## Abstract

New atomic cross-sectional data are reported in graphical and tabular form from the ionization threshold at 1378 $\overset{\circ}{\mathrm A}$ down to 525 $\overset{\circ}{\mathrm A}$. The value of the photoionization cross-section at the threshold is 0$\cdot$32 $\pm$ 0$\cdot$6 Mb falling to 0$\cdot$1 Mb in the region of 1200 $\overset{\circ}{\mathrm A}$ and thereafter is dominated by autoionizing transitions. Configuration interaction between the levels of the (4d)$^9$(5s)$^2$5p multiplet is observed which causes the $^3$D$_1$ level to interact strongly with the wings of the $^3$P$_1$ level. The oscillator strengths for the (5s)$^{2 1}$S$_0$-(4d)$^9$(5s)$^2$np$^1$P$_1$, $^3$P$_1$ and $^3$D$_1$ transitions converging on the $^2$D$_{\frac{5}{2},\frac{3}{2}}$ ionic states are reported and an oscillator strength sum approaching unity is observed. Individual line oscillator strengths are compared with Hartree-Fock calculations of Wilson (1968).