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Physical Review A

Multiphoton excitation of autoionizing states of Mg: Line-shape studies of the 3 p2 S1 state

R. E. Bonanno, Charles W. Clark, Thomas B. Lucatorto

Published Sep 1, 1986• 50 citations
Publisher

Abstract

We have observed ionization of Mg by both direct and stepwise two-photon excitation of the 3${p}^{2}$ ${\mathrm{}}^{1}$S state. The line shape of the single-color direct process is strongly modified by the resonance denominator associated with the intermediate virtual state. The measured energy and width of this resonance as determined by the stepwise two-color technique agree well with previous determinations.

DOI
10.1103/physreva.34.2082

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Journal of the Optical Society of America B•Sep 1, 1987•1 citation
We have observed ionization of Mg by both direct and stepwise two-photon excitation of the 3p2{p}^{2}p2 1{\mathrm{}}^{1}1S state. The line shape of the single-color direct process is strongly modified by the resonance denominator associated with the intermediate virtual state. The measured energy and width of this resonance as determined by the stepwise two-color technique agree well with previous determinations.
We present results from perturbative calculations on a scheme of coherent control in Mg. The scheme involves the excitation of Mg in the vicinity of an autoionizing resonance lying above the first two ionization thresholds by a two-color field composed of a fundamental frequency and its third harmonic, whose relative phase can be continuously controlled. Four as well as two-photon coherent excitation pathways, involving appropriate combinations of the two frequencies, proceed through the atomic continuum simultaneously contributing to the excitation, while three-photon ionization by the fundamental is also energetically possible. As a result, some of the excitation processes involve above-threshold ionization (ATI). The calculated ionization yields for the various groups of photoelectrons reveal significant modulation as the relative phase of the field component frequencies is varied. The modulation patterns for the different photoelectron groups are mutually shifted due to the multiphoton matrix element phases associated both with the ATI process and the multichannel nature of the final state. The effect is particularly pronounced when the autoionizing state is in near four-photon resonance with the fundamental frequency. The role of an autoionizing state as a final state as well as that of the continuum as an intermediate state in coherent control processes is discussed.
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Multiphoton ionization of Mg was studied with an excimer laser (XeCl, λ = 308 nm). The laser-intensity dependence of the number of ions indicates that the lowest-order two-photon process, which has enough energy to reach the first ionization limit, fails to dominate the ionization of Mg. The slope is 3 when a low-enough extracting field is applied and increases with the latter. A three-photon resonance with some autoionizing states is proposed as a partial explanation of this abnormal phenomenon, but the variation of the slope of the log N versus log I curve as a function of a dc electric field remains unexplained.