Vibration-dependent photoelectron angular distributions and branching ratios observed across the Cooper-minimum region of bromobenzene

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Vibrational state-resolved photoelectron anisotropy parameters, β, for the ˜X²B₁, ˜B²B₂, and ˜C²B₁ state ionizations of bromobenzene have been recorded at photon energies ranging from 20.5 to 94 eV, thus spanning the region of the expected bromine Cooper minimum (CM). The ˜X state displays no CM and its β value is also independent of vibrational level, in accord with the Franck-Condon approximation. The ˜B and ˜C state β values display the CM to differing degrees, but both show a vibrational dependence that extends to energies well below the obvious CM dip. Calculations are presented that replicate these observations. We thus demonstrate a wide-ranging Franck-Condon approximation breakdown detected in the β anisotropy parameter in the absence of any resonance. Measured and calculated vibrational branching ratios for these states are also presented. Although the ˜B state branching ratios remain constant, in accord with Franck-Condon expectations, the ˜X and (especially) the ˜C state ratios display weak, quasilinear variations across the studied range of photon energy, but with no apparent correlation with the CM position.</p></div>