We present the results of classical dynamics calculations performed to study the photodissociation of water in crystalline and amorphous ice surfaces at a surface temperature of 10 K. A modified form of a recently developed potential model for the photodissociation of a water molecule in ice [S. Andersson et al., Chem. Phys. Lett. 408, 415 (2005)] is used. Dissociation in the top six monolayers is considered. Desorption of H2O has a low probability (less than 0.5% yield per absorbed photon) for both types of ice. The final outcome strongly depends on the original position of the photodissociated molecule. For molecules in the first bilayer of crystalline ice and the corresponding layers in amorphous ice, desorption of H atoms dominates. In the second bilayer H atom desorption, trapping of the H and OH fragments in the ice, and recombination of H and OH are of roughly equal importance. Deeper into the ice H atom desorption becomes less important and trapping and recombination dominate. Motion of the photofragments is somewhat more restricted in amorphous ice. The distribution of distances traveled by H atoms in the ice peaks at 6–7 Å with a tail going to about 60 Å for both types of ice. The mobility of OH radicals is low within the ice with most probable distances traveled of 2 and 1 Å for crystalline and amorphous ices, respectively. OH is, however, quite mobile on top of the surface, where it has been found to travel more than 80 Å. Simulated absorption spectra of crystalline ice, amorphous ice, and liquid water are found to be in very good agreement with the experiments. The outcomes of photodissociation in crystalline and amorphous ices are overall similar, but with some intriguing differences in detail. The probability of H atoms desorbing is 40% higher from amorphous than from crystalline ice and the kinetic-energy distribution of the H atoms is on average 30% hotter for amorphous ice. In contrast, the probability of desorption of OH radicals from crystalline ice is much higher than that from amorphous ice.

We present new experimental and theoretical results for reactive scattering of dihydrogen from Cu(100). In the new experiments, the associative desorption of H 2 is studied in a velocity resolved and final rovibrational state selected manner, using time-of-flight techniques in combination with resonance-enhanced multi-photon ionization laser detection. Average desorption energies and rotational quadrupole alignment parameters were obtained in this way for a number of (v = 0, 1) rotational states, v being the vibrational quantum number. Results of quantum dynamics calculations based on a potential energy surface computed with a specific reaction parameter (SRP) density functional, which was derived earlier for dihydrogen interacting with Cu (111), are compared with the results of the new experiments and with the results of previous molecular beam experiments on sticking of H 2 and on rovibrationally elastic and inelastic scattering of H 2 and D 2 from Cu(100). The calculations use the Born-Oppenheimer and static surface approximations. With the functional derived semi-empirically for dihydrogen + Cu(111), a chemically accurate description is obtained of the molecular beam experiments on sticking of H 2 on Cu(100), and a highly accurate description is obtained of rovibrationally elastic and inelastic scattering of D 2 from Cu(100) and of the orientational dependence of the reaction of (v = 1, j = 2 − 4) H 2 on Cu(100). This suggests that a SRP density functional derived for H 2 interacting with a specific low index face of a metal will yield accurate results for H 2 reactively scattering from another low index face of the same metal, and that it may also yield accurate results for H 2 interacting with a defected (e.g., stepped) surface of that same metal, in a system of catalytic interest. However, the description that was obtained of the average desorption energies, of rovibrationally elastic and inelastic scattering of H 2 from Cu(100), and of the orientational dependence of reaction of (v = 0, j = 3 − 5, 8) H 2 on Cu(100) compares less well with the available experiments. More research is needed to establish whether more accurate SRP-density functional theory dynamics results can be obtained for these observables if surface atom motion is added to the dynamical model. The experimentally and theoretically found dependence of the rotational quadrupole alignment parameter on the rotational quantum number provides evidence for rotational enhancement of reaction at low translational energies.

The influence of surface morphology/porosity on the desorption kinetics of weakly bound species was investigated by depositing D2 on amorphous solid water (ASW) films grown by low temperature vapor deposition under various conditions and with differing thermal histories. A broad distribution of binding energies of the D2 monolayer on nonporous and porous ASW was measured experimentally and correlated by theoretical calculations to differences in the degree of coordination of the adsorbed H2 (D2) to H2O molecules in the ASW depending on the nature of the adsorption site, i.e., surface valleys vs surface peaks in a nanoscale rough film surface. For porous films, the effect of porosity on the desorption kinetics was observed to be a reduction in the desorption rate with film thickness and a change in peak shape. This can be partly explained by fast diffusion into the ASW pore structure via a simple one-dimensional diffusion model and by a change in binding energy statistics with increasing total effective surface area. Furthermore, the D2 desorption kinetics on thermally annealed ASW films were investigated. The main effect was seen to be a reduction in porosity and in the number of highly coordinated binding sites with anneal temperature due to ASW restructuring and pore collapse. These results contribute to the understanding of desorption from porous materials and to the development of correct models for desorption from and catalytic processes on dust grain surfaces in the interstellar medium.

Abstract. We present the results of classical trajectory calculations of the adsorption of thermal CO on the surface of compact amorphous water ice, with a view to understanding the processes governing the growth and destruction of icy mantles on dust grains in the interstellar medium and interpreting solid CO infrared spectra. The calculations are performed at normal incidence, for E i = 0.01 eV (116 K) and surface temperature T s = 90 K. The calculations predict high adsorption probabilities (∼1), with the adsorbed CO molecules having potential energies ranging from −0.15 to −0.04 eV with an average energy of −0.094 eV. In all the adsorbing trajectories, CO sits on top of the surface. No case of CO diffusion inside the ice or into a surface valley with restricted access was seen. Geometry minimizations suggest that the maximum potential energy of adsorbed CO (−0.155 eV) occurs when CO interacts with a "dangling OH" group, associated with the 2152 cm −1 band seen in laboratory solid-state CO spectra. We show that relatively few "dangling OH" groups are present on the amorphous ice surface, potentially explaining the absence of this feature in astronomical spectra. CO also interacts with "bonded OH" groups, which we associate with the 2139 cm −1 infrared feature of solid CO. Our results for CO adsorption on amorphous ice are compared with those previously obtained for CO adsorption to crystalline ice. The implications of the spectroscopic assignments are discussed in terms of the solid-CO infrared spectra observed in interstellar regions. Using the Frenkel model, the lifetime τ for which CO may remain adsorbed at the surface is calculated. At temperatures relevant to the interstellar medium, i.e. 10 K, it is longer than the age of the universe, but decreases dramatically with increasing T s , such that at T s = 90 K, τ = 300 ns. The pre-exponential factor τ ν used in the Frenkel model is found to be 0.95 ± 0.02 ps. These data are compared to recent experimental results. The astrophysical implications of these calculations are discussed, with particular reference to the CO binding sites identified on amorphous ice surfaces, their adsorption energies, probabilities and lifetimes.

Results of classical trajectory calculations on the adsorption of H atoms to amorphous solid water (ASW) ice, at a surface temperature T s of 10 K are presented. The calculations were performed for incidence energies E i ranging from 10 to 1000 K, at random incidence. The adsorption probability P s can be fitted to a simple decay function: P s = 1.0e −E i (K )/300 . Our calculations predict similar adsorption probabilities for H atoms to crystalline and ASW ice, although the average binding energy E b of the trapped H atoms calculated for ASW of 650 ± 10 K is higher than that found for crystalline ice of 400 ± 5 K. The binding energy distributions were fitted to Gaussian functions with full width half-maximum of 111 and 195 K for crystalline and amorphous ice surfaces, respectively. The variation of the H atom binding sites in the case of the ASW surface leads to broadening of the distribution of E b compared to that of crystalline ice. We have also calculated the 'hot-diffusion' distance travelled by the impinging atom over the surface before being thermalized, which is found to be about 30 Å long at E i = 100 K and increases with E i . The diffusion coefficient D of thermally trapped H atoms is calculated to be 1.09 ± 0.04 × 10 −5 cm 2 s −1 at T s = 10 K. The residence time τ of H atoms adsorbed on ASW is orders of magnitude longer than that of H atoms adsorbed on crystalline ice for the same ice T s , suggesting that H 2 formation on crystalline and non-porous ice is quite limited compared to that on porous ice. This is in good agreement with the results of experiments on H 2 formation on porous and non-porous ASW surfaces. At low T s , the long values of τ , the high values of D and the large hot distance travelled on the ASW surface before trapping the impinging H atom ensure that Langmuir-Hinshelwood and hot-atom mechanisms for H 2 formation will be effective. The data presented here will be important ingredients for models to describe the formation of H 2 on interstellar ices and reactions of H atoms with other species at the ice surface.

We present results of classical trajectory calculations on the sticking of hydrogen atoms to the basal plane (0001) face of crystalline ice, I h . The sticking probability is found to decrease with both incidence energy (E i ) and surface temperature (T s ). At the surface temperatures studied, the sticking probability can be fitted to a simple decay function: P s ) 1.5 e -E i (K)/175 at T s ) 10 K, and P s ) 0.85 e -E i (K)/175 at T s ) 70 K. In the trapped state, the adsorbed hydrogen atom is located on top of the ice surface, over the center of a surface hexagonal ring, interacting with all water molecules forming the ring. The calculated physisorption energy of the adsorbed atom is approximately 400 ( 50 K. The results of our calculations are compared with the experimental and theoretical data for amorphous ice surfaces. At T s ) 10 K, our values for the sticking probability are higher than those of Buch and Zhang [Buch, V.; Zhang, Q. Astrophys. J. 1991, 379, 647], which is attributed to differences in surface topology. Our sticking probability values are lower than those of Masuda et al. [Masuda, K.; Takahashi, J.; Mukai, T. Astron. Astrophys. 1998, 330, 243], which we attribute to an incorrect implementation of the H-H 2 O potential in their work. The experimental results available on hydrogen formation on amorphous ice are in good agreement with our results, if the assumption is made that all H-atoms that stick will recombine. Our calculations then suggest that the formation of H 2 through recombination of H-atoms adsorbed on the surface is efficient enough to compete with the cosmic destruction of H 2 .

We present the results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of ice I h , for normal and off-normal incidence at surface temperature (T s ) ) 150 K. The sticking probability decreases with the incidence energy (E i ) and with the incidence angle (θ i ) for θ i > 20°. At normal incidence, the sticking probability can be fitted to a simple decay function: P s ) 0.9 e -0.012E i (kJ/mol) . The energy transfer from the impinging molecule to the surface is found to be efficient and fast: most of E i is transferred to the surface within 0.5-1.0 ps. For off-normal incidence, the energy transfer becomes less efficient for θ i > 20°. In the case of backscattering at off-normal incidence, the hotter molecules scatter at larger angles. At high E i , no surface penetration occurs, but the impinging molecule may damage the surface significantly when it hits the surface in the center of a hexagonal ring. The energy minimization calculations suggest that CO is adsorbed either on top of an OH dangling group, or on top of the center of a water hexagonal ring, interacting mostly with an electron lone-pair oxygen atom in the first monolayer or with a four-coordinated oxygen in the second monolayer. The molecular dynamics calculations predict that the average binding energy of the adsorbed CO is 4.33 kJ/mol, with a maximum value of 10.4 kJ/mol. The results of our calculations are compared with the experimental and the previous theoretical data on the CO-ice and N 2ice systems. † Part of the special issue "Charles S. Parmenter Festschrift".

Articles you may be interested inElectron hole pair mediated vibrational excitation in CO scattering from Au(111): Incidence energy and surface temperature dependenceWe present calculations on the sticking of hyperthermal HCl to the basal plane ͑0001͒ face of ice I h at normal and off-normal incidence. The dependence of the sticking probability on the incidence energy (E i ), the angle of incidence ( i ), and the surface temperature (T s ) is discussed. Two sticking mechanisms are observed. For i р30°, penetration of the ͑0001͒ face is possible at an energy of about 100 kJ/mol, which is an order of magnitude lower than energies for which the penetration of metallic or covalently bonded crystals by atoms becomes possible. This possibility is due to the open structure of single-crystalline ice I h , in which the water molecules are arranged in superimposed hexagons, forming shafts running perpendicular to the ice surface. The penetration mechanism is operative for the entire range of T s studied ͑110-190 K͒. The second sticking mechanism, i.e., adsorption, occurs for all E i , i , and T i . For i Ͻ45°, the adsorption probability increases with i as would be expected, because the normal component of E i that needs to be transferred to the surface for sticking to occur scales with cos 2 i . However, for i у45°, the adsorption probability decreases with i . The energy transfer from HCl to the ice surface and the energy dissipation within the surface are found to be fast and efficient at normal incidence.