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Journal of Physics B Atomic Molecular and Optical Physics

Effect of the multiphoton process and spontaneous emission on laser-induced autoionisation

S F Li, F C Lin

Published Apr 28, 1989• 3 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/22/8/011

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Journal of Physics B Atomic and Molecular Physics•Oct 14, 1982•7 citations
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.
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.
The authors study doubly resonant multiphoton ionisation induced by two laser fields. The first field produces a near-resonant multiphoton transition between the ground state g and a discrete state 0. The second field is nearly resonant with a one-photon transition between 0 and an autoionising state 1. The spontaneous emission of both excited states is taken into account. The authors discuss and compare the spectral profiles expected for ionisation probability and fluorescence from state 1 when the first step laser is scanned.
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.
Two-laser multiphoton ionisation under the autoionising-like resonance condition is studied theoretically within an extended atomic model dictated by the experiment of Feldmann et al (1986). Ordinary coupling between a selected pair of bound states via the continuum only is supplemented in the author's model by Raman-type coupling involving the atomic spectrum below the ionisation threshold. In the case considered this additional Raman coupling enhances the ionisation rate to a much greater extent than the coupling via the continuum can do. The Raman coupling modifies the autoionising-like shape parameter and is decisive as far as the resonance peak profile is concerned. Exploiting this Raman coupling within the atomic model with four effective bound states, the author gives explanations for all the details of the ionisation spectrum detected by Feldmann et al, i.e. the number of resonance peaks, their strength, fine structure, shape, relative heights and sensitivity to the laser field.
The authors investigate two-photon ionisation using a generalised rate equation technique for a model in which the atom is assumed to have only two bound levels (the ground state and an excited state) together with a simple continuum of ionised states. Spontaneous emission is allowed from the excited to the ground state, and the laser is assumed to have a finite bandwidth due to phase diffusion. The photoelectron energy distribution is calculated; it is found that if the Rabi frequency exceeds the ionisation rate the energy spectrum is a sensitive function of the linewidth and spontaneous emission rate, whereas if the ionisation rate exceeds the Rabi frequency then the sensitivity of the energy spectrum to these parameters depends upon the initial state of the system. A quite small value of laser linewidth is found to considerably reduce the heights of peaks and increase the widths. Analytic and numerical results are presented.