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Physical Review A

Multiphoton ionization of rubidium atoms near the 4d 2 D quadrupole transition

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

Published Mar 1, 1986• 13 citations
Publisher

Abstract

Photoelectron angular distributions have been measured for two-photon ionization of rubidium atoms both when the first photon was in resonance with the 4d ${\mathrm{}}^{2}$D 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 ${\mathrm{}}^{2}$D resonance creates a large enhancement of the two-photon ionization photoelectron signal but causes only a modest increase in the ATI signal.

DOI
10.1103/physreva.33.2167

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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.
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.
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