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

Vibrational state-resolved photoelectron anisotropy parameters, beta, for the (X ) over tilde B-2(1), (B) over tilde B-2(2), and (C) over tilde B-2(1) 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) over tilde state displays no CM and its beta value is also independent of vibrational level, in accord with the Franck-Condon approximation. The (B) over tilde and (C) over tilde state beta 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 beta anisotropy parameter in the absence of any resonance. Measured and calculated vibrational branching ratios for these states are also presented. Although the (B) over tilde state branching ratios remain constant, in accord with Franck-Condon expectations, the (X) over tilde and (especially) the (C) over tilde state ratios display weak, quasilinear variations across the studied range of photon energy, but with no apparent correlation with the CM position.