FUNDAMENTAL INTERACTIONS OF MOLECULES (NA2, NA3) WITH INTENSE FEMTOSECOND LASER-PULSES

The real-time dynamics of multiphoton ionization and fragmentation of molecules Na2 and Na3 has been studied in molecular beam experiments e mploying ion and electron spectroscopy together with femtosecond pump- probe techniques. Experiments with Na2 and Na3 reveal unexpected featu res of the dynamics of the absorption of several photons as seen in th e one- and three-dimensional vibrational wave packet motion in differe nt potential surfaces and in high laser fields: In Na2 a second major resonance-enhanced multiphoton ionization (REMPI) process is observed, involving the excitation of two electrons and subsequent electronic a utoionization. The possibility of controlling a reaction by controllin g the duration of propagation of a wave packet on an electronically-ex cited surface is demonstrated. In high laser fields, the contributions from direct photoionization and from the second REMPI process to the total ion yield change, due to different populations in the electronic states participating in the multiphoton ionization (MPI) processes. I n addition, a vibrational wave packet motion in the electronic ground state is induced through stimulated emission pumping by the pump laser . The 4(1)SIGMA(g)+ shelf state of Na2 is given as an example for perf orming frequency spectroscopy of high-lying electronic states in the t ime domain. Pure wave packet effects, such as the spreading and the re vival of a vibrational wave packet, are investigated. The three-dimens ional wave packet motion in the Na3 reflects the normal modes in the X and B states, and shows in addition the pseudorotational motion in th e B state in real time.