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Physical Review A

Multiphoton dissociation for coherent and incoherent fields

Salvador Miret-Artés, David A. Micha

Published Dec 1, 1993• 3 citations
Publisher

Abstract

Photodissociation probabilities for coherent and incoherent laser sources are described, using number- and coherent-state representations of the light field. Previous calculations of ${\mathrm{H}}_{2}^{+}$ photodissociation in strong laser fields are used to calculate probabilities, for coherent and incoherent sources, that show increasing differences at lower light intensities.

DOI
10.1103/physreva.48.r4059

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The Journal of Chemical Physics•Sep 1, 1994•33 citations
Photodissociation probabilities for coherent and incoherent laser sources are described, using number- and coherent-state representations of the light field. Previous calculations of H2+{\mathrm{H}}_{2}^{+}H2+​ photodissociation in strong laser fields are used to calculate probabilities, for coherent and incoherent sources, that show increasing differences at lower light intensities.
A theory of molecular photofragmentation is developed for coherent and incoherent light and for the case where dissociation lifetimes of field-induced resonances are shorter than pulse lengths. Using a mixed representation for the transition operator (a description of the initial state of the field with the coherent representation and of the final state with the number representation), we show how observables are modified at different field regimes by Poisson distributions. Applications to H2+{\mathrm{H}}_{2}^{+}H2+​ and D2+{\mathrm{D}}_{2}^{+}D2+​ are presented and discussed. They include line shapes and final distributions of kinetic energies for weak, intermediate, and strong fields. Results show that the interference of resonance states plays an important role even after averages over the field phases have been carried out to account for incoherent effects.
Multiphoton spectroscopy The subject of multiphoton excitation spectroscopy began in 1931 when Göppert-Mayer [450] wrote a theoretical paper in which she calculated the transition rate for an atom in the presence of two photons rather than just one. At the time, the process seemed rather exotic, and it was reassuring that the calculated rate was so low as to guarantee that it could not readily be observed in the laboratory with conventional sources. This conclusion was reassuring because it implies that a simple perturbative theory (one photon per transition is the weak-field limit) is adequate for most purposes. The subject came to life with the advent of lasers, when it became easy to create intense beams of light. Since the probability of excitation by two photons grows as the square of the photon density, whereas the probability of single-photon excitation grows only linearly with photon density, two-photon transitions gain in relative strength with increasing intensity despite the small value of the rate coefficient. The development of multiphoton spectroscopy has followed that of lasers: as the available power has increased, so has the number of photons involved in individual transitions. More significantly, it has become apparent that the physics of the interaction between radiation and matter is not the same at high laser powers as under weak illumination, i.e. that there is a qualitative change which sets in at strong laser fields. This is normally expressed by saying that perturbative approximations break down.
Control over product probabilities and channel-specific line shapes in resonant two-photon dissociation is shown to result from quantum interference effects induced by illumination with a second laser. Control over product yield and selectivity is achieved by varying the frequency of this control laser, which need not be coherent with the excitation laser. Computations on Na2{\mathrm{Na}}_{2}Na2​ photodissociation show that control over product selectivity is extensive.
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.