Seminars Archive

A Velocity Map Imaging Analyser for two-colour and FEL Experiments

Abstract
The Velocity Map Imaging apparatus constructed at the GasPhase beamline will be described in detail. In particular the use of this apparatus to measure the Photoelectron Angular Distributions (PADs) of electrons emitted from Rydberg states of rare gases excited by synchrotron radiation and then photoionised by a synchronised linearly polarised laser light is described. The analysis of the angular distribution allowed the extraction of the beta2 and beta4 asymmetry parameters which are related to the radial matrix elements and phase differences of the outgoing electron waves in the continuum. In this work we studied the ionisation of several intermediate states in He, Ne and Ar for photoelectron energies from 20 to 100 meV. The geometry used in this experiment allows access to more parameters than revealed by previous methods [3] thus providing a better description of the photoionisation event. The experimental results have been interpreted via an angular momentum analysis method. Recent developments of the apparatus will be described and, finally, the possibility of using this apparatus at a FEL source will be discussed briefly. [1] G. Cautero et al. (2008). NIM A, 595, 447-459. [2] G.A. Garcia et al. (2001). Rev. Scient. Instr. 75, 4989-4996. [3] Mitsuke K, Hikosaka Y and Iwasaki K (2000) J. Phys. B:At. Mol. Opt. Phys. 33 391