Metastable excited states of iodide–alkyl halide cluster anions: Insights from photodetachment spectroscopy and non-Hermitian quantum chemistry

We present experimental and theoretical results for photodetachment from a series of iodide-ethyl halide cluster anions, I-·C2H5X (X = Cl, Br, I). We also analyze previously reported results for the iodide-methyl halide series I-·CH3X, as comparisons between the two series are instructive. The photoelectron angular distributions for detachment from I-·C2H5Cl, I-·C2H5Br, and I-·CH3Cl are remarkably similar to detachment from unclustered iodide, while those of I-·C2H5I, I-·CH3Br, and I-·CH3I are clearly different, suggesting that autodetachment mediated by excited anion states competes with direct detachment. In addition, the I-·C2H5I and I-·CH3I cluster anions reveal a fragmentation pathway that is clearly inconsistent with direct detachment. By means of equation-of-motion coupled-cluster theory combined with a complex absorbing potential, we identify metastable excited anion states in the detachment continuum whose positions explain why the photoelectron angular distributions of only some of the cluster anions are influenced by autodetachment. However, these resonances do not evolve into bound anion states and, therefore, do not mediate photofragmentation. Rather, our calculations reveal for each cluster a second anion state that is related to virtual states in electron-molecule scattering. For such states, the Born-Oppenheimer approximation breaks down, resulting in a characteristic shape of the adiabatic potential energy surface and finite probability for decay into the detachment continuum even in regions where the anion state is below the neutral state.