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Physical Review A

Laser-induced bound and metastable states in bound-continuum systems

Kai‐Shue Lam, Thomas F. George

Published Apr 1, 1986• 9 citations
Publisher

Abstract

A monochromatic laser is shown to be capable of inducing a bound state and multiple metastable states not originally present in the field-free spectrum of a bound-continuum system. For the bound state to exist, the field intensity must be larger than a certain frequency-dependent critical value. The widths of the metastable states are found to be in general proportional to renormalized field intensities rather than the physical field intensity. The energies can be classified as being in the metastable or unstable regimes, depending on whether the observed widths are smaller or larger than the corresponding ``bare'' widths predicted by perturbation theory. The relation of the Fano or laser-induced ``autoionization'' states to the scattering states is also demonstrated.

DOI
10.1103/physreva.33.2491

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Physical Review A•Apr 1, 1987•14 citations
A monochromatic laser is shown to be capable of inducing a bound state and multiple metastable states not originally present in the field-free spectrum of a bound-continuum system. For the bound state to exist, the field intensity must be larger than a certain frequency-dependent critical value. The widths of the metastable states are found to be in general proportional to renormalized field intensities rather than the physical field intensity. The energies can be classified as being in the metastable or unstable regimes, depending on whether the observed widths are smaller or larger than the corresponding bare'' widths predicted by perturbation theory. The relation of the Fano or laser-induced autoionization'' states to the scattering states is also demonstrated.
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.
Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.
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.
Absorption properties of a system consisting of two autoionizing states coupled strongly either by internal interactions or by external fields are investigated. The autoionizing states may themselves correspond to those induced by laser fields as in resonant multiphoton ionization. The analysis is based on the density matrices and incoherent processes like spontaneous emission and radiative recombination are included. Optical susceptibilities exist in a number of special cases for autoionizing transitions. Exact results for such optical susceptibilities are obtained. Explicit numerical results for susceptibilities, corresponding traditionally to strong-field inverse Raman effect, are given with a view to recent work on dc-field-coupled autoionizing states and on laser-induced autoionization. The behavior of both absorptive and dispersive parts is discussed for a range of parameters.