to its paramagnetic nature, presents a “doublet” structure (here unresolved) in its physisorbed state.⁴⁷ A separate film was prepared for each electron irradiation experiment in Fig. 4; each curve in Fig. 4 is thus representative of a film subjected to a single analytic XPS scan following electron irradiation at the indicated t_ir.

As seen in Fig. 4, electron irradiation leads to a broadening of the overall O1s feature to lower binding energies and to a new structure at even lower binding energies. Under electron irradiation, the intensity of the P1 and P2 structures decreases and new signals at binding energies of 535.81 eV (P3) and 533.65 eV (P4) increase. These new peaks are readily apparent after only 30 min of electron irradiation. The intensities of these peaks are plotted as a function of t_ir in Fig. 4(b). The evolution of the O1s XPS signal from a similar film, during 12 h of continuous exposure to X-rays (generated under similar operating conditions as during the analytic XPS measurements of Fig. 4), was also monitored. These results are presented as part of Fig. SM1 of the supplementary material. Very similar behaviors, i.e., the loss of P1 and P2, and the appearance of P3 and P4, are observed under both electron and X-ray irradiation. However ∼12 h of X-ray irradiation were required to achieve the same effect as 60 min of electron bombardment. While no definitive statement concerning the relative cross sections for x-ray or electron-induced damage can be made, such data clearly indicate that X-ray induced damage during (the much shorter 15 min) analytic XPS measurements of electron-irradiated films should not be expected to introduce significant additional damage.

The total O1s intensity (P1, P2, P3, P4) is indicative of the total amount of oxygen left in the films, and as seen in Fig. 4(b), decreases quickly at first and then more slowly with electron exposure. This initial loss of total O1s signal is likely due to desorption of O-containing fragments, as seen in the prompt ESD of new and more complex O containing cation species desorbing from such O₂/CH₄ ices,¹⁴ e.g., H₃O⁺ or H₃CO⁺. However, as discussed in Sec. III A, in the anion ESD, we do not see any ESD signal of CO⁻ or CO₂⁻ growing from similar O₂/CH₄ films due to the negative electron affinity of these fragments, while the O⁻ ESD signal from such mixed films or even pure O₂ films only decreases during electron impact. We also note that under electron impact the new (P3+P4) signal rises very rapidly, qualitatively matching the exponential decay of the associated (P1+P2) signal.

To tentatively identify possible reaction products formed within our electron-irradiated films, we have investigated and compared photoelectron spectra from irradiated mixtures with spectra from pure films of unirradiated candidate molecules. Figure 5 compares the C1s XPS signal from an unirradiated mixture film (25 ML, 1:3 ¹⁸O₂:CH₄) with that from an electron-irradiated sample (3.6 × 10¹⁵ electrons cm⁻²) and those from the films composed of candidate products or related molecules containing relevant oxygen and carbon functional groups. Figure 6 presents analogous data for the O1s line. In both Figs. 5 and 6, certain spectra have been shifted slightly to lower (or higher) binding energy, to compensate for charge build-up in the film and/or changes in work function due to electron irradiation, and thus bring features attributable to unreacted bonds into alignment and better indicate probable chemistry for the new structures observed.

In the uppermost panel of Fig. 5, the C1s spectrum from an unirradiated film consists of a single structure at a binding energy of 285.5 eV (Peak R₁) that is attributable to photoemission from methane’s carbon atom. Upon electron irradiation [Fig. 5(b)], this C1s feature is diminished in intensity and significantly broadened, particularly towards higher binding energy. A second structure (R₂) is apparent at a binding energy of 287 eV, while a third, weak structure (R₃) can perhaps be discerned near 289.6 eV. Note that the C1s spectrum for the electron-irradiated CH₄/O₂ mixture has been shifted to higher binding energy (ΔE = +0.6 eV) to compensate for charge build-up in the film and/or changes in work function. In doing so, the dominant structure in the C1s spectrum becomes aligned with the one seen from the unirradiated film, with which it is clearly related. The curves in Figs. 5(f)–5(h) have been shifted to lower binding energy for similar reasons.

The C1s spectra for unirradiated and irradiated CH₄/O₂ mixture films are compared with those from molecular solid films containing C—C, C=C, and C≡C functional groups, viz., ethane (C₂H₆), propene (C₃H₆), and acetylene (C₂H₂), respectively [Figs. 5(c)–5(e)]. Only minor differences in the energy and width of C1s features are observed between the unirradiated methane/O₂ mixture and each of these samples. In particular, we note that the C1s feature for propene is essentially identical to that for methane (and ethane) despite containing contributions from both C—C/H and C=C functional groups. These similarities indicate that the energy resolution of the present XPS system is effectively unable to differentiate between these functional groups and thus cannot provide information on the prevalence of unsaturated carbon bonds in the irradiated films. It is possible then that the R₁ feature in the irradiated film may contain contributions from saturated and unsaturated C atoms in newly synthesised molecules, as well as from C atoms in unreacted methane molecules.

The XPS C1s spectra of ethanol [C₂H₅OH—Fig. 5(f)], acetaldehyde [CH₃CHO—Fig. 5(g)], and ethanoic acid [CH₃COOH—Fig. 5(h)] contain contributions from C—C and, respectively, C—O and C=O, and COOH functional groups. Two distinct C1s structures are resolved in spectra for acetaldehyde and ethanoic acid, while in ethanol, a broad structure is observed that is readily de-convolved into two peaks, separated by a binding energy of ∼1.5 eV. In Fig. 5, the C1s spectra from the ethanol, acetaldehyde, and acetic acid films have been shifted to lower binding energy by 0.8 eV, 1 eV, and 1.8 eV, respectively, to bring the lower binding energy C1s feature into alignment with the peak R₁ feature seen in the spectra of unirradiated methane and ethane. When comparing spectra of Figs. 5(f)–5(h) with that from the irradiated sample, it is apparent that the R₂ feature observed in the irradiated mixture film corresponds to electron emission from C—O groups, while the energy of the very weak R₃ feature correlates well with electron emission from a COOH group.

In Fig. 6, we see that the effect of electron irradiation on the CH₄/O₂ mixture [Fig. 6(b)] is to degrade the initial O1s signal, attributable to unreacted O₂ [Fig. 6(a)], and generate a broad structure at significantly lower binding energy, which consists of at least two new lines (P₃ and P₄). Note that the O1s spectrum for the irradiated sample has been shifted to higher binding energy by 0.5 eV to bring the P1/P2 feature, attributable to unreacted O₂, into alignment with that of the unirradiated film. The most intense new feature (P₄) appears at a binding energy similar to those measured for C—O and C=O functional groups in ethanol and acetaldehyde [Figs. 6(c) and 6(d)].

The O1s spectrum for ethanoic acid [Fig. 6(e)] can be resolved into two components separated by ∼1.6 eV corresponding to emission from oxygen atoms in the O—C=O and O—C=O positions. A similar separation in energy was observed between the same O1s features in earlier experiments.⁴⁸ From a comparison between the spectra of Fig. 6, we see that the binding energy of peak P3 (535.1 eV) in Fig. 6(b) is in good agreement with the higher-lying binding energy of these two features, so it appears likely that this feature in the irradiated film is related to emission from O—C=O atoms. The O1S data are thus consistent with the electron-induced formation of C—O/C=O and COOH functional groups.

TPD was performed to identify, in situ, the electron-induced formation of more complex molecular species. In initial TPD measurements, multiple mass spectra (covering the mass range from 2 to 90 amu) were recorded as the sample films (both irradiated and unirradiated, of pure or mixture composition) were slowly warmed from 22 K to 220 K, to identify key TPD signatures of specific fragments in these thermal desorption mass spectra that differed significantly upon irradiation. The time required to record each mass spectrum was ∼70 s. Subsequently, with new sample films, the TPD measurements on electron-irradiated films were repeated while monitoring continuously the variation of these specific fragments (up to a maximum of 5 different fragment masses simultaneously) with increasing temperature of the substrate.

Figure 7 shows TPD experiments performed on irradiated films of pure methane, which should be compared with TPD results from a subset of candidate molecules (acetylene labeled A, ethane labeled B, and propylene labeled C) in Fig. 8. A full list of candidate molecules studied was given in Table I. In Fig. 7, we see TPD data for fragments of 25, 26, 27, 29, and 41 amu, which were obtained from an unirradiated 10 ML film of pure CH₄ and from a similar film that has received a fluence of 3 × 10¹⁵ electrons cm⁻². The present experimental system is not optimized for TPD, and molecules can desorb not just from the irradiated target (sample surface) region but also from some of the cryostat’s surface. For this reason, the TPD spectra do not show highly resolved sharp desorption peaks. Moreover, the rapid desorption of adsorbed molecules from the surface of the cryostat as it is initially heated, generates a sudden and transient rise in chamber pressure that masks the true TPD signal from the sample at temperatures below ∼60 K. Despite these minor shortcomings, which are the same for both irradiated and unirradiated films, considerable differences are observed between the TPD data recorded for unirradiated and irradiated films and are readily apparent.

The new structures observed in the TPD spectra from the irradiated film can be compared to the data of Fig. 8, which in some cases allows their identification. Features labeled A, B, and C, for example, appear to have analogs in spectra from acetylene, ethane, and propylene, respectively, which will be discussed below. Structures marked with an asterisk are presently unassigned.

It is recalled that Table I in the experimental section lists the candidate products, and their purities, for which we performed the TPD measurements on pristine films to compare with the results obtained from irradiated films of both pure CH₄ and ¹⁸O₂/CH₄ mixtures. The electron impact mass spectra for the candidate molecules were obtained from the National Institute of Standard and Technology (NIST) database.⁴⁹ 

The candidate product films of Fig. 8 were all of thicknesses <1 ML to better model the interactions of sub-monolayer quantities of products on the Pt substrate. This is relevant since we do not expect high yields of these products such that their TPD will largely reflect their interaction with the Pt substrate, rather than other products or the unreacted molecules in the film, which will have desorbed at lower temperature. Assignments of structures A, B, and C were made from comparisons between Figs. 7 and 8 and the NIST electron impact mass spectra of the candidate molecules.⁴⁹ While structures A and B are observed at effectively the same temperature in Fig. 7, we do not believe that they derive from a single unique product since with our candidate films we did not observe structures with the appropriate relative intensities across all four of the relevant amu channels (25, 26, 27, and 29). Instead we tentatively attribute structure A to the thermal desorption of acetylene and structure B to the thermal desorption of ethane. Both molecules exhibit desorption structures at ∼85 K, predominately in channels 25 amu and 26 for acetylene, and 27 and 29 for ethane.

Figure 9(a) shows on a log y-axis, TPD signals of selected fragment masses (29, 31, and 45) from an irradiated film of an ¹⁶O₂/CH₄ mixture (1:3, 10 ML thick, fluence of ∼3 × 10¹⁵ electrons cm⁻²), TPD results for the same masses from an unirradiated film of 1 ML of ethanol (C₂H₅¹⁶OH) on Pt [Fig 9(b)], and a film comprising of 0.5 ML of ethanol deposited on a 10 ML of unirradiated ¹⁶O₂/CH₄ [Fig. 9(c)]. Note that in these experiments, the more readily available ¹⁶O₂ has been used to replace ¹⁸O₂ and that the TPD measurements were obtained following modifications that included better temperature control and a more linear heating rate. Figures 9(d)–9(f) show on a linear y-axis scale the results for mass 31 (the strongest fragmentation channel for C₂H₅OH). A doublet structure seen in data from irradiated films at 152 and 166 K (and indicated by dashed vertical lines) is seen to coincide with a dominant structure in results for pure ethanol and ethanol spiked O₂/CH₄ mixtures. The relative intensities of the two structures vary between the pure ethanol and ethanol spiked films, indicating that the lower temperature structure may correspond to multilayer ethanol desorption, but are in impressive agreement with the irradiated films. These data are strong evidence that ethanol formation is indeed one important product when the mixed CH₄/O₂ films are irradiated with electrons. To identify the origins of the stared structures in Fig. 9, we referred to all the candidate molecules of Table I, including methanol, acetaldehyde, acetic acid, dimethyl ether, and formaldehyde, but found no correspondence.

Figure 10(a) shows the variation with electron fluence of the TPD yield for mass 33 amu associated with structures between 152 and 166 K from irradiated and unirradiated films of ¹⁸O₂/CH₄. Thus the 33 amu fragment in this film is the same oxygen containing fragment as the 31 amu fragment in the ¹⁶O₂/CH₄ films of Fig. 9, which shows no 33 amu fragment in the mass spectra for obvious reasons. These data, which are indicative of the formation of C₂H₅OH within the films, are compared to the ESD data for the C₂H⁻ fragment [Fig. 10(b)] from similar films. The similarities between these data support our hypothesis that the increase in ESD of this anion is related to increased electron attachment to O-containing molecules, such as ethanol, formed in the mixture films during electron irradiation. Moreover, the ESD data, which were obtained at 20 K, indicate that reactions between C and O atoms do not require thermal activation (e.g., by warming the film). This observation is also consistent with the XPS data presented in Figs. 3–6.

To better understand the basic radiation-driven reactions that may occur within irradiated astrophysical ices (ISM icy grains or planetary ices), we have investigated the chemistry induced by low-energy electrons and X-rays (i.e., X-ray photoelectrons) inside simple molecular ices containing CH₄ and O₂.

Unlike our earlier study¹⁴ in which film chemistry was investigated by prompt cation ESD alone, here we have used anion ESD to monitor the changes that occur at elevated fluence, e.g., prolonged irradiation times, and have sought complementary information on new products that remain in the icy films, via post-irradiation analysis using XPS and TPD. Within irradiated films containing both CH₄ and O₂, the ESD signals of anions have revealed the slow accumulation of larger species with C—C bonds, while XPS reveals the formation of C—O or C=O bonds. Comparisons between the TPD spectra of irradiated and unirradiated films show the electron-induced formation of new chemical species, the identities of which are confirmed by reference to the NIST database of electron impact mass spectra and by TPD measurements of films composed of the proposed products.

Comparisons with TPD results for pure films of propylene, ethane, and acetylene demonstrate that each of these molecules are formed in irradiated pure methane films consistent with earlier studies under high-energy electron impact, vacuum ultra-violet, and other ionizing radiations (Refs. 50–53 and references therein). TPD of irradiated and unirradiated 10 ML thick films of ¹⁸O₂/CH₄ and ¹⁶O₂/CH₄ (1:3) samples have also been acquired and compared with results for pure films of methanol, ethanol, acetaldehyde, acetic acid, dimethyl ether, and formaldehyde. To date, such measurements have only allowed the clear identification of ethanol in the irradiated samples; however, it is clear that other, as yet unidentified, complex molecular species are being formed by LEE bombardment.

This conclusion is supported by the evolution of C1s and O1s XPS features from a 25 ML thick films of CH₄/¹⁸O₂ (3:1) mixtures during ∼2 h of exposure to LEE and ∼12 h of exposure to X-rays, which demonstrates the breaking of the O—O and C—H bonds during irradiation and the formation of new C—O, C=O, and COO— bonds.

The detailed reaction pathways by which LEE mediates the observed reactions are still unclear. We hope that further measurements as a function of incident electron energy will help identify the role of transient negative ions (i.e., DEA), DD, and/or cation catalysed reactions.¹¹ Nevertheless, the impressive similarity between LEE irradiation and X-ray irradiation, seen in our XPS results, strongly supports the notion that it is the secondary LEE produced by ionizing radiation that drives the chemistry irrespective of the irradiation type. Techniques such as Reflection Absorption Infrared Spectroscopy (RAIRS) and High Resolution Electron Loss Spectroscopy (HREELS) may also provide further information in the future.¹¹ Indeed very recent work by Kundu and co-workers has also investigated LEE-induced reactions in condensed CH₄/O₂ mixtures and have observed with RAIRS and TPD the formation of ethane, methanol, and formaldehyde, though not of ethanol.⁵⁴ 

In conclusion, our results support the hypothesis that interactions of low-energy secondary electrons, radicals, and ions, formed initially during the radiolysis of matter, with atoms and molecules in the medium, may have played and may still play an important role in the chemical transformation of astrophysical and, more importantly, planetary surface ices, where they lead to the synthesis of more complex chemical species from less complex, naturally occurring components. This radiation-driven molecular synthesis may indeed represent a driving force in the original biogenesis of the molecular building blocks of life in our own solar system and, due to the ubiquitous nature of matter and radiation, may represent a key element in molecular biogenesis throughout the universe.

See supplementary material for the evolution of the O1s XPS signal from a 25 ML thick film of an ¹⁸O₂/CH₄ (1:3) mixture film held at 22 K, during 12 h of continuous exposure to X-rays.

This work has been funded by the Natural Sciences and Engineering Research Council of Canada (NSERC).