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Physical Review A

Dressed-resonance representation for strong photoexcitation of continuum states with application to laser-enhanced autoionization

Dan Agassi, Joseph H. Eberly

Published Oct 1, 1986• 12 citations
Publisher

Abstract

We introduce in the context of a simple soluble model for laser-enhanced autoionization a new representation---the dressed-resonance representation (DRR). The model includes one autoionizing state, one flat electron continuum, one continuous driving laser of arbitrary strength, and two types of relaxations, i.e., spontaneous decay of the continuum and autoionizing state back to the initial state (spontaneous recycling) and phase-jitter noise in the driving laser. The DRR is a natural extension of the dressed-states representation, employed in strong-field discrete-discrete transitions, to discrete-continuum transitions. We focus on two observables, i.e., the temporal initial state population and the photoelectron spectrum. The emphasis is on strong-field effects. The DRR yields simple, transparent, analytical expressions for the above observables in the no-relaxations limit of the model. These results are complemented by a systematic analysis of the exact solution in all qualitatively distinct physical regimes.

DOI
10.1103/physreva.34.2843

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Effect of continuum-continuum and quasibound-continuum above-threshold transitions on (1+1)-photon resonance-enhanced multiphoton autoionization

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We have studied the effect of above-threshold transitions on the (1+1)-photon resonance-enhanced ionization and autoionization processes. We have shown that the effect of the photoionization decay of the autoionizing (AI) state to the higher continuum becomes dominant when the photoionization width of the AI state is comparable to or larger than the autoionization width to within an order of magnitude, i.e., for the intermediate range of laser intensity. The continuum-continuum coupling and the higher-order transitions become effective only for much higher values of the above-threshold transition strengths, i.e., for much higher values of the laser intensity. \textcopyright{} 1996 The American Physical Society.

Physical Review A•Apr 1, 1996•2 citations
We introduce in the context of a simple soluble model for laser-enhanced autoionization a new representation---the dressed-resonance representation (DRR). The model includes one autoionizing state, one flat electron continuum, one continuous driving laser of arbitrary strength, and two types of relaxations, i.e., spontaneous decay of the continuum and autoionizing state back to the initial state (spontaneous recycling) and phase-jitter noise in the driving laser. The DRR is a natural extension of the dressed-states representation, employed in strong-field discrete-discrete transitions, to discrete-continuum transitions. We focus on two observables, i.e., the temporal initial state population and the photoelectron spectrum. The emphasis is on strong-field effects. The DRR yields simple, transparent, analytical expressions for the above observables in the no-relaxations limit of the model. These results are complemented by a systematic analysis of the exact solution in all qualitatively distinct physical regimes.
Complex dressed states and photoelectron spectra are considered for a model system containing two discrete states that are coupled by separate lasers to a continuum that supports a resonance such as an autoionizing state. Spontaneous radiative decay from the structured continuum has been included and its effects are examined. It is shown that population can be trapped within the model atom even when radiative decay is included provided (1) the two Fano q values which characterize the discrete-state--continuum interactions are equal, and (2) the two-photon resonance condition is satisfied.
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.
A projection-operator formalism is employed in studying photorecombination at energies close to an autoionizing resonance. A resonance profile is defined, and is presented without approximation for a model system consisting of one electron continuum, one autoionizing state, and one final radiatively stabilized atomic state. Effective line-profile parameters \ensuremath{\varepsilon}^ and q^ are defined, and the photorecombination resonance profile is shown to be the same as the photoionization resonance profile (for the process in which initial and final atomic states are interchanged from the photorecombination process), provided one-photon spontaneous radiative decay effects are included. The effective line-profile parameters are examined, and are shown to reduce to the standard autoionization line-profile parameters in the limit of no spontaneous radiative decay. Effective line-profile parameters are also defined in the pole approximation for two model systems featuring an isolated autoionizing state. Comparison is made with effective line-profile parameters that have been defined elsewhere in the context of photoionization into coupled electron-continuum channels. The similarity in mathematical formalism for systems supporting multiple continua, regardless of whether these continua are electron or photon continua, is discussed.
The exact solution of the Fano model, including both radiative recombination to the initial state (recycling) and off-diagonal phase-jitter relaxation, is obtained by two different methods. We derive a simple substitution rule stating that, by replacing $\ifmmode\pm\else\textpm\fi{}i{\ensuremath{\omega}}_{L}\ensuremath{\rightarrow}\ifmmode\pm\else\textpm\fi{}i{\ensuremath{\omega}}_{L}+{\ensuremath{\gamma}}_{T}$, the phase-jitter effects are included. The time dependence of the solution has a convolution form. The results for the electron spectrum exhibit an increasing elastic-peak line narrowing the faster the spontaneous decay becomes. For very fast spontaneous decay, the elastic linewidth saturates to ${\ensuremath{\gamma}}_{T}$. The relevant thresholds are discussed and demonstrated.
Near-threshold single-photon photodetachment processes are considered for simple model systems representing negative ions exposed to monochromatic radiation. The model systems considered feature an initial discrete state, the photoelectron continuum, and in some cases an excited or autodetaching discrete state. Population trapping through the formation of a bound dressed state and the photoelectron spectra are examined. Two photoelectron spectra are defined : a primary spectrum which results from decay while the laser interaction is on and a secondary spectrum which results from the destabilization of the bound dressed state if the interaction is turned off again. Careful attention is given to the passage of the poles of the Green's operator through the low-energy boundary of a continuum, and it is concluded that such poles move nonmonotonically through the boundary of an s-wave continuum, but monotonically through the boundary of a p-wave or higher-l-wave continuum. Two example systems, one featuring a single discrete state and an s-wave continuum and one featuring two discrete states and a p-wave continuum, are considered in detail.
We have studied the effect of above-threshold transitions on the (1+1)-photon resonance-enhanced ionization and autoionization processes. We have shown that the effect of the photoionization decay of the autoionizing (AI) state to the higher continuum becomes dominant when the photoionization width of the AI state is comparable to or larger than the autoionization width to within an order of magnitude, i.e., for the intermediate range of laser intensity. The continuum-continuum coupling and the higher-order transitions become effective only for much higher values of the above-threshold transition strengths, i.e., for much higher values of the laser intensity. \textcopyright{} 1996 The American Physical Society.