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Interference in the Photodissociation (Photoionization) by Two Close-Lying Frequencies

A. Lamz

Published Jan 1, 2006• 0 citations
OpenAlex

Abstract

SJ~mmary. The problem of photodissociation (photoionization) by two close-lying frequencies (two modes of a laser or two different lasers) is examined. In the case of one bound state and one continuum the problem is solved analytically and the solution is used to show that the photodissociation (photoionization) yield is reduced due to the interference between the two competing channels (i.e. absorption from one or the other photon mode). It is also shown that the population flow into the continuum happens simultaneously with a relevant photon redistribution between the two modes.

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SJ~mmary. The problem of photodissociation (photoionization) by two close-lying frequencies (two modes of a laser or two different lasers) is examined. In the case of one bound state and one continuum the problem is solved analytically and the solution is used to show that the photodissociation (photoionization) yield is reduced due to the interference between the two competing channels (i.e. absorption from one or the other photon mode). It is also shown that the population flow into the continuum happens simultaneously with a relevant photon redistribution between the two modes.
An exactly soluble model is used to describe transitions, induced by two strong laser beams, proceeding from the initial discrete state to the continuum via two channels. In the first channel the transition occurs via an intermediate, near-resonant discrete state located close to the continuum edge. In the second channel the intermediate state belongs to the continuum. Population trapping occurs due to an interference between the two channels. The model can be used to describe the photodetachment of electrons from negative ions or the photodissociation of molecules.
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 examine the suppression of 1+1 photodissociation or photoionization by laser fields. We find that atoms and molecules become more stable against photoionization or photodissociation when the intensity of the nonresonant laser field is increased above a limiting value for a two level plus continuum model system. We treat the ionization or dissociation dynamics analytically and discuss the conditions for optimum yields in a two-frequency square pulse laser field. We observe similar suppression against photodissociation for realistic sine-squared laser pulses. We demonstrate such a type of molecular suppression for 1+1 photodissociation of H2.
The theory of one- and two-photon dissociation processes with strong laser pulses for slowly varying multiple continua (‘‘direct’’ dissociation) is developed. Closed form expressions for the state preparation and evolution during and after the excitation pulse are derived. We show how saturation of the photodissociation process as a function of the laser power sets in. We also show that for direct dissociation, the fragment state distribution is independent of the laser power. The dependence of spontaneous emission during dissociation (continuum Raman and resonance fluorescence) on the pulse intensity is studied. The formulation is extended to treating resonantly enhanced two-photon dissociation with strong laser pulses. Closed-form expressions for slowly varying pulses and slowly varying continua are derived. Using these expressions, the existence of adiabatic passage to the continuum (APC) by which a system executes a complete population transfer from an initial bound state to the continuum by following two guiding light pulses is established. A simple iterative scheme for going beyond the adiabatic approximation is introduced.
We have demonstrated control over the branching ratio for photoionization into two different continuum channels of atomic barium using an interference between two distinct photoexcitation routes. Each pathway for photoionization is a two-photon process, and is resonantly enhanced by an intermediate state. We have found the unexpected situation of destructive interference when both lasers are resonant with their respective transitions. The magnitude of the interference, as well as the exceptionally broad wavelength range over which the interference persists, is also quite surprising.
The effect of interference stabilization is shown to exist in a system of two atomic levels coupled by a strong two-color laser field, the two frequencies of which are close to a two-photon Raman-type resonance between the chosen levels, with open channels of one-photon ionization from both of them. We suggest an experiment, in which a rather significant (up to 9090%90) suppression of ionization can take place and which demonstrates explicitly the interference origin of stabilization. Specific calculations are made for H and He atoms and optimal parameters of a two-color field are found. The physics of the effect and its relation with such well-known phenomena as light induced continuum structure and population trapping in a three-level system are discussed.