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The Journal of Chemical Physics

Laser control of unimolecular decay yields in the presence of collisions

Moshe Shapiro, Paul W. Brumer

Published Jun 1, 1989• 57 citations
Publisher

Abstract

We show that coherent light can be used to control the relative product yield in unimolecular photodissociation, even in the presence of collisions. That is, coherence contributions to the photodissociation, which are responsible for control over the reaction, are sufficiently robust to survive inhomogeneous broadening effects over a substantial temperature range. A number of experimental implementations for control in the presence of collisions are proposed.

DOI
10.1063/1.456334

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Canadian Journal of Chemistry•Mar 1, 1994•7 citations
We show that coherent light can be used to control the relative product yield in unimolecular photodissociation, even in the presence of collisions. That is, coherence contributions to the photodissociation, which are responsible for control over the reaction, are sufficiently robust to survive inhomogeneous broadening effects over a substantial temperature range. A number of experimental implementations for control in the presence of collisions are proposed.
A method for controlling branching unimolecular reactions, developed in collaboration with M. Shapiro of the Weizmann Institute, which uses the coherence of weak lasers, is presented. It is shown that preselected chemical products may be obtained, in preference to others, by utilizing the interference between naturally occurring processes. Our specific application then entails the dissociation of a bound superposition state with control attained by manipulating the relative intensities and phases of coherent laser beams. Enhanced yields of chemical products of interest are shown to result. Similar control is attainable for the populations of individual product quantum states. Computational results on the I*II branching ratios in the photodissociation of CH3, I and FI are presented as examples. Proposals for experiments which demonstrate coherent control present a challenge to the laser community which is well worth accepting.
A General method of controlling branching (direct and resonant) unimolecular reactions, using the coherence of lasers is presented. It is shown that pre-selected products may be obtained, in preference to others, as a result of interference be­ tween decay channels of a superposition of degenerate continuum states. It is shown that such states can be prepared by exciting a superposition of bound states using two coherent light sources with a well defined relative phase. Theoretical limits of coherent control, in direct and compound (statistical like) processes are derived. En­ hanced (often 100%) yields of ~he (chemical) products of interest are obtained. Computational results on the I /1 branching ratios in the photodissociation of FI and CH31, including the effects of initial conditions are presented. In addition, complete blocking-off of a single pre-selected quantum state of a given reaction product, resulting in population inversion with respect to all higher energetic levels, is shown to be attainable.
We show that the branching ratios into two or three channel photodissociation products can be controlled by a novel resonant two-photon vs two-photon coherent control scenario. The method allows for control even when the initial molecular system is in a mixed state, e.g., in thermal equilibrium, a significant advance for controlling reactions. Essential to the technique is the phase locking between different frequencies which results in cancellation of incoherences due to laser phase jumps. Reduction of uncontrolled background contributions also obtains. The scenario is demonstrated computationally on Na2 photodissociation, where a wide range of control is achieved over branching into two product channels [Na(3s)+Na(4s) vs Na(3s)+Na(3p)] at lower energies and over three product channels at energies above the Na(3s)+Na(3d) threshold.
The theory of coherent control of photodissociation with partially coherent laser pulses is developed and applied to the pump–dump control scenario of a collinear model of DH2. The coherence characteristics of the pump pulse are shown to be crucial for maintaining control over the product yield, whereas the coherence properties of the dump pulse are only of secondary importance. Control is shown to survive for partially coherent laser pulses, but only for a range of incoherence which precludes control with typical nanosecond pulsed dye lasers.
Extensive control over relative product yields is shown to result from weak laser photodissociation in a two-color coherent control scenario. Photodissociation takes place from a superposition state prepared by stimulated emission pumping. The method allows for control in the presence of laser phase jumps by using four-wave mixing to prepare the required laser frequencies. The near UV photodissociation of Na2, resulting in the Na(3s) + Na(4s) and the Na(3s) + Na(3d) products, is used to illustrate the method.