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Journal of the Optical Society of America B

Two-color above-threshold ionization: extension to multiple continua

Roberto Buffa, Stefano Cavalieri, R. Eramo, M. Matera

Published Mar 1, 1996• 3 citations
Publisher

Abstract

We present a discussion of the two-color above-threshold ionization process, performed with the aim of studying strong-field continuum–continuum transitions. We extend the approach of Rzażewski [ J. Opt. Soc. Am. B7, 481 ( 1990)] to obtain analytically solvable equations for the multiple continuum model analyzed by Deng Eberly [ J. Opt. Soc. Am. B2, 486 ( 1985)]. Some comments on the effect of continuum–continuum transitions on the line shape of a laser-induced continuum structure are also made.

DOI
10.1364/josab.13.000492

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We present a discussion of the two-color above-threshold ionization process, performed with the aim of studying strong-field continuum–continuum transitions. We extend the approach of Rzażewski [ J. Opt. Soc. Am. B7, 481 ( 1990)] to obtain analytically solvable equations for the multiple continuum model analyzed by Deng Eberly [ J. Opt. Soc. Am. B2, 486 ( 1985)]. Some comments on the effect of continuum–continuum transitions on the line shape of a laser-induced continuum structure are also made.
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.
We solve a model with one bound state and two continua.1 Two lasers are used to study the above threshold ionization process. The first laser couples the electron in a bound state to a continuum state. The photons are assumed to be energetic enough to promote the electron directly into the continuum far from the threshold; this avoids complications due to the Rydberg levels and the ponderomotive shifts of the threshold. The second laser mixes the electron wavefunction between two continuum states (C—C transitions); since we avoid the rotating wave approximation, multiple transitions between these continua are allowed. We find that the ionization rate is unaffected by the C—C transitions. Our model of hydrogenic atoms allows redistribution among the continua only during the ionization process and the photoelectron spectra exhibits peak suppression; i.e., the population is not the strongest at the energy of the ionization laser.
We present an analytical solution to a multilevel model of light-induced continuum structure, in which, instead of a single level, a quasicontinuum of excited levels is laser embedded into a previously structureless atomic continuum. The found solution is fully time dependent and is valid for probing strengths of the structured continuum from the ground level by another laser, which can vary from being much weaker than the embedding one to being as strong as the embedding one. For three different embeddings (weak, moderate, and strong), each one determined by a different relation between the ionization rate \ensuremath{\gamma} and the spacing \ensuremath{\Delta} of the quasicontinuum levels (\ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\ll}1, \ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\simeq}1, and \ensuremath{\Elzxh}\ensuremath{\gamma}/\ensuremath{\Delta}\ensuremath{\gg}1), we study the evolution of the ionization spectrum with increasing probing strength. We observe and interpret such strong-field effects in the spectra as (i) conversion of a Fano-like train of peaks into a train of dips; (ii) peak moving, narrowing, and lowering; and (iii) saturation, population trapping, and stabilization against ionization.
A model of atomic ionization by smooth pulses is solved with bound-continuum and energy-dependent continuum-continuum (C-C) dipole coupling. The rotating-wave approximation for the C-C transitions is avoided. In contrast with the flat continuum result, our ionization rate is independent of the C-C coupling. We also derive the photoelectron energy spectra and show that counterrotating terms make important contributions. Relation to above-threshold ionization experiments with two frequencies is discussed.