Rydberg excitation in xenon by the rising edge of a femtosecond laser pulse

Photoelectron spectra in xenon are obtained for (6+1)-photon ionization with intense laser pulses (λ=594 nm, I=5×${10}^{13}$ W/${\mathrm{cm}}^2$). By comparing spectra obtained with 300-fs and 600-fs pulses it is shown that the ground state is depleted by nondiabatic passage through high lying Rydberg states (nf) during the rise of the pulse. Spectra are well reproduced by a simple theory including both six-photon excitation to the Rydberg states and one-photon ionization out of these states.