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Physical Review A

Suppression of laser-induced collisional autoionization through electromagnetically induced transparency

Roberto Buffa

Published Jan 1, 1996• 2 citations
Publisher

Abstract

The effect of electromagnetically induced transparency on laser-induced collisional autoionization is investigated. The process, occurring only during atomic collisions and transient in nature, is characterized by the formation of a quasimolecule that can be ionized through the excitation to a pair of interfering dressed states decaying into the same continuum of ionization. Ionization suppression of the order of 70% is predicted for dressing laser intensities of the order of a few tens of MW/${\mathrm{cm}}^{2}$. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.607

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The effect of electromagnetically induced transparency on laser-induced collisional autoionization is investigated. The process, occurring only during atomic collisions and transient in nature, is characterized by the formation of a quasimolecule that can be ionized through the excitation to a pair of interfering dressed states decaying into the same continuum of ionization. Ionization suppression of the order of 70% is predicted for dressing laser intensities of the order of a few tens of MW/cm2{\mathrm{cm}}^{2}cm2. \textcopyright{} 1996 The American Physical Society.
Electromagnetically-induced-transparency-like behavior in the extreme-ultraviolet region is studied theoretically, including the effect of intense 800 nm laser dressing of He 2s2p(1Po{}^{1}\phantom{\rule{-0.16em}{0ex}}{P}^{o}1Po) and 2p2{}^{2}2(1Se){(}^{1}\phantom{\rule{-0.16em}{0ex}}{S}^{e})(1Se) autoionizing states. We present an ab initio solution of the time-dependent Schr\"odinger equation in an LSLSLS-coupling configuration-interaction basis set. The method enables a rigorous treatment of optical field ionization of these coupled autoionizing states into the N=2N=2N=2 continuum in addition to N=1N=1N=1. Our calculated transient absorption spectra show encouraging agreement with experiment.
A theoretical study of laser-induced autoionization of an atomic system in interaction with a time-dependent excitation is presented. Interference effects between the two ionization channels of the atom are found to be responsible for coherent phenomena, such as the enhancement of the photon yield, for a pulsed excitation. An analysis of the pulse-shape effects shows that pumping mechanisms of the proper dressed state, occurring during the rise time of flat-top pulses, can result in an enhancement of the photon yield. An adiabatic approximation based on the use of time-dependent complex dressed states is shown to be adequate to describe the interaction with smooth pulses.
Atomic states may be embedded in the continuum by dressing the atom by an intense laser field. This is often referred to as a laser induced continuum structure (LICS) [1]. This continuum, in general, also has a natural structure due to the doubly excited states, or autoionizing states, of the atom. We have considered the situation in which a complex atom or ion is subjected to an intense laser field such that the LICS states and autoionizing states occur simultaneously. We have have found that, by tuning the frequency and intensity of the field, the states can become degenerate, i.e. have the same position and width.
We present a theoretical study of laser-induced autoionization and laser-induced continuum structures, performed with the aim of deriving predictions on the effect of continuum-continuum transitions on the resonance line shape. The model, developed as an extension of models introduced for the description of laser-induced autoionization and laser-induced continuum structures, on one hand, and for above-threshold ionization, on the other hand, provides quantitative predictions. For specific atomic systems, the line shapes of the structures are shown not to be affected by continuum-continuum transitions, in contrast with previously reported results.