Relation between 'above-threshold' ionisation probabilities and continuum-continuum matrix elements

The authors report experimental data on 'above-threshold' multiphoton ionisation of Xe via a three-photon resonance. The ratio of the five-photon ionisation probability to that for (5+1)-photon ionisation is interpreted in terms of the measured angular distributions for both processes and the 'energy conserving' continuum-continuum matrix elements. A calculation of these matrix elements on the basis of a model potential leads to a ratio of the (5+1)-photon signal over the five-photon signal in good agreement with the experimental value of about 3*10-3 at an intensity of 1011 W cm-2.