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Journal of Physics B Atomic and Molecular Physics

Atomic ionisation by strong coherent radiation

Humberto Siqueira Brandi, L. Davidovich

Published Oct 28, 1979• 8 citations
Publisher

Abstract

The authors establish a relation between the three most frequently used non-perturbative methods proposed to study the ionisation of atoms by strong electromagnetic fields and determine the range of validity of these methods.
DOI
10.1088/0022-3700/12/20/005

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The two-photon ionisation cross section of potassium is calculated from a quasi-degenerate perturbation theory expansion in the atom-radiation field interaction. The authors assume a narrow bandwidth radiation field of moderate photon flux (1025-1029 photons/cm2 s). Resonances are treated by first diagonalising the Hamiltonian among the strongly coupled resonant states and then applying perturbation theory to include nonresonant background effects. All bound and continuum atomic states are calculated in the non-relativistic Hartree-Fock approximation and the various summations over intermediate states are handled by the inhomogeneous differential equation technique. Results for potassium show that resonance structures, including fine structure, are well resolved for a photon flux of 1027 photons/cm2 or less. As the flux increases to 1029, the resonance enhancement is substantially quenched.
Non-resonant k-photon ionisation cross sections may be calculated by kth order perturbative treatment. This method is generally considered to be good when the detuning from the closest resonance is much larger than the expected shift and width of this resonance. The aim of this paper is to discuss the validity of this criterion for several realistic cases. By diagonalising an effective Hamiltonian built on a few quasi-resonant states, we have calculated multiphoton ionisation cross sections for ground state caesium by linearly polarised light in frequency ranges containing the tuning range of the ruby laser, the Nd glass laser and its first harmonic. The study is carried out up to an intensity of 10 11 W cm -2 .
By using a perturbative treatment based on the central field approximation, the authors have calculated multiphoton ionisation probabilities in several cases of experimental interest (two-, three- and four-photon ionisation of the caesium and potassium ground states at the doubled Nd glass laser frequency and one-, two- and three-photon ionisation of some s, p, d and caesium Rydberg states). Some of these probabilities have been calculated beyond the minimum non-vanishing order. They have also performed some calculations for hydrogen. The comparison of their results with previous ones obtained by analytical methods provides a good test of the numerical techniques involved in the calculations.