Photoionization from excited states of helium

The cross sections for photoionization from the $2{}_{}{}^1{}_{}{}^{}S$, $2{}_{}{}^3{}_{}{}^{}S$, $2{}_{}{}^1{}_{}{}^{}P$, and $2{}_{}{}^3{}_{}{}^{}P$ excited states of helium are calculated for photoelectron energies below the $n=2\mathrm{}$ threshold of ${\mathrm{He}}^{+}$ using Hylleraas bound-state wave functions and $1s2s2p$ close-coupling final-state wave functions. The resonant structures associated with the lowest-lying ${}_{}{}^1{}_{}{}^{}S$, ${}_{}{}^1{}_{}{}^{}P$, ${}_{}{}^3{}_{}{}^{}P$, and ${}_{}{}^1{}_{}{}^{}D$ autoionizing states of helium are found to be characterized by large values of the line-profile parameter $q$. The cross sections and the photoelectron angular-distribution asymmetry parameters for the $P$ states are calculated for various polarization states of the target atom and the incident photon. Experiments which would lead to the separate determinations of the $S$ and $D$-wave partial photoionization cross sections are discussed.