Exploring ultra-fast charge transfer and vibronic coupling with N 1s RIXS maps of an aromatic molecule coupled to a semiconductor.

James N O’Shea,Andrew J Gibson,Karsten Handrup,Robert H Temperton,Alessandro Nicolaou,Nicolas Jaouen

doi:10.1063/1.4999135

Abstract

We present for the first time two-dimensional resonant inelastic x-ray scattering (RIXS) maps of multilayer and monolayer bi-isonicotinic acid adsorbed on the rutile TiO2(110) single crystal surface. This enables the elastic channel to be followed over the lowest unoccupied molecular orbitals resonantly excited at the N 1s absorption edge. The data also reveal ultra-fast intramolecular vibronic coupling, particularly during excitation into the lowest unoccupied molecular orbital-derived resonance. Both elastic scattering and the vibronic coupling loss features are expected to contain the channel in which the originally excited electron is directly involved in the core-hole decay process. This allows RIXS data for a molecule coupled to a wide bandgap semiconductor to be considered in the same way as the core-hole clock implementation of resonant photoemission spectroscopy (RPES). However, contrary to RPES measurements, we find no evidence for the depletion of the participator channel under the conditions of ultra-fast charge transfer from the molecule to the substrate densities of states, on the time scale of the core-hole lifetime. These results suggest that the radiative core-hole decay processes in RIXS are not significantly modified by charge transfer on the femtosecond time scale in this system.

Highlights

Ultra-fast electron transfer between a molecule and a surface to which it is coupled plays a key role in light-harvesting devices such as dye-sensitised solar cells[1] and water-splitting photoelectrochemical cells2 - to name just a couple of examples
In this paper we present for the first time two-dimensional N 1s RIXS maps of multilayer and monolayer biisonicotinic acid adsorbed on the rutile TiO2(110) single crystal surface, allowing the participator channel to be followed over the lowest unoccupied molecular orbitals
lowest unoccupied molecular orbital (LUMO) LUMO+1 LUMO+2 vibronic participator elastic participator. These correspond to the LUMO and LUMO+1 resonances of the molecule, respectively. The features within these bands below around 396 eV emission energy are attributed to inelastic x-ray scattering arising from the N 1s core-hole being filled by valence electrons from the occupied molecular orbitals

Summary

Introduction

Ultra-fast electron transfer between a molecule and a surface to which it is coupled plays a key role in light-harvesting devices such as dye-sensitised solar cells[1] and water-splitting photoelectrochemical cells2 - to name just a couple of examples. A light-harvesting molecule absorbs a photon of visible light through the excitation of an electron from the highest-occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). In RPES, the LUMO (and higher energy molecular orbitals) is instead populated by the resonant excitation of a core-electron through the absorption of a soft x-ray photon tuned to the specific energy of that resonance. This excitation is atom-specific because the transition probability is proportional to the overlap of the unoccupied molecular orbital with the specific core-level being probed. If the electron transfers away from the molecular orbital on the timescale of the core-hole lifetime the core-hole decay channel in which this electron is a direct participant will be depleted

Methods

Results

Conclusion