Probing the Spin State of Spin-Crossover Complexes on Surfaces with Vacuum Ultraviolet Angle-Resolved Photoemission Spectroscopy

Spin-crossover complexes
in direct contact with substrates have
sparked considerable interest, in particular, in view of their potential
applications in molecular electronics. While a huge number of spin-crossover
complexes is available, many of them are not robust enough to withstand
the sample preparation and/or the interaction with the substrate.
The techniques usually employed for these investigations, namely,
near-edge X-ray absorption fine structure spectroscopy and low-temperature
scanning tunneling microscopy, are not adapted for systematic studies
because of the limited access to synchrotron-radiation facilities
and complexity of the (indirect) spin determination, respectively.
Here, we detail a methodology using a (more) commonly available technique,
namely, vacuum ultraviolet (angle-resolved) photoemission spectroscopy,
to determine the spin state of layers of three different spin-crossover
complexes with thicknesses down to the submonolayer regime. We present
an approach to determine the thicknesses of the investigated layers,
relying on the inelastic mean free path of electrons determined from
combined photoemission and X-ray absorption measurements. We report
on the high-spin to low-spin relaxation dynamics of spin-crossover
(SCO) layers and the influence of the ultraviolet light on these dynamics.
While the observed relaxation processes occur on a timescale on the
order of minutes, probing spin-state dynamics on the picosecond timescale
is foreseeable with pump–probe photoemission spectroscopy.