Using vibrational branching ratios to probe shape resonances in molecular photoionization

The measurement of vibrational branching ratios in molecular photoionization can be used as a probe of the nature of resonant states, since such states are often sensitive to the geometry of the molecule. Recent computed results for BF$_{3}$ and C$_{6}$F$_{6}$ will be presented. In C$_{6}$F$_{6}$, we consider the excitation of the two symmetric stretching modes in the photoionization leading to the $C \quad ^{3}B_{2u}$ state of the ion. Two prominent shape resonances at photon energies between 18 and 20 eV respond quite differently to the excitation of the symmetric ring-breathing mode and to the symmetric C-F stretching mode. In BF$_{3}$, the excitation of both the symmetric stretching and the degenerate asymmetric stretching modes are considered in the photoionization leading to the $E \quad ^{2}$A$_{1}$' state of the ion. The symmetric stretching mode shows a relatively weak resonant enhancement in the branching ratio, whereas the asymmetric stretching mode has a much more prominent feature.