Inner-electron multiphoton ionization of barium Rydberg states with picosecond pulses

We have studied multiphoton ionization of Ba 6snl Rydberg states by intense ps pulses and have shown that the duration of the laser pulse compared to the orbit time of the Rydberg electron determines the laser-atom interaction. For high-n states with a long orbit time, the loosely bound Rydberg electron is not affected by the intense laser pulse. Instead the inner electron is ejected by absorption of five photons, leaving the ${\mathrm{Ba}}^{+}$ ion in a Rydberg state. For low-n states the outer electron is ejected and the residual ${\mathrm{Ba}}^{+}$ ion can subsequently be multiphoton ionized to ${\mathrm{Ba}}^{++}$.