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Physical Review A

Incoherent interference control of two-photon dissociation

Zhidang Chen, Moshe Shapiro, Paul W. Brumer

Published Sep 1, 1995• 38 citations
Publisher

Abstract

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 ${\mathrm{Na}}_{2}$ photodissociation show that control over product selectivity is extensive.

DOI
10.1103/physreva.52.2225

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Canadian Journal of Chemistry•Mar 1, 1994•7 citations
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.
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.
In this thesis we report an experimental study of the effect of interference between two two-photon bound-to-free excitation routes in atomic barium from the $\rm 6s\sp2\ \sp1S\sb0$ ground state. Different final products of the transition were observed by monitoring the kinetic energy spectrum of the photoelectrons using a Time-of-Flight electron detector. By tuning the frequency of one of the laser sources used for this work, we were able to change the interference from constructive to destructive, and to demonstrate a strong control over the final product states. Because it only requires two independent tunable laser fields (mutually incoherent), this technique of using quantum interference for product control is very robust. We also studied the dependence of this interference effect and product control on several experimental parameters: the density of the atomic beam, the pulse energy of the lasers, the polarization state of the lasers and the selection of intermediate states. When the density of the atomic beam was increased, we observed that the strength of the interference and the degree of product control increased significantly. When the relative polarization of the two laser fields was changed from parallel to perpendicular, the strength of interference effect increased, but the product control disappeared. In this thesis, we describe the interference, our ability to control product distributions through this interference, and the dependence of interference and product state control on the various experimental parameters.
Two-color excitation by spatially coherent and incoherent laser beams has been used to study three-photon-resonant excitation and subsequent ionization of xenon in conditions, when internally generated sum-frequency field plays an important role in excitation of atomic resonances through interfering one-photon excitation pathway. We show that the incoherence in one of the pumping fields reduces the efficiency of generated sum-frequency field, and thus suppresses the interference between the three- and the one-photon excitation channels. The degree of suppression is controlled by varying the crossing angle between coherent and incoherent laser beams. We show that ionization profiles can be analyzed on the basis of the well-studied interference of one- and three-photon transition amplitudes, but with pumping field decomposed into multiple small-scale uncorrelated domains where coherent process of four-wave mixing occurs. The gain length for a coherent process in these domains depends on the coherence degree and excitation geometry. It gives a possibility of controlling the contribution of coherent processes to the excitation of multiphoton resonances.
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.