Showing papers by "John C. Tully published in 1984"

Trajectory studies of vibrational energy transfer in gas--surface collisions

Abstract: We have applied the stochastic trajectory method to the scattering of NO from the (001) surface of an LiF crystal. The surface was represented by a 32 atom primary zone and the effects of the rest of the surface atoms were included using the Generalized Langevin Equation method. The forces between the surface atoms were treated using a shell model and the gas–surface interactions included short‐range repulsions, attractive dispersion forces, and the interaction of the dipole moment of NO and the surface ions. The dependence of the dipole moment and the dependence of the molecular polarizability on the internuclear separation of NO were also included. The calculated vibrational energy accommodation coefficient was less than 1% when trapping of the gas diatom was negligible. By modifying the vibrational frequency of the scattered diatom we have examined the dependence of vibrational energy transfer on frequency. An exponential dependence on the ratio of the molecular frequency to the surface Debye frequency was found, similar to behavior observed for vibrational relaxation in condensed phases. We have also examined the relative efficiency of the coupling of the vibrational mode to the translational and rotational modes of the gas diatom and to the phonon modes of the surface. We found that the vibrational mode is most strongly coupled to the surface phonons with a much weaker coupling to the rotational and translational modes.

Dynamics of Molecular Motion at Single-Crystal Surfaces

Abstract: Dramatic advances in our understanding of the motion of individual atoms and molecules at single-crystal surfaces have been made within the past 5 years. Recent experimental and theoretical studies of the interaction of nitric oxide with metal surfaces illustrate the depth of understanding now obtainable. General principles, applicable to a broader range of molecule-surface encounters, have begun to emerge out of the systematic and in-depth analyses of these and related studies.

Laser induced thermal desorption from surfaces

Abstract: Nonresonant laser induced desorption of adsorbed molecules from surfaces has been simulated using the stochastic trajectory technique. An NO molecule is initially bound to a cold LiF(100) surface. Rapid heating of the surface is then simulated via random forces applied to the edges of the 32 atom surface slabs. When the rate of heating is rapid compared to the rates of thermalization of the degrees of freedom of the molecule, it is found that the mean energies of the translational, rotational, and vibrational degrees of freedom of the desorbing molecule are significantly lower than those corresponding to the temperature of the surface at the instant of desorption. Additionally, the angular distribution of the desorbing molecules is found to peak towards the surface normal, and the rotational angular momentum vector is preferentially aligned parallel to the surface plane. These results shed light on recent experimental observations.

Desorption Induced by Electronic-Transitions

Abstract: Recent work on electronically induced desorption of surface atoms and molecules resulting from the impact of electrons and UV photons with surfaces is reported. Since the overwhelming majority of emitted particles are neutral, emphasis is given to studies of excited and ground state desorbed neutral atoms and molecules. In particular, first measurements of velocity distributions of desorbed particles obtained using laser induced fluorescence are discussed.

Laser Irradiation of Ge(100): An Assessment of Surface Order with He Diffraction

Abstract: We investigate the effects of Nd:YAG and excimer laser irradiation on the Ge(100) surface under UHV conditions over a temperature range 140 < T(K) < 300 using the surface sensitive probe of He atom diffraction. We study the effects of irradiation on surface damage and order using the apparent (2×1) ࢐ c(2×4) transition. We monitor surface contamination in situ. The temporal thermal response is modeled theoretically to aid in assessing the experimental results. The capability to maintain a Ge(100) surface at low temperatures free of contamination and well ordered is demonstrated.