Multiphoton ionization dynamics of barium Rydberg states in intense femtosecond pulses

Abstract

We have investigated multiphoton ionization of Ba 6snl Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the 6sns and 6snd 15{lt}n{lt}{infinity} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395{endash}425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of Ba{sup +} Rydberg states and Ba{sup 2+} photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto Ba{sup +} Rydberg states. {copyright} {ital 1998} {ital The American Physical Society}