FEMTOSECOND TIME-RESOLVED PUMP-PROBE SPECTROSCOPY OF NAI IN RARE-GAS ENVIRONMENT

In this paper we report femtosecond studies of the influence of rare-g as collisions on the dissociation reaction of NaI molecules by investi gating the coherent wave packet motion in NaI at a temperature of 670 degrees C and rare-gas pressures ranging from 0 to 1000 bar. The photo dissociation dynamics of NaI is influenced by the interaction between the excited covalent and the ionic ground state potential energy surfa ces, which cross at a certain internuclear separation. Due to the adia baticity of the so formed potential, studies of the elementary nuclear motion and reaction dynamics along the covalent (Na + I) and the ioni c (Na+ + I-) channels are possible. The probing is made for the transi tion-state complex [Na ... I](double dagger). Even at low rare-gas pr essures a considerable influence of collisions on the wave packet moti on within the adiabatic potential well is found. Only few collisions b etween NaI molecules and surrounding rare-gas atoms result in fast vib rational relaxation and loss of coherence. Furthermore, a stabilizatio n of the transition state is observed due to the decrease of the Landa u-Zener escape probability. The reaction cross section for the interac tion of the transition state molecules with helium atoms is found to b e smaller than for the interaction with argon atoms. The intensity of the detected laser-induced fluorescence arising from the transition Na <-- Na decreases with increasing rare-gas pressure. Here, the effect for argon atoms is smaller than for helium atoms.