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Quantum Electronics and Laser Science Conference

High-order processes in strong-field autoionization

Guanhua Yao, Zhizhan Xu, CHEN RONG-QING

Published May 12, 1991• 0 citations
OpenAlex

Abstract

In the last decade, laser-induced autoionization has been an active subject of Strong laser-atom interaction. The main interest in the subject arises from many interesting high-field effects in the problem, notably the stabilization of the autoionizing state (AIS) and population trapping, which leads to possible applications in lasers without inversion.1 However, many decay mechanisms may destroy this interference effect to various extents. Many authors have addressed this problem, but few 2,3 have investigated the effects of high-order processes in strong-field autoionization.

Related papers

STRONG-FIELD AUTOIONIZATION BY SQUARE LASER PULSES

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Previous studies on strong-field autoionization involving high-order ionization processes are mainly investigated for the cases of CW laser irradiations. This paper extends those studies to the cases of square laser pulses with arbitrary widths. Populations of the ground state and the line shapes of the autoionizing resonance are discussed. It is shown that strong-field autoionization and the high-order ionization processes are drastically affected not only by the intensities but also by the time of interaction between atoms and laser field.

Acta Physica Sinica•Jan 1, 1991•1 citation

Field-induced narrowing of autoionization atomic states as a way of creating population inversion

Svetlana Kotochigova

We discuss the possibility of narrowing the atomic autoionization states via their resonance mixing in a field. The results show that, in contrast to the interference process [Yu. I. Geller and A. K. Popov, Laser Stimulation of Nonlinear Resonances in Continuous Spectra (Nauka, Novosibirsk, 1981)] or to the mixing of two isolated states [A. I. Andryushin et al., Sov. Phys.---JETP 61, 678 (1985)], with mixing of multiplets one may expect substantial narrowing of autoionization states owing to their intersection with bound-electron states.

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Strong-field autoionization induced by smooth laser pulses is studied to take into account the high-order ionization processes. A second-order differential equation of the amplitude of the ground state is derived. For some special laser pulses, which include all the exponential ones, analytical results may be obtained. As an example, the explicit results of ground-state population and the photoelectron spectrum are given for an exponentially switched-on cw pulse. The multipeak structure in the photoelectron spectrum is more obviously manifested in the high spectrum resulted from the high-order ionization processes.

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Mikhail Vladimirovich Fedorov

The key subject of these lectures is Interference Stabilization (IS) of atoms in a strong laser field. Physics of the phenomenon, its various manifestations, as well as many related subtopics will be discussed. Generalizations to the case of two-colour interference stabilization will be briefly described. As it will be shown, owing to the achievable in this case very high degree of stabilization, the phenomenon appears to be very promising for making progress in such an important field as propagation of laser pulses in a gas medium. Since the first work on IS (Fedorov and Movsesian, 1988), there have been a lot of publications on this subject. I am not going to mention here all the relevant papers.

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Interferences of autoionizing lines may reduce or eliminate absorption of lower level atoms. Stimulated emission shows no such interferences, thereby allowing laser gain without population inversion.

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Autoionization of a quasicontinuum: Population trapping, self-trapping, and stabilization

Xin Chen, John A. Yeazell

Three distinct phenomena are found to affect the autoionization of a quasicontinuum. For certain initial superpositions of the states within the quasicontinuum, the autoionization may be inhibited by quantum interference and the population may be trapped. For those initial superpositions that do decay, a self-trapping effect can be observed which causes the superposition to evolve into a trapped state. The self-trapping is due to a redistribution of the population within the quasicontinuum via Raman-like transitions driven by the channel interaction with the continuum acting as the intermediate state. The physical system of a two-electron atom is studied. There the autoionizing continuum is created by an isolated core excitation that embeds a range of highly excited Rydberg states into the continuum. Another mechanism appears, in this system, that affects the autoionization. When the channel interaction is strong compared to the strength of the core excitation a stabilizing effect occurs which slows the autoionization. Analytical models examine these effects in a system with degenerate quasicontinua. Perfect population trapping, self-trapping, and stabilization are all clearly displayed by this system. Numerical models of nondegenerate quasicontinua show that the stabilization process is virtually unchanged by the lifting of the degeneracy. These models also show that, in general, the trapping effects largely disappear. However, a synchronization of the free evolution of the quasicontinuum states with Rabi oscillation of the core excitation returns them to significant roles in the decay process. A wide variety of nondecaying states can be created. Competition between these mechanisms is explored and the impact of these phenomena on experiments is considered.

Physical Review A•Aug 1, 1998•6 citations
In the last decade, laser-induced autoionization has been an active subject of Strong laser-atom interaction. The main interest in the subject arises from many interesting high-field effects in the problem, notably the stabilization of the autoionizing state (AIS) and population trapping, which leads to possible applications in lasers without inversion.1 However, many decay mechanisms may destroy this interference effect to various extents. Many authors have addressed this problem, but few 2,3 have investigated the effects of high-order processes in strong-field autoionization.
Previous studies on strong-field autoionization involving high-order ionization processes are mainly investigated for the cases of CW laser irradiations. This paper extends those studies to the cases of square laser pulses with arbitrary widths. Populations of the ground state and the line shapes of the autoionizing resonance are discussed. It is shown that strong-field autoionization and the high-order ionization processes are drastically affected not only by the intensities but also by the time of interaction between atoms and laser field.
We discuss the possibility of narrowing the atomic autoionization states via their resonance mixing in a field. The results show that, in contrast to the interference process [Yu. I. Geller and A. K. Popov, Laser Stimulation of Nonlinear Resonances in Continuous Spectra (Nauka, Novosibirsk, 1981)] or to the mixing of two isolated states [A. I. Andryushin et al., Sov. Phys.---JETP 61, 678 (1985)], with mixing of multiplets one may expect substantial narrowing of autoionization states owing to their intersection with bound-electron states.
Strong-field autoionization induced by smooth laser pulses is studied to take into account the high-order ionization processes. A second-order differential equation of the amplitude of the ground state is derived. For some special laser pulses, which include all the exponential ones, analytical results may be obtained. As an example, the explicit results of ground-state population and the photoelectron spectrum are given for an exponentially switched-on cw pulse. The multipeak structure in the photoelectron spectrum is more obviously manifested in the high spectrum resulted from the high-order ionization processes.
The key subject of these lectures is Interference Stabilization (IS) of atoms in a strong laser field. Physics of the phenomenon, its various manifestations, as well as many related subtopics will be discussed. Generalizations to the case of two-colour interference stabilization will be briefly described. As it will be shown, owing to the achievable in this case very high degree of stabilization, the phenomenon appears to be very promising for making progress in such an important field as propagation of laser pulses in a gas medium. Since the first work on IS (Fedorov and Movsesian, 1988), there have been a lot of publications on this subject. I am not going to mention here all the relevant papers.
Three distinct phenomena are found to affect the autoionization of a quasicontinuum. For certain initial superpositions of the states within the quasicontinuum, the autoionization may be inhibited by quantum interference and the population may be trapped. For those initial superpositions that do decay, a self-trapping effect can be observed which causes the superposition to evolve into a trapped state. The self-trapping is due to a redistribution of the population within the quasicontinuum via Raman-like transitions driven by the channel interaction with the continuum acting as the intermediate state. The physical system of a two-electron atom is studied. There the autoionizing continuum is created by an isolated core excitation that embeds a range of highly excited Rydberg states into the continuum. Another mechanism appears, in this system, that affects the autoionization. When the channel interaction is strong compared to the strength of the core excitation a stabilizing effect occurs which slows the autoionization. Analytical models examine these effects in a system with degenerate quasicontinua. Perfect population trapping, self-trapping, and stabilization are all clearly displayed by this system. Numerical models of nondegenerate quasicontinua show that the stabilization process is virtually unchanged by the lifting of the degeneracy. These models also show that, in general, the trapping effects largely disappear. However, a synchronization of the free evolution of the quasicontinuum states with Rabi oscillation of the core excitation returns them to significant roles in the decay process. A wide variety of nondecaying states can be created. Competition between these mechanisms is explored and the impact of these phenomena on experiments is considered.