Vibrationally promoted electron emission at a metal surface: electron kinetic energy distributions

Abstract

We report the first direct measurement of the kinetic energy of exoelectrons produced by collisions of vibrationally excited molecules with a low work function metal surface exhibiting electron excitations of 64% (most probable) and 95% (maximum) of the initial vibrational energy. This remarkable efficiency for vibrational-to-electronic energy transfer is in good agreement with previous results suggesting the coupling of multiple vibrational quanta to a single electron.Understanding the interactions of molecules with solid surfaces is important to the development of predictive theories of surface chemistry, 1,2 which up to now routinely assume electronic adiabaticity. According to this assumption, one may calculate an effective potential energy surface upon which the atoms move based on the Born-Oppenheimer approximation, where the system remains in its electronic ground state. While this approach enables highly detailed calculations to be carried out, it neglects possible energy transfer channels between nuclear motion and electronic degrees of freedom in the metal.Reports of laboratory observations of electronically nonadiabatic influences on molecule-surface interactions are now becoming increasingly available, 3-5 yet it remains an open question whether such effects are important enough to require major adjustments to the electronically adiabatic picture. 6 For example, including nonadiabatic influences such as electronic friction, employing a weak coupling approximation, has been remarkably successful in addressing the inadequacies of the adiabatic assumption in describing vibrational lifetimes of small molecules on metal surfaces. 7 Specifically, adiabatic theories predict millisecond lifetimes whereas electronic friction calculations result in picosecond lifetimes, in good agreement with experiment. 8 The successes of friction theory suggest that major adjustments to the adiabatic picture might not be necessary.Recently, multi-quantum vibrational relaxation, 9 vibrationally promoted electron emission 10,11 and electron mediated vibrational overtone excitation 12 in molecule-surface scattering have been reported by our group. It is unclear if these phenomena can be described by electronic friction theories. For example, multi-quantum vibrational relaxation of NO(v = 14,15) on Au(111) is at least semi-quantitatively reproduced by friction-like theories. 13 However, good agreement with experiment is also found using a multi-state (independent electron surface hopping) model, 14-16 where a single electron transfer mechanism is operative.Putting it another way, a fundamental unknown is the fraction of molecular vibration that can be converted to single electron excitation and vice versa. For example, one might envision that a single highly excited electron in a solid could transfer nearly all of its excitation energy to a molecule at the surface. If that were possible, time reversal suggests that a highly excited molecule would be able to transfer all (or nearly all) of its excitation energy to a single electr...

Multiquantum Vibrational Excitation of NO Scattered from Au(111): Quantitative Comparison of Benchmark Data to Ab Initio Theories of Nonadiabatic Molecule–Surface Interactions

“…This study revealed very efficient multiquantum transfer of molecular vibration to the metal, in contrast to results for the same molecule scattered from an insulator, where vibration was transferred inefficiently 3b. 4 Similar experiments on low work function materials even showed vibrationally promoted exoelectron emission 3d–f…”

Incidence energy dependent state-to-state time-of-flight measurements of NO(v = 3) collisions with Au(111): the fate of incidence vibrational and translational energy

“…There is now considerable experimental evidence that vibrational excitation and de-excitation in NO collisions with Au and Ag surfaces are due to the nonadiabatic coupling to the electronhole pairs in the metal. [3][4][5][6]8,9,[29][30][31][32][33] In such a strongly electronically nonadiabatic system, translational motion was historically considered a spectator to the vibrational energy transfer. 12,14 While increased incidence energy of translation can increase the nonadiabatic coupling strength, it was thought to not directly exchange energy with the vibrational DOF.…”

On the determination of absolute vibrational excitation probabilities in molecule-surface scattering: Case study of NO on Au(111)

“…The ample evidence of energy exchange between molecular vibrations and metallic EHPs gained from molecular beam-surface scattering experiments includes studies of (single and multi-quantum) vibrational excitation, 8,9,12 (single and multi-quantum) vibrational relaxation, 4,28 and electron emission 5,6,29 occurring in the course of molecule-surface encounters. Specifically, vibrational excitation exhibiting an Arrhenius-like surface temperature dependence with activation energy equal to the vibrational excitation energy is often considered to be a signature a) Author to whom correspondence should be addressed.…”