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Canadian Journal of Chemistry

Two-color coherent control with SEP preparation: electronic branching in Na2 photodissociation

Jeffrey Dods, Paul W. Brumer, Moshe Shapiro

Published Mar 1, 1994• 7 citations
Publisher

Abstract

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.

DOI
10.1139/v94-124

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•Jan 1, 1986•0 citations
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.
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.
Control over the product branching ratio in the photodissociation of Na2{\mathrm{a}}_{2}a2​ into Na(3s)+Na(3p)\mathrm{Na}(3s)+\mathrm{Na}(3p)Na(3s)+Na(3p) and Na(3s)+Na(3d)\mathrm{Na}(3s)+\mathrm{Na}(3d)Na(3s)+Na(3d) is demonstrated using a two-photon incoherent interference control scenario. Ordinary pulsed nanosecond lasers are used and Na2{\mathrm{a}}_{2}a2​ is at thermal equilibrium in a heat pipe. Results show a depletion in the Na(3d)\mathrm{Na}(3d)Na(3d) product of at least 25% and a concomitant increase in the Na(3p)\mathrm{Na}(3p)Na(3p) yield as the relative frequency of the two lasers is scanned.
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.
We report branching ratios between the ground and excited states of iodine atoms in the photodissociation of sodium iodide. We employ wave packet propagation techniques to study the optimal production of polarized iodine atoms and find experimentally realizable laser parameters to control the outcome. Application of a learning algorithm shows that the product branching can be controlled by suitably varying the time delay, the chirp, and the relative phase of the pump and control laser pulses. Periodic modulation of the polarized iodine atom branching ratio as a function of the delay between the firing of the two ultrashort laser pulses provides interesting insights into the photodissociation process.
For the first time by means of two-photon hybrid excitation in the Na2-Na system, which is distinguished from the usual two-photon excitation in atomic vapor, tunable UV coherent radiation (between 337.6 and 339.1 nm and between 358.0 and 359.6 nm) and tunable infrared stimulated radiation (3.38-3.46 µm) were generated. By optically pumping with a pulsed dye laser scanning from 614.5 to 617.4 nm for sodium vapor in a heat-pipe oven, Na2 was excited by the first photon absorption from the X ′ Σ g + to the A ′ Σ u + state. The energy was transferred to Na by Na2*-Na collision to populate the levels 3p1/2,3/2 in Na, which was followed by another photon excitation from the same beam up to near the 5S level, resulting in SERS (3P1/2-4P and 3P3/2-4P) and OPSE(5S → 4P and 4P → 4S) radiation. Assuming the detunings of pumping frequency ω1, from ω(5S,3P3/2) and ω(5s,3p1/2) to be Δ1 and Δ2, respectively, the three UV coherent radiation components were supposed to be generated by the following mixing processes: ωUV = 2ω1 − ω(5S − 4P); ω U V ′ = 2 ω 1 − [ ω ( 5 S → 4 P ) + Δ 1 ] ; ω U V ′ ′ = 2 ω 1 − [ ω ( 5 S → 4 P ) + Δ 2 ] , and also ω U V ′ ′ ′ = 2 ω 1 − ω ( 4 P → 4 S ) . The SERS and the UV signals were tuned as the pumping wavelength was tuned. The measured wavelengths agreed well with the calculated values.
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.