High-energy resonant cross sections for simultaneous electron-photon excitation of the 3s state of hydrogen

High-energy cross sections for the excitation of the 3s state of the hydrogen atom by simultaneous collision with electrons and photons are calculated under the second-order Bethe-Born approximation, both as functions of electron energy and of laser intensity. Resonant structures are shown to arise as a function of laser-photon frequency. The behaviour of these resonances is described and their origin explained. It is concluded that these resonances are characteristic of the simultaneous electron-photon excitation process itself and can occur for any atomic target, under suitable conditions.