O/CO/H2O ices

Discussion

1. 
   Distance-dependent radiation chemistry
   

As discussed above, previous studies^{8a, 9} have reported the simultaneous radiation-induced hydrogenation (mainly to methanol) and oxidation (mainly to CO₂) of CO in CO/H₂O films. On the other hand, only hydrogenation products (HCO, CH₂O and CH₃OH) were observed in our previous study when CO is positioned deep enough (60  400 ML) from an ASW/vacuum interface that was exposed to 100 eV electrons.¹⁰ Here, we have explored smaller cap layer coverages (4 ML ≤ _Cap ≤ 60 ML) for the buried CO layer. For these smaller cap layer coverages, oxidation products such as CO₂ and formate are observed, and they become the dominant products for _Cap < 15 ML (Fig. 3). The fraction of methanol amongst the products decreases with decreasing _Cap and becomes negligible for _Cap < 10 ML (Fig. 3). These results show that placing CO at different positions within the ASW films allows one to map zones with very different radiation chemistry – preferential oxidation in the near-surface region and preferential hydrogenation further into the ASW films. Figure 6 illustrates these zones within the ASW film exposed to energetic electrons from the vacuum side.

Figure 6. Schematic illustration of the oxidation and hydrogenation “zones” within an ASW film that is irradiated with 100 eV electrons. Ionizations and electronic excitations of H₂O by the incident electrons generate oxidizing and reducing species in the region near the ASW/vacuum interface. A significant part of the primary products react, recombine or desorb into the vacuum. However some higher mobility products, such as hydrogen atoms or hydronium ions, can diffuse into the film, creating a zone of preferential hydrogenation for a deeply buried CO. Conversely, the region near the vacuum interface is enriched with oxidizing products, such as OH radicals, that are less mobile and less likely to desorb. CO layers placed in this region are preferentially oxidized. Thus by changing the location of the CO layer, one can tune the chemical environment that the CO encounters within the irradiated ASW films.

The various reactions occurring within irradiated ASW films suggest a plausible explanation for the different reactions zones depicted in Figure 6. In particular, H atom desorption from,²⁵ and diffusion into,¹⁰ electron-irradiated ASW films play a significant role in creating an oxidizing region near the surface of the film and a reducing region further in. When an energetic electron penetrates into the ASW film, it initiates a cascade of events. For 100 eV electrons, ionizations and excitations of water molecules are most likely within the first few water layers and the majority of these events occur within the first ~20 ML.²⁶ Electronically excited water molecules, formed by direct excitation or via electron – ion recombination, can dissociate leading to H atom desorption, while leaving behind an OH.²⁵ H atoms can also diffuse into the ASW films,¹⁰ again leaving behind an OH. As a result, the surface and near surface region of electron-irradiated ASW films becomes enriched with hydroxyls and other oxidizing species such as H₂O₂ and HO₂.^60,61 H₂²⁷ and O^{25a, 25c} also desorb during electron-irradiation but their yields are smaller, while H₂O desorption does not change the balance of oxidizing and reducing agents. Since the electron-stimulated desorption of O₂ proceeds through a sequence of reactions involving OH, H₂O₂ and HO₂ intermediates,^{11d, 11e, 28} it also provides evidence of an oxidizing environment near the water/vacuum interface. In addition to contributing to the creation of the oxidation zone, H atom diffusion into the ASW films creates the reducing zone within the film. As result, CO (or other molecules) deposited at different places within the ASW film can be oxidized or hydrogenated, as seen experimentally.

2. 
   Insights into mechanism of radiation-induced oxidation in CO/H₂O systems
   

The major mechanism of radiation-induced oxidation in CO/H₂O systems is associated with the reaction of OH radicals with CO: ^{9b, 9c, 9h, 21b, 23, 29}

(2)

The data presented here support this reaction. For example, CO₂ was produced at 20 K when CO was dosed onto pre-irradiated ASW films (Fig. 5a). The low temperature at which the reaction is observed suggests that the CO₂ is produced in a barrierless reaction between CO and one of the products of that remains on the ASW surface after electron irradiation. Previous studies of the O₂ production in electron-irradiated ASW films, indicated that OH, H₂O₂ and HO₂ are involved in a series of reactions leading to O₂.^{11d, 11e} Those studies showed that the concentration of OH was the largest for films irradiated at 20 K and decreased at higher temperatures due to increases in thermally-activated reactions involving OH (e.g. OH + OH  H₂O₂ and OH + H₂O₂  HO₂ + H₂O). In contrast, the concentration of H₂O₂ and HO₂ both are expected to increase at higher annealing or irradiation temperatures. Thus, the observed decrease in CO₂ production versus annealing temperature for pre-irradiated ASW films (Fig. 5b) closely matches the expected decrease in the OH concentration at the surface of the ASW films versus temperature,^11e and supports the reaction 2 as a pathway for CO₂ production. Because the reaction CO + O  CO₂ has a significant activation barrier,^{23c, 30} this reaction is unlikely to be important for the CO₂ produced by CO adsorption on pre-irradiated ASW films (Fig. 5). However, since electron-irradiation of water films produces energetic oxygen atoms,^{25a, 25c} this reaction could potentially play a role in experiments with irradiated co-adsorbed CO and ASW.

For _Cap < 35 ML, formate (HCO₂^-) and the unidentified peak near 1700 cm^-1 are observed in the irradiated films. There is not much information in literature on irradiated CO + H₂O ices regarding these peaks, and there may be several reasons for this. First, these peaks are relatively broad and they overlap with the intense band of the H₂O bending mode. Since the shape and intensity of the H₂O bending mode band changes under electron irradiation, it is difficult to observe these features unless one looks at the difference between IR spectra for irradiated films with and without buried CO. Second, most of our irradiations are performed at 100 K, which is the optimal temperature to maximize both of these peaks. Most of the previous studies were performed at significantly lower temperatures (e.g. 10 – 20 K), where these peaks are much smaller. Other parameters, such as the radiation type, fluence, etc. may also make these peaks harder to observe.

In our experiments, the reactions leading to the production of formate are uncertain. However, a potential reaction pathway starts with the reaction of CO and OH. This reaction involves the formation of an energized HOCO* radical (in either cis- or trans- forms) that can decompose into reactants, proceed to products, or become thermally stabilized through collisions with a third species:^22-23

CO + OH  HOCO*

CO₂ + H

HOCO

(3)

The HOCO radical is extremely reactive, producing CO₂ when reacting with OH, HO₂, O₂, or O, or producing CO when reacting with H radicals.²² Trans-HOCO can also isomerize to the formyloxyl radical, HCO₂, which is metastable with respect to dissociation into CO₂ + H:³¹

(4)

The formyloxyl radical, which has been studied in the gas-phase via photo-electron detachment from the formate anion (HCO₂^-), is also very reactive. It has a large electron affinity (3.5 eV) for converting to formate.^31a Thus if the formyloxyl radical forms in the CO + OH reaction, it could be efficiently converted to the formate anion by capturing an electron, which are readily available in the zone of primary ionizations and excitations (see Fig. 6):

(5)

The formyloxyl radical can also be created from CO₂ reacting with H atoms,^31b which could explain the formate peak in IR spectra of irradiated CO₂ on ASW (Fig. S1):

(6)

Unfortunately, we know of no studies presenting the IR spectra of formyloxyl radicals in ice or other matrices, and we do not have enough information to assign the 1700 cm^-1 peak in the IR spectra of CO or CO₂ irradiated in ASW to the formyloxyl radical or any other product. Obviously, more work needs to be done to assign this peak and understand the mechanism of its formation.

For CO layers within ~20 ML of the ASW/vacuum interface, the oxidation of CO to CO₂ is weakly temperature dependent (Fig. 4a), while for more deeply buried CO layers the hydrogenation of CO to methanol depends more strongly on the temperature (Fig. 4b). The weak temperature dependence for CO oxidation probably reflects several factors. First, the ionizations and excitations that lead to the production of OH do not depend strongly on temperature,^11e and since the CO + OH reaction can proceed even at low temperatures (Fig. 5), CO oxidation is relatively efficient even at 20 K. Second, non-thermal effects within the penetration range of the incident electrons – such as electron-stimulated diffusion of various reaction products^11d – may sustain the reactions at low temperatures where otherwise the low diffusion rates of the reactants would limit the reaction rate. In contrast, for more deeply buried CO layers where the hydrogenation of CO to methanol is the dominant reaction, the reactants are thermalized and the H + CO reaction has a barrier³² that introduces a stronger temperature dependence to the reaction.¹⁰ It is also likely that the slow diffusion of H atoms into the bulk at 20 K limits the reaction rate.³²

5. 
   Summary
   

We have investigated the electron-stimulated reactions of CO layers buried in ASW films with 4 ML ≤ _Cap ≤ 60 ML. Depending on _Cap, both oxidation and reduction of the CO can be observed with infrared spectroscopy. For _Cap < 15 ML, CO₂ is the main oxidation product, accompanied by formate anion (HCO₂^-). For _cap ~ 10 ML where the CO₂ yield is maximized, it accounts for approximately 45% of all converted CO. Hydrogenation (to methanol) becomes the most important reaction channel for _Cap > 15 ML, and above ~ 35 ML the CO - methanol conversion efficiency is close to 100 %. The observed distance – dependent radiation chemistry is associated with mobile hydrogen atoms diffusing deeper into the film and leaving behind a zone of preferential oxidation closer to the surface, while the less-mobile OH radicals remain in closer to the surface initiating the oxidation reactions.

Acknowledgements:

This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. The work was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830. RJM thanks the Dalton Cumbrian Facility program in part funded by the Nuclear Decommissioning Authority for financial support for her research visit to PNNL.

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