A chemical view on X-ray photoelectron spectroscopy: the ESCA molecule and surface-to-bulk XPS shifts

Artikel i vetenskaplig tidskrift, 2018

In this paper we remind the readerofasimple, intuitive pic-
ture of chemical shifts in X-ray photoelectron spectroscopy
(XPS) as the differenceinchemical bonding between the
probedatom and itsneighbor to the right in the periodic
table, the so called Z + 1approximation. We use the classical
ESCA molecule, ethyl trifluoroacetate, and 4d-transition metals
to explicitly demonstrate agreement between core-level shifts
computed as differences between final core-hole states and
the approach where each core-ionized atom is replaced by a
Z + 1atom. In this final state, or total energy picture, the XPS
shift arises due to the more or less unfavorable chemical bond-
ing of the effective nitrogen in the carbon geometry for the
ESCA molecule. Surfacecore level shifts in metals are deter-
mined by whether the Z + 1atom as an alloy segregatesto
the surface or is more soluble in the bulk. As further illustration
of this more chemical picture, we compare the geometry of
C1s and O1s core-ionized CO with that of, respec tively,NO
+
and CF
+
.The scope is not to proposeanew methodtocom-
pute XPS shifts but rather to stress the validity of this simple
interpretation.