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The Journal of Chemical Physics

Atomic and molecular stabilization in two-frequency laser fields

Avijit Datta, Santanu Bhattacharyya, Sungyul Lee, Bongsoo Kim

Published Jul 17, 2003• 3 citations
Publisher

Abstract

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.

DOI
10.1063/1.1587117

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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.
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.
The existing theoretical and experimental works on interference stabilization of Rydberg atoms are overviewed. Physical origin of the phenomenon, its main features and the conditions of existence, as well as the main theoretical approaches used for its description are discussed. These ideas are used to describe interference stabilization of molecules with respect to photodissociation. Specific calculations are carried out for the hydrogen molecular ion H2+. The main features of strong‐field photodissociation are described. The conditions of stabilization and destabilization of a molecule are found and related to features of the initial‐state molecular vibrational wave function.
We present the effects of frequency chirping of laser pulses on (1+1)-photon resonance-enhanced dissociation dynamics of H2.{\mathrm{H}}_{2}.H2​. The dissociation occurs via two closely spaced nonadiabatically coupled intermediate levels which are in one-photon resonance or near resonance with the initial level. Predissociating levels embedded into continua are considered. When the first laser field is sufficiently intense and suitably chirped, the dissociation probability is enhanced by adiabatic rapid passage through the avoided crossing arising from the frequency swept radiative interaction. The whole population of the ground level can be effectively transferred to the intermediate levels by this technique facilitating the dissociation process by the second field. We also report the effect of frequency detuning and chirp width on the dissociation probability. Widths of the two peaks of the dissociation line shape increase with an increase in chirp width, resulting in the possibility of control in the dissociation yield. When the first field is a laser pulse of low intensity and constant frequency and the second laser frequency is chirped, predissociating levels take important parts in the dissociation dynamics and we obtain a signature of the nonadiabatic effect of the first step on the second step of photodissociation dynamics. This feature is due to the presence of the predissociating levels and the nonadiabatic mixing of two intermediate levels. All these results can be explained in terms of the adiabatic dressed levels.
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.
Two laser pulses that overlap in time have been successfully used for coherent excitation of a molecule or atom to a desired state. An intermediate state that is at or near resonance with the laser frequencies has been used in past experiments and calculations. Here we replace this intermediate state by a continuum of intermediate states. An analytic solution of a simple model suggests that the continuum can be used in such an excitation scheme, provided that the laser pulses are arranged in the so-called counterintuitive order. We use a special case of this solution to relate this work to ionization suppression and coherent population trapping.