Photoionization of neon between 100 and 2000 eV: Single and multiple processes, angular distributions, and subshell cross sections

Abstract

All aspects of photoionization in the soft-x-ray region are taken into account, and a complete partitioning of the photoionization cross section of neon is given in terms of single-electron processes in $2p$, $2s$, and $1s$ subshells and multiple-electron processes involving these subshells. The various processes, including their angular dependences, are identified and studied by the technique of photoelectron spectrometry. The partition relies solely on experimental evidence. Absolute subshell cross sections for the emission of a single electron are compared with current theoretical predictions: The single-particle, frozen-structure model (Cooper, 1962) that uses the Herman-Skillman potential overestimated ${\ensuremath{\sigma}}_{2p}$ by up to 15%, ${\ensuremath{\sigma}}_{2s}$ by (25-35)%, and ${\ensuremath{\sigma}}_{1s}$ by about 20%; the random-phase-approximation-with-exchange model (Amusia, 1972) that includes multielectron correlation and uses Hartree-Fock wave functions predictions correctly ${\ensuremath{\sigma}}_{2s}$ at $110<h\ensuremath{\nu}<220$ eV, where comparative data exist. The absolute cross section for double ionization in the $L$ shell is 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}20}$ ${\mathrm{cm}}^{2}$ at $h\ensuremath{\nu}=278$ eV as compared with the theoretical value of 4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}20}$ ${\mathrm{cm}}^{2}$. The energy dependence of simultaneous excitation and ionization processes in the $L$ shell is reported. A finite threshold value is observed and a plateau at higher energy is indicated. For $h\ensuremath{\nu}>130$ eV, $\ensuremath{\epsilon}l,{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ transitions are found to be most probable in which the continuum electron changes its angular momentum, $\ensuremath{\Delta}l=\ifmmode\pm\else\textpm\fi{}1$, and the excited electron retains its momentum, $\ensuremath{\Delta}l=0$, namely $2{p}^{6}\ensuremath{\rightarrow}2{p}^{4}\ensuremath{\epsilon}d,np$. Anisotropy parameters $\ensuremath{\beta}$ for $2p$ electrons agree well with theoretical results; however an unexplained maximum near $\ensuremath{\theta}={0}^{\ensuremath{\circ}}$ at $h\ensuremath{\nu}>1$ keV is found for the angular distributions of $2p$ photoelectrons.