Photoionization of Na: Resonance enhancement of the single-channel cross section.

Abstract

Total and partial photoionization cross sections for the ground state of Na leaving the residual ${\mathrm{Na}}^{+}$ ions in the three (2${\mathit{p}}^{6}$ $^{1}$S, 2${\mathit{p}}^{5}$3s $^{3,1}$P \ifmmode^\circ\else\textdegree\fi{}) and seven (2${\mathit{p}}^{6}$ $^{1}$S, 2${\mathit{p}}^{5}$3s $^{3,1}$P \ifmmode^\circ\else\textdegree\fi{}, 2${\mathit{p}}^{5}$3p $^{3}$S${,}^{3,1}$D${,}^{1}$P) lowest LS states are calculated using the R-matrix method. Additionally, a 12-state R-matrix calculation is performed leaving the residual ${\mathrm{Na}}^{+}$ ions in LS states of the 2${\mathit{p}}^{6}$, 2${\mathit{p}}^{5}$3s, and 2${\mathit{p}}^{5}$3p manifolds and 2${\mathit{p}}^{5}$3d ${(}^{3,1}$P \ifmmode^\circ\else\textdegree\fi{}${,}^{1}$D\ifmmode^\circ\else\textdegree\fi{}) to determine the cross-section magnitudes of the satellite levels 2${\mathit{p}}^{5}$3p $^{1}$S and 2${\mathit{p}}^{5}$3d $^{3,1}$P \ifmmode^\circ\else\textdegree\fi{} and to assess their influence on the total cross section.Correlation and polarization effects are taken into account through the use of extensive configuration-interaction target wave functions. The results cover both the single- and multiple-channel photon energy regions (2--3.8 Ry). We find that the three-state total cross section is characterized by resonance structure dom- inated by an isolated 2${\mathit{p}}^{5}$3${\mathit{s}}^{2}$ $^{2}$P \ifmmode^\circ\else\textdegree\fi{} resonance at 2.37 Ry (1.62 Mb) and a main resonance in the multiple-channel energy region at 2.84 Ry (21 Mb) due entirely to the 2${\mathit{p}}^{5}$3s $^{3}$P \ifmmode^\circ\else\textdegree\fi{} level. The 2${\mathit{p}}^{5}$3s $^{3,1}$P \ifmmode^\circ\else\textdegree\fi{} satellite lines have unusually large cross sections which dominate that of the main line throughout the energy range. The three-, seven-, and 12-state calculations predict essentially the same behavior for the total cross section. Various partial photoionization cross sections are presented to elucidate the predicted strong satellite enhancement of the single-channel total cross section. We conclude from comparison of the three-, seven-, and 12-state results that coupling effects are important in the multiple-channel region and that the three-state calculation predicts the total cross-section behavior reasonably well over a wide energy range.