Selectivity of the Br 3 d − 1 Auger decays in HBr

The Auger decay of the spin-orbit and molecular-field split Br $3{d}^{\ensuremath{-}1}$ core holes in HBr is investigated, both by a photoelectron--Auger-electron coincidence experiment and by ab initio calculations based on the one-center approximation. The branching ratios for the Auger decay of the five different core-hole states to the $4p{(\ensuremath{\sigma},\ensuremath{\pi})}^{\ensuremath{-}2}$ dicationic final states are determined. Experimental and theoretical data are in good agreement and conform to results for the $4p{\ensuremath{\pi}}^{\ensuremath{-}2}$ final states from a previous analysis of the high-resolution conventional Auger-electron spectrum. The branching ratios for the Br $3{d}^{\ensuremath{-}1}$ Auger decay to the $4p{(\ensuremath{\sigma},\ensuremath{\pi})}^{\ensuremath{-}2}$ with \ensuremath{\Sigma} symmetry follow the propensity rule of ${L}_{2,3}VV$ Auger decay [S. Svensson, A. Ausmees, S. J. Osborne, G. Bray, F. Gel'mukhanov, H. \AA{}gren, A. Naves de Brito, O.-P. Sairanen, A. Kivim\"aki, E. N\~ommiste, H. Aksela, and S. Aksela, Phys. Rev. Lett. 72, 3021 (1994)] stating that the oriented core holes decay preferentially by involving a valence electron from an orbital with the same spatial orientation. For the ${M}_{4,5}VV$ decay in HBr this propensity rule has to be supplemented by the requirement that the Auger-electron channel and the other valence orbital have the same preferential orientation. We also probe the influence of the Auger kinetic energy on the distortion of the photoline caused by the postcollision interaction effect. For small kinetic energies, differences between experimental results and theoretical predictions are identified.