Two-color photoionization of atoms in low and high radiation fields

Abstract

Atomic photoionization was studied by means of electron spectroscopy using two photon sources of different wavelengths for the excitation. In one part of the experiments, alkaline atoms were excited by an optical laser of high spectral resolution prior to their photoionization by short wavelength radiation from the synchrotron radiation source BESSY. These low field studies concentrated on measurements of the linear alignment dichroism in the angular distribution (LADAD) and of the linear magnetic dichroism (LMDAD) in the 2p ionization of laser-polarized sodium atoms. In another type of experiments, the high intensity of a femtosecond optical laser was used for the investigation of dressed atoms, i.e. of the photoionization dynamics in strong external fields. In particular, the above threshold ionization (ATI) of rare gases was studied at the free electron laser facility FLASH in Hamburg, Germany. Measurements of the linear dichroism, here in the direct two-photon ionization of the 1s-shell in He, were performed in order to determine the partial photoionization cross sections related to the emission of electrons with s- and d- symmetry, respectively.