Absence of resonances in femtosecond photoionisation spectra

Abstract
Xenon was ionised by 100 fs UV pulses. The electron spectra showed a decreasing energy of the photoelectrons with increasing light intensity. No resonances were observed in the electron spectrum. The absence of resonances is understood in terms of the ratio of the pulse duration and the characteristic time (2 pi n3 in au) of the intermediate state. From the observed spectra it appears that the four-photon ionisation of xenon at 309 nm is not saturated at an intensity of 1.5*1013 W cm-2.