COLLISIONAL ENHANCEMENT OF RYDBERG LIFETIMES OBSERVED IN VIBRATIONAL WAVE-PACKET EXPERIMENTS

Evidence for lifetime enhancement of Rydberg states by interactions wi th surrounding ions is obtained from femtosecond pump-probe experiment s on the vibrational wave packet dynamics of the I-2(B)-state, in whic h either detection of I-2(+) ions or zero-kinetic energy (ZEKE) electr ons was used. The use of the ZEKE technique leads to the appearance of new frequency components in the Fourier transform power spectra of pu mp-probe time delay scans, as well as the observation of enhanced peak -to-valley ratios in these scans. These observed frequency components do not correspond to any energy level spacings in the molecule; they c orrespond to a sum of two energy level spacings, suggesting a nonlinea r detection mechanism in the ZEKE technique (i.e., interaction with io ns). Additional evidence for our interpretation is presented through e xperiments in which additional ions were formed through nonresonant mu ltiphoton ionization of I-2 with a picosecond 266 nm laser, as well as experiments in which both the decay time of the ZEKE signal and the a ppearance of the pump-probe time delay scans as a function of the puls ed field time delay were studied. Theoretical wave packet calculations which support the conclusions are presented. (C) 1995 American Instit ute of Physics.