Photoionization of iodine atoms: Angular distributions and relative partial photoionization cross-sections in the energy region 11.0-23.0 eV

Relative partial photoionization cross-sections and angular distribution parameters, β, have been measured for the first, I+(3P2)<-I(2P3/2), and fourth, I+(1D2)<-I(2P3/2), (5p)-1 photoelectron (PE) bands of atomic iodine, by performing angle-resolved constant-ionic-state (CIS) measurements on these PE bands in the photon energy range 11.0-23.0 eV. Three Rydberg series, two ns and one nd series, which converge to the I+3P1 limit at 11.33 eV and four Rydberg series, two ns and two nd series, which converge to the I+1D2 limit at 12.15 eV were observed in the first PE band CIS spectra. The fourth band CIS spectrum showed structure in the 12.9-14.1eV photon energy range, which is also seen in the first band CIS spectra. This structure arises from excitation to ns and nd Rydberg states that are parts of series converging to the I+1S0 limit we reported on earlier, as well as 5s→5p excitations in the photon energy range 17.5-22.5 eV. These atomic iodine CIS spectra show reasonably good agreement with the equivalent spectra obtained for atomic bromine. The β-plots for the first PE band recorded up to the I+3P1 and I+1D2 limits only show resonances corresponding to some of the 5p→nd excitations observed in the first band CIS spectra scanned to the I+1D2 limit (12.15 eV). These plots are interpreted in terms of an angular momentum transfer model with the positive values of β obtained on resonances corresponding to parity allowed jt=1 and 3 channels and the off-resonance negative β values corresponding to parity unfavored channels, where jt is the quantum number for angular momentum transfer between the molecule, and the ion and photoelectron. The β-plots recorded for iodine are significantly different from those obtained for atomic bromine. Comparison of the experimental CIS spectra and β-plots with available theoretical results highlights the need for higher level calculations which include factors such as configuration interaction in the initial and final states, relativistic effects including spin-orbit interaction, and autoionization via resonant Rydberg states.