Observation of Direct Multiquantum Vibrational Excitation in Gas-Surface Scattering: NH3on Au(111)

Abstract

Vibrationally excited NH3 molecules are observed in the scattering of NH3 (v ~0) from Au(lll). We observe up to three quanta of vibrational excitation in the vi ''umbrella'' vibrational mode of NH3. The vibrational excitation probability scales linearly with the kinetic energy of the incident beam above a threshold energy corresponding to the quanta excited, and is independent of the surface temperature, T s , over the range 300 K< T s < 800 K. These results are discussed in terms of an electronically adiabatic collisional mechanism for vibrational excitation.PACS numbers: 79.20. Rf, 34.50.Ez, 34.50.Pi Heterogeneous catalysis, corrosion, microelectronic fabrication, aerodynamic drag, and heat transfer are processes with great economic and technological importance. The detailed molecular dynamics of the interactions between molecules and surfaces form the microscopic basis for these macroscopic processes. Recently molecular-beam-surface scattering experiments and theoretical techniques have been utilized to study the fundamental aspects of rotational excitation 1 " 11 and vibrational relaxation 12 " 14 during gas-surface encounters. However, the detailed study of vibrational excitation via such encounters has received relatively little attention. 15 " 20 A detailed understanding of vibrational excitation at surfaces is requisite to a dynamical description of surface chemical reactions, since molecular vibrational degrees of freedom are directly coupled to the dissociative reaction coordinate. Most previous experimental studies 15 " 17 of vibrational excitation have been concerned with excitation following trapping. One important exception is an elegant study by Rettner et ai 20 on the NO/Ag(lll) system in which significant vibrational excitation is observed in a direct encounter. These results have been interpreted with an electron-hole-pair deexcitation mechanism first proposed by Gadzuk 18 and more recently addressed by Newns. 19 In this Letter we report the first observation of multiquantum vibrational excitation in a direct gas-surface encounter. The NH3/AUGH) system was chosen for study since it has revealed interesting new dynamical information concerning rotationally inelastic scattering 11 and because NH3 has several normal vibrational modes. The experiment consists of scattering of a well-collimated, rotationally cold ( < 20 K), supersonic NH3 beam from an atomically clean Au(lll) surface. The scattered angular, rotational, and vibrational distributions are detected in a quantum-resolved manner by use of two-photon-resonant, three-photon-ionization [(2 -r-l)MPI] laser spectroscopy and molecular-beam scattering techniques. A detailed description of the experimental apparatus and the (2+ 1)MPI laser spectroscopy is published elsewhere. 10,21,22 Up to three quanta of vibrational excitation in the V2 "umbrella" mode of NH3 are observed in the scattered beam. In order to examine quantitatively the dependence of the observed vibrational excitation upon the energy and angle of the incident molecular beam, we ...

An advanced molecule-surface scattering instrument for study of vibrational energy transfer in gas-solid collisions

“…Vibrational excitation or de-excitation of gas molecules may result in surface phonon destruction or creation and/or electronic de-excitation or excitation. [10][11][12][13][14][15] Vibrational motion of the gas molecules may influence trapping, 14,16 exhibit isotope dependent effects, 17,18 and play a vital role in vibrationally promoted surface reactions, 14,19,20 electron transfer, 21 and even electron emission. 22 Much effort has been devoted to the study of electronically nonadiabatic influences in vibrational energy transfer at solid surfaces, but this has been accomplished for only a few molecule-surface systems.…”

Electron hole pair mediated vibrational excitation in CO scattering from Au(111): Incidence energy and surface temperature dependence

“…[1][2][3] For example, Kay and co-workers 4 have shown that the vibrational excitation probability of NH 3 (umbrella mode) scattered from a Au(111) surface increases monotonically with the translational incidence energy (E i ) above a threshold close to the vibrational excitation energy. The surface temperature (T s ) had no influence on the vibrational excitation probability.…”

On the temperature dependence of electronically non-adiabatic vibrational energy transfer in molecule–surface collisions

“…1 has been previously recognized as a signature of an electronically non-adiabatic energy transfer mediated by free electrons in the metal surface. 1,18,19,21 However, a complete interpretation of the physical meaning of the Arrhenius parameters derived from experiment has been lacking.…”