Sticking of HCl to ice at hyperthermal energies: Dependence on incidence energy, incidence angle, and surface temperature

The Journal of Chemical Physics

2004

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We present results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of crystalline ice Ih and to the surface of amorphous ice Ia. The calculations were performed for normal incidence at a surface temperature Ts = 90 K for ice Ia, and at Ts = 90 and 150 K for ice Ih. For both surfaces, the sticking probability can be fitted to a simple exponentially decaying function of the incidence energy, Ei: Ps = 1.0e(-Ei(kJ/mol)/90(kJ/mol)) at Ts = 90 K. The energy transfer from the impinging molecule to the crystalline and the amorphous surface is found to be quite efficient, in agreement with the results of molecular beam experiments on the scattering of the similar molecule, N2, from crystalline and amorphous ice. However, the energy transfer is less efficient for amorphous than for crystalline ice. Our calculations predict that the sticking probability decreases with Ts for CO scattering from crystalline ice, as the energy transfer from the impinging molecule to the warmer surfaces becomes less efficient. At high Ei (up to 193 kJ/mol), no surface penetration occurs in the case of crystalline ice. However, for CO colliding with the amorphous surface, a penetrating trajectory was observed to occur into a large water pore. The molecular dynamics calculations predict that the average potential energy of CO adsorbed to ice Ih is -10.1 +/- 0.2 and -8.4 +/- 0.2 kJ/mol for CO adsorbed to ice Ia. These values are in agreement with previous experimental and theoretical data. The distribution of the potential energy of CO adsorbed to ice Ia was found to be wider (with a standard deviation sigma of 2.4 kJ/mol) than that of CO interacting with ice Ih (sigma = 2.0 kJ/mol). In collisions with ice Ia, the CO molecules scatter at larger angles and over a wider distribution of angles than in collisions with ice Ih.

Section: Introductionsupporting

confidence: 83%

Sticking of CO to crystalline and amorphous ice surfaces

Al-Halabi

Dishoeck

The Journal of Chemical Physics

2004

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“…Essentially, we followed the same approach as used previously to study the adsorption of HCl and CO to crystalline and amorphous ice at thermal and hyperthermal energies, where E i varies between 0.1 and 2.0 eV (Kroes & Clary 1992;Al-Halabi et al 2001.…”

Section: Methodsmentioning

confidence: 99%

Adsorption of CO on amorphous water-ice surfaces

Al-Halabi

Fraser

et al. 2004

A&A

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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.

“…The H atom photodesorption probabilities are larger than those for D in the H 2 O and D 2 O amorphous ice systems ( 39,40 states that the energy transfer from the X atom to a water molecule (which is assumed to be stationary initially) is dependent on the mass ratio of the two colliding objects, as follows:…”

Section: A X(=hd) Atom Photodesorptionmentioning

confidence: 99%

Isotope effects on the photodesorption processes of X2O (X = H,D) and HOD ice

Koning

The Journal of Chemical Physics

Arasa

2013

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To investigate the isotope effects on the photodesorption processes of X 2 O (X = H,D) ice, molecular dynamics calculations have been performed on the ultraviolet photodissociation of an H 2 O or a D 2 O molecule in an H 2 O or a D 2 O amorphous ice surface, and on HOD photodissociation in an H 2 O amorphous ice surface, where the photodissociated molecules were located in the top four or five monolayers at ice temperatures of 10, 20, 30, 60, and 90 K. Three photodesorption processes can occur upon X 2 O photodissociation: X atom photodesorption, OX radical photodesorption, and X 2 O (or HOD) molecule photodesorption. X 2 O (or HOD) photodesorption can occur after recombination of X and OX, or after an energetic X atom photofragment kicks a surrounding X 2 O molecule from the ice surface. Isotope effects are observed for the X atom and the OX radical photodesorption as well as for the kick-out photodesorption. However, no isotope effects were noticeable for the photodesorption of recombined X 2 O molecules. The average D atom photodesorption probabilities are about a factor 0.9 smaller than those for the H atom, regardless of the isotope of the surrounding ice system. Also, the kick-out mechanism is more likely to occur if a D photofragment is created upon dissociation than if an H atom is created. These observations can be explained by more efficient energy transfer from the D atom to water molecules than from the H atom. Reasoning based on the X 2 O phonon frequencies associated with the librational modes and energy transfer efficiencies explain why the OX radical photodesorption probabilities are noticeably larger if the OX radical desorbs from a D 2 O ice system than from an H 2 O ice system. Also, the OX radical photodesorption is more probable upon dissociation of DOX (X = H,D) than upon dissociation of HOX (X = H,D), because the initial kinetic energy of the OX radical is larger if the dissociation products are D + OX than H + OX. The branching ratio of OD OH desorption following photodissociation of an HOD molecule in ice (about 1.0) is much lower than the OD OH branching ratio in gas-phase HOD photodissociation. This may lead to differences in isotope fractionation in OH(g) formation in dense and diffuse clouds in the interstellar medium.