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Physical Review Letters

Photoelectron Angular Distributions for Near-Threshold Two-Photon Ionization of Cesium and Rubidium Atoms

Adila Dodhy, R. N. Compton, John A. D. Stockdale

Published Feb 4, 1985• 38 citations
Publisher

Abstract

Photoelectron angular distributions have been measured for nonresonant two-photon ionization of cesium and rubidium atoms just above the ionization threshold. The photoelectron energies ranged from 25 to 100 meV. The results are compared with theoretical estimates based on nonrelativistic atomic wave functions. Initial results are also presented for above-threshold ionization in cesium.

DOI
10.1103/physrevlett.54.422

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Annual Meeting Optical Society of America•Jan 1, 1992•0 citations
Photoelectron angular distributions have been measured for nonresonant two-photon ionization of cesium and rubidium atoms just above the ionization threshold. The photoelectron energies ranged from 25 to 100 meV. The results are compared with theoretical estimates based on nonrelativistic atomic wave functions. Initial results are also presented for above-threshold ionization in cesium.
We present both experimental and theoretical studies of resonantly enhanced multiphoton ionization of cesium atoms. Photoelectron angular distributions for two-photon ionization via the one-photon-allowed 7p2P12,327p^{2}P_{\frac{1}{2},\frac{3}{2}}7p2P21​,23​​ and 8p2P12,328p^{2}P_{\frac{1}{2},\frac{3}{2}}8p2P21​,23​​ intermediate states and three-photon ionization via the two-photon-allowed 8d2D52,328d^{2}D_{\frac{5}{2},\frac{3}{2}}8d2D25​,23​​ resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of ~ 0.1 eV and angular resolution of ~ \ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}. A straightforward calculation based upon a finestructure scheme gives excellent agreement with the measured angular distributions for the 2P12^{2}P_{\frac{1}{2}}2P21​​ states. The laser pulse duration is comparable to the hyperfine precession period which allows the hyperfine coupling to partially destroy the anisotropy initially produced in the 2P32^{2}P_{\frac{3}{2}}2P23​​ resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the 2P32^{2}P_{\frac{3}{2}}2P23​​ data.
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.
Photoelectron angular distributions have been measured for two-photon ionization of rubidium atoms both when the first photon was in resonance with the 4d 2{\mathrm{}}^{2}2D state via an electric quadrupole transition and when it was detuned from it. Angular distributions have also been obtained for the first and second above-threshold ionization (ATI) photoelectron peaks. Tuning to the 4d 2{\mathrm{}}^{2}2D resonance creates a large enhancement of the two-photon ionization photoelectron signal but causes only a modest increase in the ATI signal.
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.