Photodissociation of a HCl molecule adsorbed on ice (original) (raw)
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The Journal of Chemical Physics, 2009
The 3D photodissociation dynamics of HCl and HF molecules adsorbed on ice is studied by quantum and classical simulations. The quantum calculations are carried out with the multiconfiguration time-dependent Hartree ͑MCTDH͒ approach. Dynamical observables like angular distributions in the momentum space of the H fragments, absorption cross sections are computed. The results are compared with our previous 2D studies. As expected, less encapsulation of the H atom between the ice surface and the halogen atom is obtained in the 3D study, resulting in less pronounced interference structures in the photoabsorption cross sections and in a decrease of the classical rainbow peaks observed in the 2D scheme. Although the amplitudes of the oscillations corresponding to quantum interferences in the asymptotic angular distribution of the H fragment are different between the 2D and 3D results, the qualitative pattern of the oscillations is similar in the 2D and 3D approaches. In addition, a good agreement is observed for the angular distribution between the classical and the quantum calculations.
Classical and quantum studies of the photodissociation of a HX (X=Cl,F) molecule adsorbed on ice
The Journal of Chemical Physics, 2007
The photodissociation dynamics of a HX ͑X=Cl,F͒ molecule adsorbed on a hexagonal ice surface at T = 0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures.
The depletion of stratospheric ozone at high latitudes is caused by gas-phase catalytic cycles involving active chlorine compounds produced through heterogeneous chemical reactions involving HCl. In this Letter, results aimed at understanding the activation of chlorine on ice cloud particle surfaces are presented. We focus speci®cally on HCl interacting with the ice surface prior to reaction. We conclude that, in regions with a low surface density of dangling OH groups, HCl may be either ionically or molecularly adsorbed, but in regions with a high surface density of dangling OH groups, HCl is ionically adsorbed. Ó
A theoretical study of hydrogen diffraction following photodissociation of adsorbed molecules
The Journal of Chemical Physics, 1992
A new probe of surface structure is presented which is based on the photodissociation of hydrogen from an adsorbate molecule. The event creates an atomic hydrogen fragment, positioned between the adsorbate layer and the solid surface. Due to its light mass, the hydrogen dynamics is quantum mechanical in nature. A useful image is of the hydrogenic wave function behaving like a liquid able to fill all cracks. The coherent character of the hydrogenic wave function is crucial in the ability of the photodissociation experiment to act as a probe. A series of case studies has been carried out whose aim is to reveal the relation between the structure of the surface and the asymptotic energy resolved angular distribution of the hydrogen fragment. The dynamics of the hydrogen atom motion was modeled by the time dependent Schrijdinger equation. The cases studied include the dissociation of a single HBr adsorbate on flat and corrugated surfaces. A broad specular peak was observed, in addition to diffraction peaks which can be correlated with the corrugation. Moreover, selective adsorption peaks, which can be correlated with the attractive part of the surface potential, have been identified. Systems in which the hydrogenic wave function scatters from several adsorbates were also investigated. It was found that the scattering is dominated by the trapping of the wave function by unstable periodic orbits. The quantization rules of these periodic orbits have been identified, creating a link between the structure of the adsorbates and the asymptotic angular distributions.
Photodissociation processes in the HCl molecule
Journal of Chemical Physics, 1982
Various ab initio methods have been employed for the study of photodissociation processes in the HCI molecule. Potential curves for selected singlet and triplet states and dipole transition moments between singlet states have been calculated. The transition moments vary significantly with internuclear distance for all states studied. The lifetime of the B 1,2' + state is predicted to be 3 ns. The calculations show that photodissociation of Hel occurs by absorption into the repulsive A III state and by absorption into the bound C III state, followed by predissociation. The theoretical photodissociation cross sections for the A III state and oscillator strengths for the C I II state are in good agreement with experimental data. The contributions from other excited states are investigated. The photodissociation rate of HCl in diffuse interstellar clouds is computed.
Ultrafast photochemistry of methyl hydroperoxide on ice particles
Proceedings of the National Academy of Sciences, 2010
Simulations show that photodissociation of methyl hydroperoxide, CH 3 OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH 3 OOH is found to prefer the exterior of the cluster, with the CH 3 O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH 3 O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH 3 O to form CH 3 OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH 2 O, H 2 O, H 2 , CO, CH 3 OH, and H 2 O 2 .
The Astrophysical Journal, 2002
In this paper, we report on the formation of molecular hydrogen on different types of amorphous water ice. We show that mass spectra of desorbing molecules upon formation are sensitive to the way in which ice is deposited on a cold substrate, to its thermal history, and to the action of UV photons. Implications that these results bear on H 2 formation in dense quiescent clouds are presented and discussed.
Molecular dynamics simulation of the photodissociation of adsorbed HCl on a MgO (001) surface
The Journal of Chemical Physics, 1995
The photodissociation of HCl/MgO ͑001͒ is studied by classical molecular dynamics of a single adsorbate system including the substrate phonon modes. An important quantum effect is accounted for by taking the hydrogen coordinates and momenta in the initial state from a vibrational ground state wave function. In the angular distribution of the scattered photofragments characteristic structures due to rainbows, scattering shadow and resonances are found, that are already well described within the rigid surface approximation. The hydrogen kinetic energy release also shows a pronounced peak structure corresponding to different energy transfer mechanisms and is significantly affected by inclusion of energy transfer to the phonon modes. Due to multiple collisions with the surface and the chlorine, the hydrogen can lose more than 3.5 eV of its 4.7 eV excess energy. The angular resolved energy spectrum is explained by several types of trajectories connected with the above mechanisms. The results suggest further that the different mechanisms can be separated in an experiment.
Transient absorption of vibrationally excited ice Ih
The Journal of Chemical Physics, 2008
The ultrafast dynamics of HDO:D2O ice Ih at 180 K is studied by mid-infrared ultrafast pump-probe spectroscopy. The vibrational relaxation of HDO:D 2 O ice is observed to proceed via an intermediate state, which has a blue-shifted absorption spectrum. Polarisation resolved measurements reveal that the interediate state is part of the intramolecular relaxation pathway of the HDO molecule. In addition, slow dynamics on a time scale of the order of 10-100 ps is observed, related to thermally induced collective reorganisations of the ice lattice. The transient absorption line shape is analysed within a Lippincott-Schröder model for the O-H stretch potential. This analysis identifies the main mechanism behind the strong spectral broadening of the v OH = 1 → 2 transition.