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Physical Review A

Two-photon resonant three-photon ionization of the nd D2 states of cesium, rubidium, and sodium: Photoelectron angular distributions

Adila Dodhy, John A. D. Stockdale, R. N. Compton, Xian Tang, P. Lambropoulos, A. Lyras

Published Apr 1, 1987• 9 citations
Publisher

Abstract

Energy-resolved photoelectron angular distributions have been measured for the nd ${}^{2}$D 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 ${}^{2}$D 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}$${D}_{5/2}$,3/2 levels. In addition, the high laser powers \ensuremath{\sim}${10}^{8}$ W/${\mathrm{cm}}^{2}$ 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.

DOI
10.1103/physreva.35.2878

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The influence of the dynamic Stark effect on photoelectron angular distributions for two-photon-resonant three-photon ionization of sodium by way of the 42D4^{2}D42D fine-structure components has been investigated. Owing to the dynamic Stark shift of the atomic transition frequency, the angular distributions expected at low laser intensities were modified as different transitions were intensity tuned into and out of resonance with the laser frequency. The distributions are in good qualitative agreement with theory.
Experimental and theoretical results are presented for resonantly enhanced three-photon ionization of cesium atoms in the wavelength range from 830 to 950 nm. The accidental near resonance of the 6p2P32,126p^{2}P_{\frac{3}{2},\frac{1}{2}}6p2P23​,21​​ levels at the one-photon energy and the near resonance at the two-photon level with the 6d2D32,526d^{2}D_{\frac{3}{2},\frac{5}{2}}6d2D23​,25​​ states gives rise to large ionization signals and interesting anomalies in the relative intensities of the fine-structure levels. "Hybrid resonances" are observed which involve the production of 6p2P126p ^{2}P_{\frac{1}{2}}6p2P21​​ atoms from molecular cesium. One hybrid resonance involves ionization of the 6p2P126p ^{2}P_{\frac{1}{2}}6p2P21​​ state via an electric quadrupole transition ($6p ^{2}P_{\frac{1}{2}}\ensuremath{-}7p ^{2}P_{\frac{3}{2}}$). Both experimental and theoretical photoelectron angular distributions are compared for all of the observed resonances except for the 6p2P326p ^{2}P_{\frac{3}{2}}6p2P23​​ state where the signals were too weak for such a measurement. Agreement between experiment and theory is fair for three-photon ionization via the 6p2P126p^{2}P_{\frac{1}{2}}6p2P21​​ state. Comparison of experiment and theory for the case of the 6d2D32,526d^{2}D_{\frac{3}{2},\frac{5}{2}}6d2D23​,25​​ states indicates that hyperfine coupling during the laser-pulse duration may be important. The angular distributions for the hybrid resonances show the general behavior predicted by theory but the features appear broadened.
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