Molecular ATI and ATD with Femtosecond Laser Pulses

Abstract

In a typical ATI experiment the excited states are populated due to ac Stark shifts. Here we report ATI experiments on the sodium dimer molecule where all in the ionization process involved electronic states are in resonance with the light field. Additionally, the vibrational wave packet dynamics on the electronic potential curves introduce a time scale for ionization out of these states1. Using intense femtosecond pump-probe lasers we were able to show that the vibrational wave packet motion in different excited electronic states of the neutral molecule is found in the time resolved ATI photoelectron peak. In addition to that we found that the the coherent coupling of the involved neutral electronic states 2 3 is beautifully reflected in the ATI spectra. A further unexpected result is that even in the high intensity laser fields - leading to the ATI structure in the photoelectron spectra - we do not observe additional ionic fragmentation channels. Our results indicate that no matter how many photons are additionally absorbed in the continuum, the final electronic and vibrational ionic states are always the same. This finding has far reaching consequences for femtosecond experiments in cluster physics.