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 7p²P1/2,3/2 and 8p²P1/2,3/2 intermediate states and three-photon ionization via the two-photon-allowed 8d²D5/2,3/2 resonant intermediate states are reported. The photoelectrons are energy analyzed with a spherical-sector electrostatic energy analyzer with an energy resolution of ~ 0.1 eV and angular resolution of ~ ±{}2^∘{}. A straightforward calculation based upon a finestructure scheme gives excellent agreement with the measured angular distributions for the ²P1/2 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 ²P3/2 resonant intermediate states. Quantitative calculations including hyperfine coupling take this into account and provide an expression which gives a reasonable fit to the ²P3/2 data.