Can photoemission tomography be useful for small, strongly-interacting adsorbate systems?

Larissa Egger,Michael G Ramsey,Daniel Lüftner,Peter Puschnig,Mathias Richter,F Stefan Tautz,Simon Weiss,Xiaosheng Yang,Georg Koller,Serguei Soubatch,Alexander Gottwald,Bernd Kollmann,Philipp Hurdax

doi:10.1088/1367-2630/ab0781

Abstract

Molecular orbital tomography, also termed photoemission tomography, which considers the final state as a simple plane wave, has been very successful in describing the photoemisson distribution of large adsorbates on noble metal surfaces. Here, following a suggestion by Bradshaw and Woodruff (2015 New J. Phys. 17 013033), we consider a small and strongly-interacting system, benzene adsorbed on palladium (110), to consider the extent of the problems that can arise with the final state simplification. Our angle-resolved photoemission experiments, supported by density functional theory calculations, substantiate and refine the previously determined adsorption geometry and reveal an energetic splitting of the frontier π-orbital due to a symmetry breaking which has remained unnoticed before. We find that, despite the small size of benzene and the comparably strong interaction with palladium, the overall appearance of the photoemission angular distributions can basically be understood within a plane wave final state approximation and yields a deeper understanding of the electronic structure of the interface. There are, however, noticeable deviations between measured and simulated angular patterns which we ascribe to molecule-substrate interactions and effects beyond a plane-wave final state description.

Highlights

In recent years, photoemission tomography (PT) has evolved as a powerful technique to study the geometric and electronic structure of oriented layers of organic molecules [1, 2]
We observe a clear energy splitting of the π1 and π2 states, which comprise the formerly degenerate highest occupied molecular orbital (HOMO) of the gas phase benzene ring. This splitting has been overlooked in the early photoemission study of benzene/Pd(110) [29] and could be revealed in our work, despite worse energy resolution, because of (a) the availability of angle-resolved ultraviolet photoemission spectroscopy (ARUPS) data over a wide angular range and (b) the predictions of the angular emission patterns of π1 and π2
The density functional theory (DFT) total energy results alone do not give a compelling result for the orientation, because the total energy difference between the along and across alignment of only 0.1 eV is within the typical error bar expected from uncertainties in the exchange-correlation treatment including van-der-Waals corrections

Summary

Introduction

Photoemission tomography (PT) has evolved as a powerful technique to study the geometric and electronic structure of oriented layers of organic molecules [1, 2]. PT has found many interesting applications including the assignment of molecular orbital densities in momentum and real space [3,4,5,6,7,8], the deconvolution of spectra into individual orbital contributions beyond the limits of energy resolution [9,10,11,12], or the extraction of detailed geometric information [13,14,15,16,17,18] These successes have been on relatively large molecules adsorbed on noble metal surfaces and the usefulness of PT to smaller molecules and on more reactive surfaces has not been investigated. The first will cause the momentum maps, i.e. k-space distribution of the orbitals, to be less distinct, and the second could lead to scattering becoming dominant in the angular distribution

Methods

Results

Conclusion