Molecules in intense femtosecond laser fields

The interaction of small molecules (Na-2, Na-3) and larger molecules (C-60, C-70 and Fe(CO)(5)) with intense femtosecond laser fields is investigated experimentally. The experiments are performed by combining molecular beam and femtosecond pump-probe techniques with ion-and photoelectron detection. The recently developed femtosecond time resolved photoelectron spectroscopy is used to observe molecular dynamics over a wide range of internuclear distances. A main focus of the paper regarding small molecules is the change of the dynamics of molecular multiphoton ionization from the low laser intensity regime, where perturbation theory still can be applied, to the high laser intensity regime, where the electronic states participating in the multiphoton ionization processes are coupled coherently in Rabi-type processes. In this laser intensity regime, the influence of vibrational wavepacket dynamics on molecular above threshold ionization is also observed. Experiments on larger molecules are performed in a laser intensity regime where the generally accepted description of molecular multiphoton excitation has to be changed and recently proposed mechanisms due to the high field strenghts are discussed to explain the observations.