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Journal of Physics B Atomic and Molecular Physics

Population trapping in the laser excitation of a realistic autoionising level

P. Thornton Greenland

Published Sep 28, 1982• 28 citations
Publisher

Abstract

Photoionisation by a strong laser tuned to the neighbourhood of an autoionising level is discussed. An explicit model for the continuum dipole strength is used which reproduces the important features of a real atom. Within the context of this model the problem is solved exactly. The exact solution is related to the usual pole approximation via a Taylor expansion which also yields an improved pole approximation. Population trapping is observed to occur and the general condition for it is derived. An interpretation of population trapping as the strong field relic of the Fano minimum is given. Some remarks are made on the consequences of population trapping for electron-ion scattering in the presence of a laser.

DOI
10.1088/0022-3700/15/18/015

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Journal of Physics B Atomic and Molecular Physics•Dec 14, 1986•18 citations
Photoionisation by a strong laser tuned to the neighbourhood of an autoionising level is discussed. An explicit model for the continuum dipole strength is used which reproduces the important features of a real atom. Within the context of this model the problem is solved exactly. The exact solution is related to the usual pole approximation via a Taylor expansion which also yields an improved pole approximation. Population trapping is observed to occur and the general condition for it is derived. An interpretation of population trapping as the strong field relic of the Fano minimum is given. Some remarks are made on the consequences of population trapping for electron-ion scattering in the presence of a laser.
We study a theoretical model of photoionization. An initially occupied ground electronic state is ionized to a flat continuum by two-photon excitation. There are two near-resonant intermediate excitation states. The line-narrowing confluence effect and population trapping are studied. We show that we cannot define an effective Hamiltonian to relate our model to the Fano model. In connection with recent studies of laser-induced autoionization (LIA), we show several differences between our model and LIA. For example, we show that the zero in the absorption spectrum and in the photoelectron spectrum has different features compared with that in the Fano model [ Phys. Rev.124, 1866 ( 1961)]. We also show that a laser-power-independent confluence condition and a complete population-trapping condition can be found in our model.
Laser-induced autoionisation is discussed in the simple model in which the autoionising level is embedded in several atomic continua. The coupling to the second continuum can drastically diminish the characteristic narrowing in the photoelectron spectra near the confluence of coherences point. The results seem to be important for possible experimental verification of this narrowing, as in the real atom such coupling is hardly avoidable. The similarity of the effects caused by the spontaneous emission and by the coupling to the second continuum is pointed out.
Strong laser-field-induced autoionisation in the presence of both photoionising and radiative decay of the autoionising state (AS) is investigated, focusing on the laser intensity dependence of the photoemission and photoelectron spectra. In contrast to previous predictions, power broadening and increasing reduction of the doublet peak heights with field strength are found in the photoemission spectrum. Similar effects leading to considerable suppression and even complete disappearance of the lowest-order peaks in the photoelectron spectrum, together with peak switching, are also demonstrated, which are closely related to above-threshold ionisation. In addition, it is suggested that the total number of energetic photoelectrons may serve as an alternative to measuring the atomic parameters of the AS. All these effects are attributed to the presence of the strong 'probe': laser-induced decay of the AS.