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Physical Review Letters

Product State Control through Interfering Excitation Routes

Feng Wang, Ce Chen, Daniel S. Elliott

Published Sep 16, 1996• 47 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.77.2416

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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.

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Physical Review Letters•Nov 3, 1997•11 citations
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.
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 present the results of our study on coherent control of photoionization of atomic barium. Our study focused on the understanding of the controllability, especially due to the effect of the coherent interaction between the atomic system and the laser field. The first half of the study investigates the mechanisms of the control behind the previously observed laser phase-insensitive product state control. The controllability of this excitation scheme, two-color two-photon resonantly enhanced excitation, was analyzed from two aspects, the role of ac Stark shift introduced by the strong laser field and the multi-pathway quantum mechanical interferences. We have analyzed the excitation scheme from the analysis of the photoelectron angular distribution measured using the excitation scheme and the monitoring of the intermediate state population. Analysis of the data as well as the numerical simulation showed clear understanding of the role of two mechanisms in the product state control reported. We also investigated the control of the phase lag during the product state control. We conducted the control of the phase lag in the study of asymmetric photoelectron angular distribution, which arises from the concurrent even-odd parity outgoing electron wave excitation. The phase lag was controlled in full range, 2π, and the results were analyzed in terms of the role of autoionizing resonance structures as well as the nature of outgoing electron waves at different locations of the autoionizing resonances.
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.
We report photoionization measurements that help us interpret mechanisms leading to the control of the branching ratio for resonantly enhanced, two-color, two-photon ionization experiments. We measure intermediate state populations and photoelectron angular distributions to explore the ionization dynamics. Our measurements support the interpretation that ac Stark shifts are the primary mechanism of the product state control previously observed in a similar experimental setup. We discuss the role of interference in the ionization continuum and the effect of the ac Stark shift on the product state control.
A quantitative analysis of part of the experimental data reported by Wang, Chen and Elliott [1,3] who studied in barium coherent control through two-color resonant interfering paths is reported. Dynamics of the two-color photoionization process, described as an adiabatic process in the rotating wave approximation, is governed by the coherent excitation of the 6s6p and 6s7p 1P1 intermediate states. Interference effects are found to play a minor role. The required atomic parameters are obtained from a theoretical approach based on a combination of jj-coupled eigenchannel R-matrix and Multichannel Quantum Defect Theory.
We investigate the variation of the photoionization yields into different ionic channels by means of the one-photon-near-resonant two-photon ionization scheme under two-color excitation. We present a general formulation and the results of specific calculations pertaining to the Ca atom with realistic parameters. A significant change of the ionization into Ca+{\mathrm{Ca}}^{+}Ca+ 4s4s4s, 3d3d3d, and 4p4p4p channels has been observed as a detuning is varied, which agrees qualitatively well with the observation by Wang, Chen, and Elliott [Phys. Rev. Lett. 77, 2416 (1996)] for the Ba atom. The importance of the laser intensity effects is also addressed.