Multiphoton ionisation cross sections: a correlation function approach

N-photon ionisation cross sections obtained by perturbation theory treatments are expressed in terms of Fourier transforms of appropriate autocorrelation functions. As expected, the N=1 case is related to the well known electric dipole-dipole autocorrelation function, while correlation of increasingly complicated operators is involved as N=2,3,.... A simple application of the formalism to the evaluation of one- and two-photon cross sections for a schematic model of the negative ion H- is presented. (author)