Final rotational state distributions from NO(vi = 11) in collisions with Au(111): the magnitude of vibrational energy transfer depends on orientation in molecule-surface collisions.

Bastian C Krüger,Alec M Wodtke,Tim Schäfer,Nils Bartels

doi:10.1039/c6cp02100j

Abstract

When NO molecules collide at a Au(111) surface, their interaction is controlled by several factors; especially important are the molecules' orientation with respect to the surface (N-first vs. O-first) and their distance of closest approach. In fact, the former may control the latter as N-first orientations are attractive and O-first orientations are repulsive. In this work, we employ electric fields to control the molecules' incidence orientation in combination with rotational rainbow scattering detection. Specifically, we report final rotational state distributions of oriented NO(vi = 11) molecules scattered from Au(111) for final vibrational states between vf = 4 and 11. For O-first collisions, the interaction potential is highly repulsive preventing the close approach and scattering results in high-J rainbows. By contrast, these rainbows are not seen for the more intimate collisions possible for attractive N-first orientations. In this way, we reveal the influence of orientation and the distance of closest approach on vibrational relaxation of NO(vi = 11) in collisions with a Au(111) surface. We also elucidate the influence of steering forces which cause the O-first oriented molecules to rotate to an N-first orientation during their approach to the surface. The experiments show that when NO collides at the surface with the N-atom first, on average more than half of the initial vibrational energy is lost; whereas O-first oriented collisions lose much less vibrational energy. These observations qualitatively confirm theoretical predictions of electronically non-adiabatic NO interactions at Au(111).

Highlights

NO scattering from Au(111) has been investigated by the independent electron surface hopping (IESH) method,[9] an algorithm for propagating classical trajectories[12] on an electron transfer (Newns–Anderson) Hamiltonian, hybridized to the metal electronic continuum.[13]
We report final rotational state distributions of oriented NO(vi = 11) molecules scattered from Au(111) for final vibrational states between vf = 4 and 11
IESH gives good agreement with many experimental results,[6] but due to inaccuracies in the interaction potential used for the calculation, it does not always compare favorably with experiment when compared in a one-toone fashion.[14,15]

Summary

Introduction

NO scattering from Au(111) has been investigated by the independent electron surface hopping (IESH) method,[9] an algorithm for propagating classical trajectories[12] on an electron transfer (Newns–Anderson) Hamiltonian, hybridized to the metal electronic continuum.[13]. Orientation distributions are so broad that a nominal N-first distribution contains some O-first oriented molecules.[3,16] Beyond this, to find good agreement between experiment and theory, the theory would need to accurately describe the weak forces in the entrance channel that govern dynamical steering, which it cannot yet do.[17] we seek an alternative experimental approach to testing the qualitative predictions of the IESH theory as they apply to the vibrational relaxation of highly vibrationally excited NO in collisions with a Au(111) surface. We use the 245–315 nm output of a commercially available OPO laser system (Continuum Sunlite Ex, 3 GHz bandwidth, 2 mJ per pulse@255 nm) to record rovibrationally resolved REMPI spectra of scattered molecules These spectra contain all necessary information to derive rotational and vibrational distributions of ground electronic state NO with vibrational quantum numbers ranging from 4 to 11. In order to derive the rotational state distributions we analyze the REMPI data by fitting simulated spectra to the experiment as explained in more detail in the ESI.†

Results and discussion

Lower panel

Conclusions