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Physical Review A

Radiative collision-induced electron continuum-continuum scattering

Munir Hasan Nayfeh, David B. Geohegan

Published Sep 1, 1983• 0 citations
Publisher

Abstract

The photoionization of an atom is examined in the presence of a foreign atom. We find that a two-photon radiative collision process induces large cross sections for electron continuum-continuum scattering. The enhancement is caused by an intensity-induced collisional shift which, when used to cancel out the collisional dephasing between the initial and final scattering states at wide range of internuclear separations, enhances the electron continuum-continuum scattering process.

DOI
10.1103/physreva.28.1395

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The photoionization of an atom is examined in the presence of a foreign atom. We find that a two-photon radiative collision process induces large cross sections for electron continuum-continuum scattering. The enhancement is caused by an intensity-induced collisional shift which, when used to cancel out the collisional dephasing between the initial and final scattering states at wide range of internuclear separations, enhances the electron continuum-continuum scattering process.
Two-color two-photon photoionization of hydrogen and alkali atoms is presented. The method based on a perturbation expansion is valid for low to moderately intense radiation fields. Three processes are considered leading to the photoionization. For the first process both photons are below threshold, and for the second process one photon is above the threshold with an energy less than the final energy of the photoelectron. The last process is identical to the continuum Raman (Compton) scattering where the first photon is absorbed high in the continuum and the second low energy photon is scattered. Photoionization cross sections of hydrogen and the alkali atoms are given as functions of the first photon energy and the photoelectron energy.
Semiclassical expressions of two-photon ionization of atoms induced by radiative collisions are derived. The dependence of the ionization yield on the atomic forces, field intensity, and energy gap is derived. Although absorption tends to decrease as the field intensity rises due to stimulated emission at the second crossing, the two-photon ionization yield can be nearly saturated at the first crossing, thus enhancing the absorption. Both regions, ionization in single collisions and ionization between collisions, are treated. In the latter we find that saturation of the ionization can be achieved at much reduced intensities. This process promises an extremely sensitive method for studying radiative collisions, especially when absorption or fluorescence becomes extremely weak.
Process of photon scattering on atoms is considered at energy above the ionization threshold. Influence of the resonant structure in the continuum, induced by a laser field due to coupling with the level 41 S in helium, on the cross sections of elastic and inelastic scattering of a probe radiation on atom in the meta-stable state 21 S is theoretically investigated. In the rotating wave approximation approach dependences of resonant scattering cross sections on the probe and laser radiation characteristics are obtained. The frequency and polarization dependences of the scattering cross sections to the final states of atom 11 S, 21 S and 41 S are calculated.
Formulas are derived for the differential and integral cross sections associated with laser-induced excitation transfer. The theory is patterned closely after our recent treatment of laser-induced chemi-ionization. It is found that cross sections specific to single-photon absorption are proportional to the square of the cosine of the angle between the laser polarization and the initial relative velocity of the two colliding atoms. The distortion by a laser of the cross section for an elastic scattering event also is treated. Finally, a cross-section formula is derived for a collision-induced two-photon absorption, mediated by a single intermediate electronic state.
We have measured the near-threshold cross sections for the photodetachment of ${\mathrm{Li}}^{\ensuremath{-}}$ into the ${e}^{\ensuremath{-}}(ks,d)\text{\ensuremath{-}}\mathrm{Li}(2{p}_{1∕2})$, ${e}^{\ensuremath{-}}(ks,d)\text{\ensuremath{-}}\mathrm{Li}(3p)$, ${e}^{\ensuremath{-}}(kp)\text{\ensuremath{-}}\mathrm{Li}(3s)$, and ${e}^{\ensuremath{-}}(kp)\text{\ensuremath{-}}\mathrm{Li}(4s)$ continuum channels and the photodetachment of ${\mathrm{K}}^{\ensuremath{-}}$ into the ${e}^{\ensuremath{-}}(ks,d)\text{\ensuremath{-}}\mathrm{K}(4{p}_{3∕2})$ continuum channel. The data was analyzed using both a top-of-barrier model and a modified effective range theory. Both approaches explicitly take into account the significant polarization potential in the final continuum channels. The study involving both experiment and theory enabled us to investigate the effect of the polarization on near-threshold photodetachment. Furthermore, since photodetachment and electron scattering are related in the half-collision concept, we were able to determine low-energy electron scattering quantities such as phase shifts, elastic cross sections, and sss wave scattering lengths for electron scattering on excited Li and K atoms. The results allowed us to predict a virtual state in the ${e}^{\ensuremath{-}}(ks)\text{\ensuremath{-}}\mathrm{Li}(3p)$ system.