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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Multiphoton ionization of Ba atoms with photoelectron spectroscopy

A. Yu. Elizarov

Published Mar 15, 1999• 0 citations
Publisher

Abstract

An experimental and theoretically study is performed of the angular photoelectron distribution for three-photon ionization of Ba atoms through the 2(omega) -excited intermediate state 6p2 (1So) and the auto- ionized state 6p8s(3P1). Rotation of the polarization plane of dye-laser radiation allowed us to investigate the photoelectron angular distribution. Electrons were counted with the help of a time-of-flight electron spectrometer. Employing the density-matrix formalism, expressions have been obtained for angular dependence of the differential ionizations probability. Possible experiments are discussed.

DOI
10.1117/12.342367

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An experimental and theoretically study is performed of the angular photoelectron distribution for three-photon ionization of Ba atoms through the 2(omega) -excited intermediate state 6p2 (1So) and the auto- ionized state 6p8s(3P1). Rotation of the polarization plane of dye-laser radiation allowed us to investigate the photoelectron angular distribution. Electrons were counted with the help of a time-of-flight electron spectrometer. Employing the density-matrix formalism, expressions have been obtained for angular dependence of the differential ionizations probability. Possible experiments are discussed.
An experimental and theoretical study is performed of the angular photoelectron distribution for three-photon ionization of Ba atoms through the 2ω-excited intermediate state 6p2(1S0) and the auto-ionized state 6p8s(3P1). Rotation of the polarization plane of dye-laser radiation allowed us to investigate the photoelectron angular distribution. Electrons were counted with the help of a time-of-flight electron spectrometer. Employing the density-matrix formalism, expressions have been obtained for angular dependence of the differential ionization probability. Possible experiments are discussed.
An experimental and theoretical study is performed of the angular photoelectron distribution for three-photon ionization of Ba atoms through the 2 ω -excited intermediate state 6 p 2 ( 1 S 0 ) and the auto-ionized state 6 p 8 s ( 3 P 1 ). Rotation of the polarization plane of dye-laser radiation allowed us to investigate the photoelectron angular distribution. Electrons were counted with the help of a time-of-flight electron spectrometer. The density-matrix formalism is used to obtain expressions for the angular dependence of the differential ionization probability. Possible experiments are discussed.
The theory of multiphoton ionization for an atomic system of arbitrary complexity is developed using a density matrix formalism. An expression is obtained which determines the differential N-photon ionization cross section as a function of the polarization states of the target atom and the incident radiation. The parameters which characterize the photo-electron angular distribution are related to the general reduced matrix elements for the N-photon transition. Two-photon ionization of unpolarized atoms is treated as an illustration of the use of the theory. The dependence of the multiphoton ionization cross section on the polarization state of the incident radiation, which has been observed in two- and three-photon ionization of Cs, is accounted for by the theory. Finally, the photoelectron spin polarization produced by the multiphoton ionization of unpolarized atoms, like the analogous polarization resulting from single-photon ionization, is found to depend on the circular polarization of the incident radiation.
A theoretical investigation has been carried out on the dependence of the photoelectron angular distributions on laser intensity for the case of two-photon-resonant three-photon ionization of sodium. Good overall agreement is obtained with the experimental anisotropy parameters for ionization via the 4D3/2{D}_{3/2}D3/2​ and 4D5/2{D}_{5/2}D5/2​ intermediate states. The comparison of the theoretical results with the experimental data also clearly shows that the hyperfine splitting of the ground state has to be taken into account at high laser intensities although the low-intensity angular distributions do not depend on the initial ground hyperfine state.
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.
In this paper, we present the theory of photoelectron angular distributions in resonant multiphoton ionization that rigorously takes into account the saturation, ac Stark-shift, and the laser-linewidth effects. The influence of these effects on the distribution is incorporated through the coupling equations between the angle-resolved ionization probabilities and the (time-dependent) bound-state density-matrix elements. General expressions for the angular distributions are derived and several properties of these distributions are arrived at from these expressions. Finally, numerical results are presented to illustrate the effects of saturation, of ac Stark shifts, and of the laser line shape.