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Physical Review Letters

Experimental Observation of Laser Control: Electronic Branching in the Photodissociation of N a2

A. Shnitman, I. Sofer, Ilya Golub, Amnon Yogev, M. Shapiro, Z. Chen, Paul W. Brumer

Published Apr 15, 1996• 141 citations
Publisher

Abstract

Control over the product branching ratio in the photodissociation of N${\mathrm{a}}_{2}$ into $\mathrm{Na}(3s)+\mathrm{Na}(3p)$ and $\mathrm{Na}(3s)+\mathrm{Na}(3d)$ is demonstrated using a two-photon incoherent interference control scenario. Ordinary pulsed nanosecond lasers are used and N${\mathrm{a}}_{2}$ is at thermal equilibrium in a heat pipe. Results show a depletion in the $\mathrm{Na}(3d)$ product of at least 25% and a concomitant increase in the $\mathrm{Na}(3p)$ yield as the relative frequency of the two lasers is scanned.

DOI
10.1103/physrevlett.76.2886

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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.
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.
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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.
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 have extended our studies of control of the branching ratio for photoionization of atomic barium. This control is based on an interference between two distinct photoexcitation routes, each of which is a resonantly enhanced two-photon process. We report our experimental studies of the dependence of the interference on atomic beam density, laser power, selection of intermediate states, and relative polarization of the two field components. We observe that the potential for product control increases dramatically with increasing atomic beam density and depends strongly on the laser pulse energies as well.
We have demonstrated control over the branching ratio for photoionization into two different continuum channels of atomic barium using an interference between two distinct photoexcitation routes. Each pathway for photoionization is a two-photon process, and is resonantly enhanced by an intermediate state. We have found the unexpected situation of destructive interference when both lasers are resonant with their respective transitions. The magnitude of the interference, as well as the exceptionally broad wavelength range over which the interference persists, is also quite surprising.