The total absorption cross section of He and the ${\mathrm{He}}^{+}$(1s) partial cross section have been measured using photoelectron spectroscopy and synchrotron radiation in the photon-energy region of the 1$s^2$→3s3p resonance. The effects of autoionization are relatively small, causing deviations of 5–10 % from the nonresonant cross sections. These new measurements, along with a previous study of the 3s3p resonance behavior of the ${\mathrm{He}}^{+}$(n=2) satellite cross section, branching ratio, and angular-distribution parameter, allow the complete determination of all the parameters governing the 1$s^2$→3s3p resonance. From these experimentally determined parameters we have derived all of the relevant oscillator strengths and individual dipole and Coulomb matrix elements which describe He photoionization in the vicinity of the 3s3p doubly excited state.

• Received 6 March 1987

DOI:https://doi.org/10.1103/PhysRevA.36.2112