Image potential states of ultrathin NiO films: A time-resolved two-photon photoemission study

TL;DR

This study investigates the image potential states of ultrathin NiO films on Ag(001) using time-resolved two-photon photoemission, revealing how their lifetimes depend on film thickness and electronic structure.

Contribution

It provides the first detailed measurement of IPS lifetimes in ultrathin NiO films and analyzes their dependence on film thickness and electronic coupling.

Findings

IPS identified for 2-4 ML NiO films

Lifetimes of IPS vary with film thickness

Coupling to electronic structure affects IPS lifetimes

Abstract

Well-ordered ultrathin films of NiO have been prepared on an Ag(001) substrate using molecular beam epitaxy. With the help of angle-resolved two-photon photoemission (2PPE) a series of image potential states (IPS) for film thicknesses of 2--4 monolayers (ML) has been identified. By time-resolved 2PPE, the lifetimes of the first three IPS and their dependence on the oxide film thickness have been determined. While the lifetimes of the (n=1) IPS are all in the range of 27--42 fs, the values for the (n=2) IPS decrease from 85 fs for 2 ML to 33 fs for 4 ML. These differences are discussed in terms of a coupling to the layer-dependent electronic structure of the NiO ultrathin films.