fs Time-Resolved Photoelectron Spectroscopy
Femtosecond time-resolved photoelectron
spectroscopy (TRPES) is a relatively new approach to the study of rapid
excited state electronic relaxation processes. The general scheme
involves preparation of an excited state, dynamical evolution and a
time-delayed probe through ionization. In TRPES, photoelectron spectra are
measured as a function of the pump-probe delay thus providing dynamic and spectroscopic information on excited state relaxation
processes.
A pump pulse
excites a molecule from the ground state to a bright excited state A
which might be non-adiabatically coupled to an excited state B. A
and B have different electronic character and preferentially ionize
into ionic states A+ and B+, respectively.
In a TRPES spectrum two photoelectron bands, a and b, will be
observed. The population in state A decreases and hence the
photoelectron band a shows a decay. At the same time the population
in state B increases and photoelectron band b rises. Deactivation
pathways might consist of several internal conversion steps and TRPES
provides a unique way to directly identify participating electronically
excited states.
Photoelectron Spectroscopy as a Probe
Scheme:
TRPES is especially suited to
processes involving both charge and energy flow in excited molecules as
photoelectron spectroscopy is sensitive to both electronic configurational changes (via ionization
correlations) and vibrational dynamics (via Franck-Condon factors). Koopman’s
theorem provides an elementary picture for ionization correlations in the
case of single photon, single active electron ionization of a given
molecular orbital: emission of an independent outer electron occurs without
simultaneous reorganization of the core. Similarly for TRPES, the
probabilities of partial ionization into specific continuum electronic
states can differ drastically with respect to the molecular orbital nature
of the excited state. Vibrational energy flow can be monitored through
changes in the structure of the photoelectron bands due to changes in the
Franck-Condon overlap (Dv=0 propensity rule).
Advantages of photoelectron spectroscopy compared to other
probe techniques include:
•
analyze outgoing photoelectron as to its
kinetic energy
®
differential technique (contains
spectral and dynamical information)
•
simultaneous detection of ions can provide mass
info (coincidence techniques)
•
ionization is always allowed ®
there are no dark states
•
charged particle detection extremely sensitive
•
ionic states well understood (ab initio & He(I) PES)
fs Time-Resolved Photoelectron Photoion Coincidence
Spectroscopy:
TRPES is
readily combined with ion time-of-flight detection to obtain mass
information. Under certain expansion conditions, molecular clusters of a
variety of sizes can be produced in a molecular beam. To record a TRPES
spectrum of a certain cluster size it becomes necessary to identify the ion
core (i.e. its mass) that the photoelectron originated from.
Experimentally, this can be accomplished by photoionizing a single
molecule/cluster per laser pulse and detecting the photoelectron and
photoion in coincidence (PEPICO). To avoid any ambiguities the count-rate
is typically reduced to <<1 count per shot as PEPICO spectrometers
have a limited collection and detection efficiency.
