| We have observed the ion and electron spectra produced by the multiphoton ionization of barium and sodium atoms, using picosecond laser radiation in the range 560-650 nm. Barium, a two-valence-electron atom, is one of the simplest cases in which to study the multiple ionization process. Both the ion and electron spectra suggest that the dominant process in double ionization of Ba 6s{dollar}sp2{dollar} atoms, in our wavelength and intensity regime, is sequential. This was also verified for the double ionization of Ba 6snl Rydberg atoms. In this case however, the ionization process depends on the n-state. When the round-trip time of the Rydberg electron is smaller than the pulse width, the Rydberg electron is photoionized first, followed by photoionization of the Rydberg electron. On the other hand, when the round-trip time of the Rydberg electron is greater than the laser pulse width, the inner electron is photoionized first, followed by field ionization of the outer electron. The electron spectra produced by multiphoton ionization of the two atomic systems in consideration, show AC Stark shifts of the order of 10{dollar}sp3{dollar} cm{dollar}sp{lcub}-1{rcub}{dollar}. A comprehensive study of the effect is carried out in sodium, demonstrating a good agreement between theory and experiment. We show the importance of the atomic structure in the ionization process, which takes place at lower intensities than the peak intensity of the laser. |
