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The Journal of Chemical Physics

Laser control of photopredissociation angular distributions and photoionization in Cl2 by two color interference

André D. Bandrauk, Jean-Marc Gauthier, James F. McCann

Published Jan 1, 1994• 16 citations
Publisher

Abstract

A coupled equations approach based on a general artificial channel method is used to demonstrate two color laser coherent control of photopredissociation and photoionization in Cl2. Photopredissociation of the 1Πg states of Cl2 is suppressed by a two plus four photon interference process 2ω2=4ω4, whereas the photoionization is controlled by interference between a six photon transition and a four photon transition 6ω4=2ω2+2ω4 using the same 1Πg photopredissociation resonances as intermediates. Photopredissociation angular distributions illustrate the suppression of the corresponding transition in selective rotational states.

DOI
10.1063/1.466947

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A coupled equations approach based on a general artificial channel method is used to demonstrate two color laser coherent control of photopredissociation and photoionization in Cl2. Photopredissociation of the 1Πg states of Cl2 is suppressed by a two plus four photon interference process 2ω2=4ω4, whereas the photoionization is controlled by interference between a six photon transition and a four photon transition 6ω4=2ω2+2ω4 using the same 1Πg photopredissociation resonances as intermediates. Photopredissociation angular distributions illustrate the suppression of the corresponding transition in selective rotational states.
Prospects for the coherent control of optically-induced processes have attracted a great deal of interest of late. In this talk we will discuss our recent demonstration of the first example of two-pathway coherent control on a bound-free transition in a molecular system. In this demonstration, we controlled the asymmetry of the angular distribution of photoelectrons produced by photoionization of NO. Preparation of the NO in its A{sup 2}{Sigma}, state allowed isolation of a single rotational state, which was subsequently ionized via interfering one- and two-photon interactions. We discuss the role of intermediate-state resonances and laser mode quality on two-pathway control processes.
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.
We present a theoretical study of the main characteristics of two-color photoionization spectra obtained when atoms are subjected to an intense radiation pulse containing the fundamental of an infrared laser and one of its higher harmonics. The frequency of the latter is chosen to be high enough that the atom can be ionized by a single photon. The calculation is performed by numerical integration of the time-dependent Schrödinger equation for a (three-dimensional) hydrogen atom. We present the general properties of the spectra and angular distributions as well as the possible occurrence of interferences between the laser-assisted process and above-threshold ionization. Our results indicate that such interference effects, which can lead to partial coherent control of the photoionization process, can be observed at moderate laser intensities within the reach of currently operated laser and harmonic sources.
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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.