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Physical Review A

Photoemission spectra in intense laser field induced autoionization

G. S. Agarwal, Stanley L. Haan, Keith R. Burnett, J. Cooper

Published Oct 1, 1982• 47 citations
Publisher

Abstract

Photoemission spectra from autoionizing states in the presence of a strong laser field are calculated. Such spectra have very sharp features near "confluence" (at the Fano minimum) and are very similar, in structure, to photoelectron spectra. The total intensity, as a function of laser intensity or detuning, has a peak near confluence, which can be used, among other things, for the accurate determination of Fano asymmetry parameter $q$.

DOI
10.1103/physreva.26.2277

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Physical Review Letters•Apr 23, 1984•5 citations
Photoemission spectra from autoionizing states in the presence of a strong laser field are calculated. Such spectra have very sharp features near "confluence" (at the Fano minimum) and are very similar, in structure, to photoelectron spectra. The total intensity, as a function of laser intensity or detuning, has a peak near confluence, which can be used, among other things, for the accurate determination of Fano asymmetry parameter qqq.
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.
Spectral features of the radiation emitted by autoionizing states in the presence of a strong laser field are calculated under the condition of multiple photon absorption and emissions. The method is based on the exact solution of the master equation for the recycled system. The decay spectra are found to exhibit sharp maxima at the laser frequency and, for large values of the Fano qqq value, sidebands which are reminiscent of the triplet which one finds in the context of a two-level atom.
The fluorescence produced by an autoionizing state under strong pumping by a coherent laser field is considered. It is shown how the spectrum of emitted photons and the total number of photons are related to the two-time and single-time atomic expectation values, and explicit expressions are derived for them. Numerical results for the time development of the number of photons ejected are presented for finite times and in the long-time limit. The photon spectra are shown to exhibit ac Stark splitting of the states involved. The spectrum is a doublet for transitions to a stable final state and a triplet for transitions connected back to the initial, strongly driven state. The decay of an autoionizing state in the absence of a laser driving field is considered, and the square of the Fano qqq value is interpreted as the ratio of two decay rates.
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.
Photoelectron ionization spectra of an autoionization system interacting with a neighboring two-level atom are studied using the method of canonical transformation. Conditions for the occurrence of a Fano zero are exactly derived, together with its frequency. Ionization spectra are typically composed of at most four peaks in this case. Sharp peaks occur not far from the Fano zero due to the confluence of bound-free coherences. Spectral peaks close to the Fano zero are suppressed. Also dynamical zeros appearing once per the Rabi period in conditioned photoelectron ionization spectra are observed.