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Journal of Physics B Atomic and Molecular Physics

Resonance lineshape and photoelectron spectrum in power broadened two-photon ionisation

L Armstrong, Stephen V. O'Neil

Published Mar 28, 1980• 17 citations
Publisher

Abstract

The authors study the resonance lineshape in two-photon ionisation of atoms when the power broadening due to the bound-bound transitions is by far the dominant broadening mechanism. The authors consider the effect on the lineshape of turn-on of the laser-atom interaction, spontaneous emission and laser linewidth, and show that several different types of lineshapes can result depending on the conditions involved. The authors also consider the photoelectron spectrum, and show that it is also sensitive to these same parameters.

DOI
10.1088/0022-3700/13/6/018

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Journal of Physics B Atomic and Molecular Physics•Aug 28, 1980•7 citations
The authors study the resonance lineshape in two-photon ionisation of atoms when the power broadening due to the bound-bound transitions is by far the dominant broadening mechanism. The authors consider the effect on the lineshape of turn-on of the laser-atom interaction, spontaneous emission and laser linewidth, and show that several different types of lineshapes can result depending on the conditions involved. The authors also consider the photoelectron spectrum, and show that it is also sensitive to these same parameters.
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.
According to laser theory the lineshape of a laser broadened by Ornstein-Uhlenbeck phase fluctuations is non-Lorentzian. The equations describing the interaction of such a laser with atomic systems are cast into a form which has the same structure as the density-matrix equations in the absence of fluctuations but with modified detunings, damping parameters and interaction matrix elements. By these means the authors derive substitution rules for taking into account the effect of these non-Lorentzian lineshapes which generalize those already known for Lorentzian lineshapes arising from Wiener-Levy fluctuations. The theory is applied to two-photon ionisation and analytic expressions are obtained for the rate of this process. For the extended two-level model the authors' results agree with those of Yeh and Eberly (1981) in the appropriate limit, but they also consider a more general model which leads to enhanced single-photon ionisation rates.
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 author examines some simple models for multiphoton ionisation which can be constructed when the bound-free transition rates are much smaller than the bound-bound transition rates and considers how to proceed when this condition is relaxed. The author identifies a regime where the transition rate is time independent. If the laser has a sufficiently broad linewidth one can use a rate equation approach; this assertion is justified and applied to two-, three- and N-photon ionisation. If the laser is narrow-band, a density matrix method has to be used and the effects of different laser lineshapes are discussed. In three-photon ionisation the role of optical pumping effects is emphasised. Explicit expressions for the multiphoton ionisation probabilities are presented and finally some extensions of the method are briefly discussed.
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.
Investigates theoretically a new aspect of radiative collisions on two-photon ionisation of colliding atoms of different species A and B. The authors derive the main characteristics of the resonance appearing in the two-photon ionisation cross section of atom B when the laser frequency is tuned on a resonance transition of atom A which cannot itself be ionised by two-photon absorption. Under some realistic conditions and for reasonable values of the light energy density flux F ( approximately 1 MW cm -2 ), the cross section of the process considered varies as F 2/5 while its width varies as F 1/2 . These predictions are in reasonable agreement with a recent observation on the mixture Sr(A)-Cs(B) by Brechignac et al. (1980).