Resonant Interatomic Coulombic Decay in HeNe: Electron Angular Emission Distributions

We present a joint experimental and theoretical study of resonant interatomic Coulombic decay (RICD) in HeNe employing high resolution cold target recoil ion momentum spectroscopy and ab initio electronic structure and nuclear dynamics calculations. In particular, laboratory- and molecular-frame angular emission distributions of RICD electrons are examined in detail. The exciting-photon energy-dependent anisotropy parameter beta(omega), measured for decay events that populate bound HeNe+ ions, is in agreement with the calculations performed for the ground ionic state X-2 Sigma(+)(1/2). A contribution from the a(2)Pi(3/2) final ionic state is found to be negligible. For the He + Ne + fragmentation channel, the observed laboratory-frame angular distribution of RICD electrons is explained by a slow homogeneous dissociation of bound vibrational levels of the final ionic state Lambda(2)Pi(1/2) into vibrational continua of the lower lying states X-2 Sigma(+)(1/2) and a(2)Pi(3/2). Our calculations predict that the angular distributions of RICD electrons in the body-fixed dipole plane provide direct access to the electronic character (i.e., symmetry) of intermediate vibronic resonances. However, because of the very slow dissociation of the Lambda(2)Pi(1/2) state, the molecular-frame angular distributions of RICD electrons in the He + Ne+ fragmentation channel are inaccessible to our coincidence experiment.