Absorption of additional photons in the multiphoton ionisation continuum of xenon at 1064, 532 and 440 nm

The energy spectrum of electrons produced by multiphoton ionisation of xenon atoms has been measured, using a new type of electron energy analyser with an acceptance angle of 2 pi sr. Absorption of additional photons above the minimum number required for ionisation has been observed in the non-resonant as well as in the resonant multiphoton ionisation process. In the resonant proces the AC Stark shift is used to link wavelength detuning to the light intensity, in order to find the probability of extra photon absorption as a function of light intensity.