Photoelectron angular distributions from multiphoton ionization of cesium atoms

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 7p 2P/sub 1/2,3/2/ and 8p 2P/sub 1/2,3/2/ intermediate states and three-photon ionization via the two-photon-allowed 8d 2D/sub 5/2,3/2/ resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of approx.0.1 eV and angular resolution of approx. +- 20. A straightforward calculation based upon a fine-structure scheme gives excellent agreement with the measured angular distributions for the 2P/sub 1/2/ 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 2P/sub 3/2/ resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the 2P/sub 3/2/ data