Current Research

Experimental Approach

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 (Δv=0 propensity rule).

 

 

Advantages of photoelectron spectroscopy compared to other probe techniques include:

· analyze outgoing photoelectron as to its kinetic energy
→ differential technique provides spectral and dynamical information
     (integrated techniques only provide dynamics)

· 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.

Department of Physics and Astronomy

Ullrich Group