Towards resolving the hyperfine structure in ions by photoelectron spectroscopy

Abstract

Pulsed field ionization experiments have been performed on high Rydberg states of the O2 and the H2O molecules and of several isotopes of the krypton atom. Using a high resolution extreme ultraviolet (XUV) laser system pulsed field ionization zero kinetic energy (PFI-ZEKE) photoelectron spectra of the O+2 X2g,(v+ = 2) O2X3-g(v = 0) and the H2O+X2B1 X H2OX1A1 transitions have been recorded at a resolution of 0.3 cm-1. These spectra represent the first photoelectron spectra that provide information on the energy level structure of molecular ions that goes beyond that contained in the rotational structure, and provide new, fundamental information on molecular photoionization. The electron spin splittings in the ground neutral state of O2 have been fully resolved and the results are compared with ab initio predictions of the threshold photoelectron spectrum by Braunstein, M., McKoy, V., and Dixit, S. N. (1992, J. chem. Phys., 96, 5726). The spin-rotation splittings of the rotational levels of the H2O+ X2B1 state also have been resolved, and the intensity distributions in the photoelectron spectrum are influenced by the relative orientation of the electron spin and rotational angular mometum vectors. Pulsed field ionization spectra of high Rydberg states of the krypton atom have been measured at a resolution of 250 kHz, and the hyperfine structure in the spectrum of the Kr isotope has been resolved at n 50. From these measurements it is inferred that the measurement of hyperfine structure intervals in ions by photoelectron spectroscopy and pulsed field ionization techniques may become feasible in the near future.