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Journal of Physics B Atomic and Molecular Physics

Laser-induced autoionisation in the presence of additional atomic continua

Jakub Zakrzewski

Published Mar 14, 1984• 8 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/17/5/012

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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.
Atoms in an autoionisation state can be excited to higher states in the continuum as well as performing spontaneous transitions to bound states by laser-induced autoionisation. Both effects are investigated by means of the reaction matrix. Results obtained by Armstrong et al (1978) using the same method are extended. The shape of the photoelectron spectrum is different from that of the photon spectrum emitted from the autoionisation state but is similar to that of the photon spectrum emitted from the same bound state which is excited to the ionisation states in the former case. It is predicted that the shape of the fluorescence yield with the pump laser detuning is Lorentzian. The multiphoton process contributes to the photoelectron spectrum in a complicated way.
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.
The atoms at autoionization state can be excited to higher continuum state, as well as transit to bond states by spontaneous emission a photon in laser induced autoionization. The effect of these processes on the laser induced autoionization process has been investigated by means of using the reaction matrix. The effect of spontaneous emission process is small in general case, but its spectrum shows that the emission is contributed mainly from the spontaneous Raman scattering enhanced by the cutoioni-zation state, and the Dressed atom picture was shown clearly. The multiphoton process makes the photoelectron spectra and laser induced autoionization process very complicated. For example, the photoelectron spectra of one and two-photon process have multi-maxium structure.
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 continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.