The influence of the bromine atom Cooper minimum on the photoelectron angular distributions and branching ratios of the four outermost bands of bromobenzene

Angle resolved photoelectron spectra of the $X ̃ 2 B 1$⁠, $A ̃ 2 A 2$⁠, $B ̃ 2 B 2$⁠, and $C ̃ 2 B 1$ states of bromobenzene have been recorded over the excitation range 20.5-94 eV using linearly polarized synchrotron radiation. The photoelectron anisotropy parameters and electronic branching ratios derived from these spectra have been compared to theoretical predictions obtained with the continuum multiple scattering approach. This comparison shows that ionization from the 8b₂ orbital and, to a lesser extent, the 4b₁ orbital is influenced by the Cooper minimum associated with the bromine atom. The 8b₂ and 4b₁ orbitals are nominally bromine lone-pairs, but the latter orbital interacts strongly with the π-orbitals in the benzene ring and this leads to a reduced atomic character. Simulations of the $X ̃ 2 B 1$⁠, $B ̃ 2 B 2$⁠, and $C ̃ 2 B 1$ state photoelectron bands have enabled most of the vibrational structures appearing in the experimental spectra to be assigned. Many of the photoelectron peaks exhibit an asymmetric shape with a tail towards low binding energy. This asymmetry has been examined in the simulations of the vibrationally unexcited peak, due mainly to the adiabatic transition, in the $X ̃ 2 B 1$ state photoelectron band. The simulations show that the asymmetric profile arises from hot-band transitions. The inclusion of transitions originating from thermally populated levels results in a satisfactory agreement between the experimental and simulated peak shapes.