Evaluation of molecular orbital symmetry via oxygen-induced charge transfer quenching at a metal-organic interface

TL;DR

This study demonstrates that oxygen adsorption on metal surfaces preserves molecular geometry, enabling the use of NEXAFS spectroscopy to determine molecular orbital symmetries and charge transfer characteristics at metal-organic interfaces.

Contribution

The paper introduces a method to analyze molecular orbital symmetry at metal-organic interfaces using oxygen-induced surface modifications and NEXAFS spectroscopy.

Findings

Oxygen adsorption preserves molecular geometry on metal surfaces.

Charge transfer at the interface is quenched by oxygen, maintaining molecular order.

Distinct orbital symmetries ({c0}* and {c3}*) are identified and analyzed.

Abstract

Thin molecular films under model conditions are often exploited as benchmarks and case studies to investigate the electronic and structural changes occurring on the surface of metallic electrodes. Here we show that the modification of a metallic surface induced by oxygen adsorption allows the preservation of the geometry of a molecular adlayer, giving access to the determination of molecular orbital symmetries by means of near-edge x-ray absorption fine structure spectroscopy, NEXAFS. As a prototypical example, we exploited Nickel Tetraphenyl Porphyrin molecules deposited on a bare and on an oxygen pre-covered Cu(100) surface. We find that adsorbed atomic oxygen quenches the charge transfer at the metal-organic interface but, in contrast to a thin film sample, maintains the ordered adsorption geometry of the organic molecules. In this way, it is possible to disentangle {\pi}* and {\sigma}* symmetry orbitals, hence estimating the relative oscillator strength of core level transitions directly from the experimental data, as well as to evaluate and localize the degree of charge transfer in a coupled system. In particular, we neatly single out the {\sigma}* contribution associated with the N 1s transition to the mixed N 2px,y-Ni 3dx2-y2 orbital, which falls close to the leading {\pi}*-symmetry LUMO resonance.