Molecular dynamics simulation of the photodissociation of adsorbed HCl on a MgO (001) surface (original) (raw)
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Mixed quantum/classical simulation of the photolysis of HCl on MgO(001)
The Journal of Chemical Physics, 1996
Quantum and mixed quantum/classical calculations of the photolysis of a HCl adsorbate on a MgO surface are reported. In the quantum calculation of the hydrogen dynamics ͑with rigid surface and chlorine atoms͒ a strong oscillatory structure is found in the angular distribution of the photofragmented hydrogen as well as in the absorption spectrum. These resonances are caused by temporary trapping of the hydrogen atom between the chlorine atom and the surface and reflect the initial perpendicular adsorption geometry. Corrugation of the surface potential leads to a significant modification of these interference patterns, which exist even for a flat surface. Within a mixed quantum/classical time-dependent self-consistent field ͑Q/C TDSCF͒ propagation the influence of surface degrees of freedom on the interference patterns is investigated. The thermal motion of the surface and inelastic collisions of the hydrogen atom with the surface and the chlorine atom washes out most of the oscillatory structure. In the fully angular and energy resolved spectra nevertheless clearly distinguishable peaks are seen. They can be used in practice to extract information about adsorption geometry and surface potential parameters.
Local surface structure probed by photodissociation in the system: quantal and classical simulations
Surface Science, 1995
The photodissociation of HC1 on MgO(001 ) is studied by classical and quantum methods. The quantum aspect resulting from the hydrogen zero-point motion is also modeled in the classical simulation and has an important influence on the dynamics through the initial distribution of tilt angles from the surface normal. The angular distribution of the scattered photofragments and the kinetic energy release of hydrogen show characteristic structures due to rainbows, scattering shadow and resonances. Information about surface potential and adsorbate geometry can be obtained from them.
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.
1999
The photodissociation of HCl adsorbed on the surface of an Ar 12 cluster is studied by semiclassical molecular dynamics simulations, using a surface-hopping approach for the nonadiabatic transitions. The DIM method is used to construct the 12 potential energy surfaces that are involved, and the nonadiabatic couplings. The results are compared with previous studies on HCl embedded inside Ar clusters and on the triatomic Ar-HCl cluster. The main findings are the following: ͑1͒ There is a yield of about 1% for recombination onto the ground electronic state of HCl, roughly the same as for HCl embedded inside Ar 12 . ͑2͒ Photodissociation lifetimes much longer than for Ar-HCl are found. ͑3͒ The kinetic energy distribution of the H atom shows large energy transfer to the cluster, greater than in the case of HCl in the embedded geometry in ͑Ar͒ 12 HCl. ͑4͒ An interesting mechanism leads to the formation of some fraction of very ''hot'' Cl atoms. ͑5͒ About 10% of the Cl is left trapped in (Ar) m Cl clusters. ͑6͒ The branching ratio P 1/2 : P 3/2 for the Cl atoms that leave the cluster shows electronic cooling compared to the isolated HCl molecule case. The results throw light on the role of local geometry in photodissociation/recombination processes, and in particular on the mechanisms pertinent in the case of surface-adsorbed species. The nature of the results, showing strong cage effects at the surface geometries is to a large extent a consequence of the encapsulation of the H atom, obtained for the structure of the ͑Ar͒ 12 HCl cluster.
Simulation of coherent control of hydroxyl formed due to HCl adsorption on MgO(001)
Chemical Physics, 2006
A laser-excitation scheme is proposed to excite the stretching vibration of the hydroxyl ion formed at the surface of MgO(0 0 1) due to HCl adsorption as example for a photoinduced reaction in a molecule in contact with a crystal. A dissipation operator was constructed using a dissipation lifetime obtained from classical molecular dynamics simulations. The optimal control technique is used to design laser fields that maximise the yield of hydroxyl molecules in a vibrationally highly excited state. The main frequencies and chirp parameters of the predicted fields are extracted. Predicted pulses involve considerable contributions from double excitations. Taking dissipation into account, a lowering of the excitation yield is predicted but only by 50%, so that the proposed process remains effective. A radiative mechanism is proposed to desorb hydrogen from the vibrationally excited state of OH À .
Photodissociation of a HCl molecule adsorbed on ice
Chemical Physics Letters, 2005
This work is a two-dimensional study of the photodissociation of a HCl molecule adsorbed on a proton ordered hexagonal ice surface. We apply the multi-configuration time-dependent Hartree method to study the dynamics of the hydrogen photofragment. The calculated absorption spectrum exhibits structures which are related to a temporary trapping of the hydrogen between the Cl atom and the ice surface. The angular distribution of the emitted hydrogen shows structures which are attributed to a surface rainbow effect. Moreover the results indicate that the photodissociation of HCl can lead to a diffusion of the hydrogen fragment into an ice cavity.
The Journal of Chemical Physics, 1995
We investigate the usefulness of density functional theory ͑DFT͒ for calculating excited state potential energy surfaces. In the DFT calculations, the generalized gradient approximation ͑GGA͒ is used. As a test case, the photodissociation of H 2 O through the first excited à 1 B 1 state was considered. Two-dimensional potential energy surfaces were obtained for both the X 1 A 1 ground state and the first excited state. Wave packet calculations employing these surfaces were used to obtain both the absorption spectrum and partial photodissociation cross sections, which are resolved with respect to the final vibrational state of the OH fragment. Comparisons are made with a previously calculated high level ab initio potential energy surface, with dynamics calculations using that surface, and with experiment. The vertical excitation energy for the (X 1 A 1 →à 1 B 1) transition calculated using DFT is in good agreement with the previous ab initio calculations. The absorption spectrum and the partial cross sections obtained with the DFT treatment are in good agreement with experiment.
Chemisorption of HCl to the MgO(001) surface: A DFT study
Physical Chemistry Chemical Physics, 2006
We use plane wave and embedded cluster ab initio density functional calculations to study adsorption, dissociation and diffusion of the HCl molecule on the MgO(001) surface. The two methods yield comparable results for adsorption of an isolated HCl molecule and complement each other when considering charged species and coverage effects. We find dissociative chemisorption at a coverage smaller than 0.5 monolayer with a Cl À ion electrostatically coupled to the OH À ion at the surface oxygen site. The adsorption energy of the Cl À Á Á Á(OH) À complex is 1.5 eV and the activation energy of Cl À diffusion away from OH À is 0.6 eV. There is no significant activation energy for rotation of Cl À around the adsorption site. At rising coverage, an increase in dipole-dipole repulsion between HCl molecules leads to a lowering of the adsorption energy per HCl and a change of binding towards hydrogen-bridge type as well as a lowering of the activation energy for Cl À diffusion. OH À formed in the surface due to HCl adsorption has a stretch frequency of 3083 cm À1 with Cl À associated and 3648 cm À1 with Cl À removed.
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.
The Journal of Physical Chemistry B, 1997
We report a quasi-classical trajectory study of a chemical reaction between H and CO 2 at the LiF(001) surface. The reaction is initiated by photodissociation of well-aligned HBr(ad) at 193 nm, which produces a "hot" H atom directed toward a nearby CO 2 (ad). Single molecules of each reactant are placed on a static surface, and a full-dimensional HCO 2 potential derived from ab initio calculations is used. The adsorbate-substrate and the adsorbate-adsorbate potentials consist of both nonelectrostatic and electrostatic contributions. Several energetically favorable adsorption configurations are determined by a Monte Carlo method. Quasi-classical trajectories are calculated at 80 K for four different adsorption configurations. We find that the reactivity at some configurations is significantly enhanced compared with the corresponding gas-phase simulation. The calculated impact parameters and incident angles of the surface-aligned collisions indicate that the enhanced reactivity can be largely attributed to the closeness and alignment of the coadsorbates on the surface. Owing to the long-lived complex, product distributions, with the exception of a departure angle, show little memory with regard to the initial configuration and are similar to those obtained in the gas phase. A significant number of the unreacted hydrogen atoms retain sufficient energy to make subsequent reaction with other coadsorbates a possibility. We find evidence of several dynamic features pertinent to the use of the surface as a template for reactivity enhancement, including scattering at the surface, the squeezed atom effect, chattering, and caging.