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Physical Review A

Effect of a laser field on resonant photoionization and recombination processes

Leonard Rosenberg

Published May 1, 1995• 3 citations
Publisher

Abstract

The effect of a low-frequency laser field on photoionization and radiative recombination processes is studied theoretically, with particular attention paid to the combined influence of the electron-ion Coulomb interaction and the laser field on continuum states of the system. A procedure for representing the asymptotic states of the system, with both long-range interactions properly accounted for, is described. The transition rates are strongly affected by the existence of a Rydberg series of resonances. Methods developed for describing similar resonant processs that occur in the field-free case are generalized here to include the effects of the external laser field. A simple model, based on parametrizations of the type employed in quantum-defect theory, is used to study the modification of resonant structures in the cross section introduced by the external field. In the weak-field limit, a low-frequency approximation for bound-state Compton scattering is obtained, with resonance effects accounted for.

DOI
10.1103/physreva.51.3676

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The effect of a low-frequency laser field on photoionization and radiative recombination processes is studied theoretically, with particular attention paid to the combined influence of the electron-ion Coulomb interaction and the laser field on continuum states of the system. A procedure for representing the asymptotic states of the system, with both long-range interactions properly accounted for, is described. The transition rates are strongly affected by the existence of a Rydberg series of resonances. Methods developed for describing similar resonant processs that occur in the field-free case are generalized here to include the effects of the external laser field. A simple model, based on parametrizations of the type employed in quantum-defect theory, is used to study the modification of resonant structures in the cross section introduced by the external field. In the weak-field limit, a low-frequency approximation for bound-state Compton scattering is obtained, with resonance effects accounted for.
The theory of resonant reactions, in the projection-operator formulation of Feshbach, is generalized to account for the presence of an external electromagnetic field. The theory is used as the basis for the construction of low-frequency approximations for the transition amplitude. Results obtained here for scattering in a laser field confirm earlier versions of the low-frequency approximation when the resonances are isolated. However, if there are several closely spaced resonances additional terms must be included (their importance magnified by the appearance of near singularities) which account for the effect of radiative transitions between pairs of nearly degenerate resonant states. The weak-field limit of this result yields a low-frequency approximation for single-photon spontaneous bremsstrahlung which, through the inclusion of correction terms associated with closely spaced resonances, provides an improvement over the Feshbach-Yennie version derived some time ago. A separate treatment is required to deal with the limiting case of a static external field and this is worked out here in the context of a time-dependent formulation of the scattering problem. Linear and quadratic Stark splitting of the resonance positions, and resonance broadening due to the tunneling mechanism, are expected to play a significant role in the static limit and these effects are included in the approximation derived here for the transition amplitude.
We develop a theory describing laser excitation of a Rydberg series by an intense laser field. Our theory is based on the fact that the radiative coupling is restricted to a region around the atomic nucleus (the reaction zone) which is small in comparison with the extent of the excited Rydberg states. The physical processes inside the reaction zone are characterized by a few parameters which are slowly varying functions of energy across the Rydberg threshold. This finite range of the radiative coupling allows us to apply methods from quantum-defect theory and to treat the laser interaction with the Rydberg series and the adjoining electron continuum as a whole. Within this approach we derive analytical expressions for transition probabilities as a function of time in either a dressed-state representation which is appropriate as long as only a few Rydberg states are excited or a multiple-scattering expansion which is particularly suited for a description of the excitation process close to threshold. The physical picture emerging in this limit is one of a radial electronic wave packet which is generated within the reaction zone and moves in the Coulomb potential of the ionic core. Every time it returns to the inner turning point of its orbit, i.e., to the reaction zone, it is either deexcited back to the initial atomic state or is scattered by the ionic core.
A theory describing the radiative decay of an autoionizing state under strong pumping by a coherent field is developed. The theory systematically takes into account the radiative decay of the unperturbed continuum. The problem at hand corresponds to the case of strongly coupled bound states decaying to electron and photon continua with the two continua also weakly coupled to each other. A master equation describing the time evolution of the atomic system is derived, and its exact solution under arbitrary initial conditions is given. The effect of radiative decay on the Fano profiles and photoelectron spectra is analyzed in detail. The time development of the system is also examined. The radiative decay of the autoionizing state and the unperturbed continuum changes the spectra in a significant way. The characteristics of the spectra are correlated with the dressed states (with complex energies) of the system. The changes in the structure of the dressed states as a function of the system parameters such as the spontaneous-emission rate and laser intensity are discussed in detail.
In a system consisting of an electron and a highly charged ion, interaction with the radiation field is strong, and radiation-damping effects must be accurately taken into account. The present work provides a numerical method to study the radiation-damping effect in the processes of photoionization, radiative recombination, and electron scattering. A simple calculation is made for Fe24+{\mathrm{Fe}}^{24+}Fe24+, Xe52+{\mathrm{Xe}}^{52+}Xe52+(1s2s)+h\ensuremath{\nu}\ensuremath{\leftrightarrows}Fe25+{\mathrm{Fe}}^{25+}Fe25+, Xe53+{\mathrm{Xe}}^{53+}Xe53+(1s)+e. Special emphasis is placed on the nonapplicability of conventional first-order perturbation theory to photoinduced autoionization of a highly charged ion. The interference effect between the resonant and the nonresonant parts is found to be negligible in the present cases.
Excitation and photoionization of Rydberg wave packets are described. Interference of bound-free transitions from many coherently populated Rydberg levels is shown to affect the dependence of the photoionization probability on the duration of interaction. Strong-field interference effects in such a system are described also. The most important of these is the field-induced interference stabilization of Rydberg atoms in a strong ionizing field. The intensity range in which this effect can exist is determined. The results derived are used for a qualitative model describing supercontinuum emission in the process of multiphoton ionization.