Abstract

In the present work we have studied multi-photon ionization with a 5 ps laser pulse starting from bound barium 6sns and 6snd Rydberg states. For low n states in which the radial roundtrip time for the Rydberg electron is short compared to the duration of the laser pulse, Ba⁺⁺ is produced by the now familiar sequential multiphoton ionization process (Fig. 1b). The Rydberg electron is ejected by one-photon ab­sorption, and the resulting Ba⁺ ion absorbs five photons to produce Ba⁺⁺. In contrast, for high n-states with a round trip time longer than the laser pulse, the intense laser field does not result in ejection of the weakly bound Rydberg electron. For these high n states only a small fraction of the wavefunction will pass by the core during the laser pulse and since ionization takes place close to the core, ionization of the Rydberg electron is unlikely. Instead the strongly bound 6s electron is multiphoton ionized and the Rydberg electron is projected onto the Ba⁺ n′1 Rydberg states of the same radial size (Fig. 1a). Its quantum state changes due to the electron shake-up associated with the sudden change in the Coulomb potential of the core. We have determined experimentally the relation between the principal quantum number of the initial Ba Rydberg state and that of the final Ba⁺ Rydberg state.

© 1992 IQEC