Two-colour experiments in the gas phase

First experiments on atomic photoionization and molecular dissociation have been performed by taking advantage of the unprecedented characteristics of the free electron laser in Hamburg (FLASH) combined with a separate near-infrared (NIR) femtosecond laser. In a series of two-colour experiments, the photoionization of rare gases in the presence of a strong NIR dressing field as well as the polarization dependence of this process were investigated systematically. A detailed analysis of the partial cross sections for the two-colour two-photon ionization process was carried out for low dressing fields. Higher dressing fields gave rise to multi-photon processes, which were observed and analysed without undesirable interferences, a beneficial consequence of the monochromaticity of the FLASH radiation. The experimental results were compared with theoretical descriptions for two-colour above-threshold ionization obtained by employing second-order perturbation theory and the 'soft-photon' approximation. In addition, complementary information was obtained on the sequential two-photon double ionization of Ne, which was made possible by the short and intense FLASH pulses. As a starting point for future time-resolved studies of molecular dissociation, a proof-of-principle experiment on the hydrogen diatomic system was carried out. In a typical pump–probe arrangement, excited neutral fragments, which were formed during photo-induced dissociation by the FLASH radiation, were identified via single- and multi-photon ionization induced by the time-delayed optical laser.