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Journal of Physics B Atomic and Molecular Physics

Electron-atom ionisation in a resonant electromagnetic field

P. Cavaliere, G. Ferrante, C Leone

Published Feb 14, 1982• 15 citations
Publisher

Abstract

Electron-atom ionisation in the presence of a resonant EM field is considered theoretically, taking the hydrogen atom as an example. The radiation field is taken to have a frequency matching the 1s-2p transition frequency. The theoretical treatment is based on the Coulomb-Born wave approximation and on the two-level atom model. The physical picture arising from the resulting field-assisted triple differential cross section (TDC) is that of ionisation events taking place simultaneously from the ground and the excited 2p states, the role of the radiation field being mainly that of pumping target atoms.

DOI
10.1088/0022-3700/15/3/029

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Electron-atom ionisation in the presence of a resonant EM field is considered theoretically, taking the hydrogen atom as an example. The radiation field is taken to have a frequency matching the 1s-2p transition frequency. The theoretical treatment is based on the Coulomb-Born wave approximation and on the two-level atom model. The physical picture arising from the resulting field-assisted triple differential cross section (TDC) is that of ionisation events taking place simultaneously from the ground and the excited 2p states, the role of the radiation field being mainly that of pumping target atoms.
Investigates theoretically a new aspect of radiative collisions on two-photon ionisation of colliding atoms of different species A and B. The authors derive the main characteristics of the resonance appearing in the two-photon ionisation cross section of atom B when the laser frequency is tuned on a resonance transition of atom A which cannot itself be ionised by two-photon absorption. Under some realistic conditions and for reasonable values of the light energy density flux F ( approximately 1 MW cm -2 ), the cross section of the process considered varies as F 2/5 while its width varies as F 1/2 . These predictions are in reasonable agreement with a recent observation on the mixture Sr(A)-Cs(B) by Brechignac et al. (1980).
Two-photon ionisation of atomic hydrogen in the ground state with linearly and circularly polarised light is considered. In the region above the one-photon ionisation threshold expressions for the two-photon ionisation transition matrix elements are obtained using the non-relativistic dipole approximation. The two-photon ionisation rate is calculated above and below the one-photon ionisation threshold. Above threshold the total cross section for two-photon ionisation has no resonances and diminishes more rapidly than the photoionisation cross section as the wavelength decreases.
Perturbation theory of multiphoton ionisation due to a classical electromagnetic field is modified to allow for intermediate resonances with bound states. Complex energies, generally associated with resonances, do not enter into this formalism. For a monochromatic field of frequency u, a constant ionisation rate can then be defined unambiguously and only such continuum states are excited which correspond to the absorption of energy in integral multiples of hu. As an application, differential and total cross sections for the two-photon ionisation of hydrogen, for frequencies below the n = 3 resonance region, are obtained in closed form. Existing data for generalised cross sections, calculated numerically using the complex coordinate method, are in good agreement with the present results. Finally, the mean fractional ionisation resulting from a pulse of finite duration is estimated on the basis of the associated power spectrum. For short pulses, the time dependence of ionisation exhibits a departure from that expected of a time-independent rate.
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.