Photoionization of the 4$f$ and 6$s$ Subshells of Atomic Thulium in the Region of the 5$p$ Excitations

The relative partial cross sections, $\sigma$, and the angular distribution parameters, $\beta$, of the photoelectrons originating from the $4f$ and $6s$ subshells of atomic thulium have been measured in the region of the $5p$ excitations (23~eV -- 38~eV). In qualitative agreement with the early absorption work of Tracy [1], our $\sigma$ curves show two regions of strong resonance enhancement, one around a photon energy of 27~eV corresponding to $5p_{3/2}$ excitations and the other near 32~eV corresponding to $5p_{1/2}$ excitations. The $\beta$ curves show considerable variation between the allowed extremes of -1.0 and 2.0 for many of the $4f$ photolines. In addition, the $6s$ photoline shows very marked deviations from $\beta = 2.0$ which is indicative of an open-shell atom. We also compare our calculated $4f$ partial cross sections, using a modified version of the Cowan code [2], with experiment. While there is good qualitative agreement between theory and experiment, quantitative details are not reproduced: i) the $5p_{3/2}$ excitations are too narrow and ii) have an intensity larger than what is observed experimentally. [1] D. H. Tracy, Proc. R. Soc. Lond. A {\bf 357}, 485 (1977), [2] M. Martins, J. Phys. B, {\bf 34}, 1321 (2001).