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Physical Review Letters

Manipulation of the line shape and final products of autoionization through the phase of the electric fields

Takashi Nakajima, P. Lambropoulos

Published Feb 22, 1993• 88 citations
Publisher

Abstract

We show that the autoionization line shape can be modified by the choice of two laser intensities and the relative phase. Under well specified conditions, proper choice of the phase can lead to selective cancellation of the transition to the discrete or the continuum part of the state. Application of the idea to a multichannel problem in the rare gases also provides significant effects.

DOI
10.1103/physrevlett.70.1081

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We show that the autoionization line shape can be modified by the choice of two laser intensities and the relative phase. Under well specified conditions, proper choice of the phase can lead to selective cancellation of the transition to the discrete or the continuum part of the state. Application of the idea to a multichannel problem in the rare gases also provides significant effects.
We present a formal theory and detailed calculations for phase-dependent laser-atom interactions involving autoionizing states. First, through simple models, we demonstrate that the simultaneous one- and three-photon excitation of one or two neighboring autoionizing states can exhibit profound changes of the line shape, as the relative phase of the two fields is varied from 0 to \ensuremath{\pi}. Through a proper choice of the field intensities and the phase, we obtain analytical results showing that one can cancel the transition to the discrete or the continuum part of the wave function, thereby leading to a flat or a completely symmetric line shape, respectively. At higher intensities, additional effects come into play, providing additional coupling between the discrete and continuum parts, which also exhibits a phase dependence. Finally, our theory is applied to a much more complex situation in Xe, involving many channels, not amenable to simple analytical expressions, but exhibiting nevertheless equally profound effects, including a modification of the branching ratio of two different products. The theory, which is here developed for atomic autoionization, is in fact fairly general and should pertain to related problems in any system involving discrete states embedded in continua.
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.
The effect of a laser field on the cross section of photoionization of an atom by probe radiation in the region of closely lying autoionizing resonances is studied theoretically. The cross sections in the region of the overlapping autoionizing Rydberg states 5p 5(2 P 1/2)6d′ J = 1 and 5p 5(2 P 1/2)8s′ J = 1 resonantly coupled with the discrete state 5p 5(2 P 3/2)7p[1/2]1 in the xenon atom are calculated. The behavior of the shape and position of resonances in relation to the frequency and intensity of laser radiation is studied.
Experimental investigations of the laser-induced continuum structure in the vicinity of autoionizing states (AIS's) of Ca are reported for both ionization and third-harmonic generation. The shape of the induced structures is shown to be strongly affected by the presence of the AIS's, as well as exhibiting a detuning- and laser-intensity-dependent transition from a window resonance dip to a peak. The observed modification of the continuum is discussed in terms of interfering excitation channels.
The change of the spectral parameters of a single autoionization resonance with concentration in gaseous media is studied in terms of the isotropic Lorentz-Lorenz model. It is shown that the effect of a local field may result in a shift, narrowing, or broadening of the resonance, as well as in a change in its amplitude and the character of interference of the discrete state with the continuum. By appropriate selection or variation of the partial pressures of substances in the mixture, the separation of a particular regime is possible. An analogy to the laser-induced narrowing of autoionization resonances is drawn.