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Product state control through interfering optical ionization routes

Feng Yun Wang

Published Jan 1, 1996• 0 citations
OpenAlex

Abstract

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.

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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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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.

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Photoelectron angular distributions and phase interference in alkali atoms

Yi-Yian Yin

In this thesis we report an experimental study of the interference between one- and two-photon ionization processes from the ground state of rubidium atom. This interference is observed by measuring the photoelectron angular distribution under the influence of fundamental and second harmonic laser fields. Externally controlling the relative phase and amplitude between these two laser fields, we obtained interference in the angular distribution patterns. This study represents the first observation of the phase interference in photoionization angular distributions. The photoelectron angular distributions resulting from the single-photon ionization of cesium and rubidium from its ground state to the continuum were measured. This is the first set of measurements of the single photon photoelectron angular distributions at a variety of energies in vicinity of the Cooper minimum from a single initial state of alkali metals. The results show a strong variation of asymmetry parameter $\beta$ from its nonrelativistic value of 2. Spin-orbit coupling displaces the minimum in the cross section for excitation to the $\rm \varepsilon P\sb{1/2}$ and $\rm \varepsilon P\sb{3/2}$ and results in a rapid variation of the angular distribution with photon energy in this region. Through the measurement of two-photon ionization angular distributions the quantum defects and spin-orbit coupling were investigated. These studies help us to understand the phase interference interaction, and the data in those studies would be used to analyze the interference processes.

•Jan 1, 1992•0 citations
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 6s\sp2 \sp1S\sb0\rm 6s\sp2\ \sp1S\sb06s\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 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.
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.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
Coherent control of interfering one- and two-photon processes has for decades been the subject of research to achieve the redirection of photocurrent. The present study develops two-pathway coherent control of ground-state helium atom above-threshold photoionization for energies up to the N=2N=2N=2 threshold, based on a multichannel quantum defect and R-matrix calculation. Three parameters are controlled in our treatment: the optical interference phase $\mathrm{\ensuremath{\Delta}}\mathrm{\ensuremath{\Phi}}$, the reduced electric field strength $\ensuremath{\chi}={\mathcal{E}}_{\ensuremath{\omega}}^{2}/{\mathcal{E}}_{2\ensuremath{\omega}}$, and the final state energy $\ensuremath{\epsilon}$. A small energy change near a resonance is shown to flip the emission direction of photoelectrons with high efficiency, through an example where 9090%90 of photoelectrons whose energy is near the 2p21Se2{p}^{2}{\phantom{\rule{4pt}{0ex}}}^{1}{S}^{e}2p21Se resonance flip their emission direction. However, the large fraction of photoelectrons ionized at the intermediate state energy, which are not influenced by the optical control, make this control scheme challenging to realize experimentally.
In this thesis we report an experimental study of the interference between one- and two-photon ionization processes from the ground state of rubidium atom. This interference is observed by measuring the photoelectron angular distribution under the influence of fundamental and second harmonic laser fields. Externally controlling the relative phase and amplitude between these two laser fields, we obtained interference in the angular distribution patterns. This study represents the first observation of the phase interference in photoionization angular distributions. The photoelectron angular distributions resulting from the single-photon ionization of cesium and rubidium from its ground state to the continuum were measured. This is the first set of measurements of the single photon photoelectron angular distributions at a variety of energies in vicinity of the Cooper minimum from a single initial state of alkali metals. The results show a strong variation of asymmetry parameter β\betaβ from its nonrelativistic value of 2. Spin-orbit coupling displaces the minimum in the cross section for excitation to the $\rm \varepsilon P\sb{1/2}$ and $\rm \varepsilon P\sb{3/2}$ and results in a rapid variation of the angular distribution with photon energy in this region. Through the measurement of two-photon ionization angular distributions the quantum defects and spin-orbit coupling were investigated. These studies help us to understand the phase interference interaction, and the data in those studies would be used to analyze the interference processes.