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Journal of Physics B Atomic and Molecular Physics

The influence of radiative collisions on two-photon atomic ionisation

Michele Crance, S. Feneuille

Published Aug 28, 1980• 7 citations
Publisher

Abstract

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).

DOI
10.1088/0022-3700/13/16/014

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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).
Laser-induced collisional ionisation of caesium is observed in a strontium-caesium vapour mixture. In their initial state both atomic species are in the ground level. The ionisation of caesium is achieved when the system is illuminated by an intense laser light, the frequency of which is tuned across the resonance line of strontium. The process involves photoabsorption during the collisional interaction. It is monitored by the observation of optical fluorescence after Cs + , e - recombination. The laser-induced collisional ionisation profile, i.e. the variation of the ionisation yield against the laser wavelength is recorded under various experimental conditions.
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.
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.
The first analysis is made of stimulated Raman scattering and two-photon absorption (emission) processes resulting in a change in the states of two colliding particles. These processes are caused the by interaction between dipole moments induced as a result of polarization of the particles by the radiation. Equations are derived to describe multiphoton transitions during a collision between two particles in a radiation field. Using the approximation of rectilinear trajectories, expressions are derived and analyzed for the cross section of collisional two-photon transitions in weak and strong fields. It is shown that in fields of ≈103 cgs esu, the cross section of these transitions may reach 10–4 cm2
An expression is derived for the amount of two-photon ionisation produced by sub-nanosecond laser pulses under resonance conditions. On this time scale damping effects can be neglected and the ionisation probability becomes proportional to the cube of the pulse duration. A similar prediction for resonance multiphoton processes of higher order leads to a qualitative agreement with recent experimental observations. Finally, it is demonstrated that the same result can be obtained from a simple analysis of a two-step model.
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.