Photoionization of jet-cooled HI with coherent vacuum ultraviolet radiation: Evidence for Hund’s case (e)

In the range of 83 500–89 200 cm−1, the photoionization spectrum of HI is strongly influenced by autoionization of Rydberg series converging to the higher lying 2Π1/2 component of the spin–orbit split 2Π ionic ground state. The photoelectron yield spectrum, exhibiting the various autoionization processes, has been measured on a rotationally cooled sample (Trot∼13 K), with a spectral resolution of the exciting vacuum ultraviolet radiation of 0.5 cm−1. Using a multichannel quantum-defect theory treatment explicitly including rotation, we calculate spin–orbit and rotational autoionization fine structure in the spectrum. In order to find a qualitative agreement of theory and experiment, the Rydberg states of HI have to be described in terms of a transition in angular momentum coupling cases, going from Hund’s case (c) to case (e).