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Annual Meeting Optical Society of America

One- and two-photon photoelectron angular distribution measurements in the heavy alkali metal atoms

Daniel S. Elliott, Yi-Yian Yin

Published Jan 1, 1992• 0 citations
Publisher

Abstract

We have measured photoelectron angular distributions for single photon ionization of atomic rubidium and cesium and for two photon ionization of rubidium. The single-photon measurements show clear variation as a function of wavelength of the exciting laser due to the spin-orbit interaction. From these measurements we derive the Fano parameter, a measure of the relative strength of the dipole transition amplitudes from the ground state to the j = 1/2 and j = 3/2 continuum p states. These results are in excellent agreement with, and in some cases of much smaller uncertainty than, similar results obtained through measurements of the Fano effect and related polarization measurements. The two-photon measurements also vary with wavelength because of their dependence on relative cross sections and phase shifts of the p → d and p → s transitions, and the fine structure of the continuum d-state.

DOI
10.1364/oam.1992.tuz31

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We have measured photoelectron angular distributions for single photon ionization of atomic rubidium and cesium and for two photon ionization of rubidium. The single-photon measurements show clear variation as a function of wavelength of the exciting laser due to the spin-orbit interaction. From these measurements we derive the Fano parameter, a measure of the relative strength of the dipole transition amplitudes from the ground state to the j = 1/2 and j = 3/2 continuum p states. These results are in excellent agreement with, and in some cases of much smaller uncertainty than, similar results obtained through measurements of the Fano effect and related polarization measurements. The two-photon measurements also vary with wavelength because of their dependence on relative cross sections and phase shifts of the p → d and p → s transitions, and the fine structure of the continuum d-state.
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.
We report in detail our complete characterization of atomic photoionization via a single measurement, namely, that of the two-photon photoelectron angular distribution using elliptically polarized light. Through these measurements we are able to determine the atomic parameters necessary to unambiguously describe two-photon ionization of atomic rubidium. These parameters include relative cross sections for photoionization into various outgoing channels as well as the phase difference between the continuum S and D partial waves. We present a derivation of the angular distribution with arbitrarily polarized light in terms of these cross sections and phases, demonstrate the advantages of using elliptically polarized light, discuss details of the experiment, and report the results of our measurements in this paper. While measured phase differences are in excellent agreement with expected values over the entire range of photoelectron kinetic energy of our measurements from 0 to 0.5 eV, ratios of cross sections for photoionization into the two D channels are anomalous, suggesting unexpected fine structure effects in these continua.
We present our study on complete measurements of two-photon ionization in atomic rubidium. In this study we developed an effective technique to measure the angular distributions of photoelectrons. Through the measurements with this technique and using elliptically-polarized light we have successfully determined the atomic parameters—the relative cross sections for various ionization channels as well as the phase difference between s- and d-continuum waves. The measured phase differences are in excellent agreement with expected values over the entire range of photoelectron kinetic energy from 0 to 0.5 eV. The measured relative cross sections for ionization into S and D channels are also in good agreement with theoretical data. Both the relative cross sections and phase differences show a slow variation as a function of photoelectron energy. The ratios of cross sections of the two D channels are not in agreement with the expected value, suggesting fine structure effects. We also studied a quantum mechanical interference using two-color laser fields and observed the extremely asymmetric photoelectron angular distribution produced by interfering even- and odd-order photoionization processes in atomic rubidium as the optical phase was varied. We observed these effects with both perpendicularly and parallel-polarized fields components. It is worthy to note that the quantum interference observed with perpendicularly-polarized two-color fields is unique. Based on the analysis of measured angular distributions we derived the phase difference between even and odd continuum waves for the first time. The measured value of the phase difference is in good agreement with the calculated value from quantum defect theory.
We report in detail our complete characterization of atomic photoionization via a single measurement, namely, that of the two-photon photoelectron angular distribution using elliptically polarized light. Through these measurements we are able to determine the atomic parameters necessary to unambiguously describe two-photon ionization of atomic rubidium. These parameters include relative cross sections for photoionization into various outgoing channels as well as the phase difference between the continuum S and D partial waves. We present a derivation of the angular distribution with arbitrarily polarized light in terms of these cross sections and phases, demonstrate the advantages of using elliptically polarized light, discuss details of the experiment, and report the results of our measurements in this paper. While measured phase differences are in excellent agreement with expected values over the entire range of photoelectron kinetic energy of our measurements from 0 to 0.5 eV, ratios of cross sections for photoionization into the two D channels are anomalous, suggesting unexpected fine structure effects in these continua.
Energy-resolved photoelectron angular distributions have been measured for the nd 2{}^{2}2D states (n=11--23) in cesium and rubidium atoms for two-photon resonant, three-photon ionization under conditions where the spin-orbit fine structure was not resolved. The photoelectron angular distributions reveal strong fine-structure mixing effects when the time duration of the laser pulse is longer than the time corresponding to the inverse of the fine-structure splitting of the nd states involved. Three-photon ionization photoelectron angular distributions for sodium atoms have been studied both experimentally and theoretically for laser frequencies in which resonance enhancement occurs via the two-photon excited nd 2{}^{2}2D states (n=5--9). The laser pulse duration is \ensuremath{\sim}6 ns which is comparable to or less than the fine-structure mixing time for the unperturbed fine-structure nd 2{}^{2}2D5/2{D}_{5/2}D5/2​,3/2 levels. In addition, the high laser powers \ensuremath{\sim}108{10}^{8}108 W/cm2{\mathrm{cm}}^{2}cm2 employed result in alterations of the energy separations of the fine-structure levels (and cause corresponding changes in the mixing times) due to the ac Stark effect. A detailed theoretical analysis is presented and good agreement is obtained with the experimental results.