Metastable He2- and its autodetachment spectra: An accurate coupled-cluster study

The unusual autodetachment spectrum of metastable He2- is studied theoretically, using high-level coupled-cluster methods and hybrid numerical and Slater orbital basis sets. By obtaining accurate curves for the repulsive wall of the ground He2(X g+1) state, the excited He2(a u+3) state, and the metastable He2-(4 g) state, we are able to provide an alternative explanation for the experimental observations, which had cast doubt on the veracity of the accepted curve for the repulsive part of the He2(X1g+) potential. We attribute the experimental peak at 15.78±0.13 eV to transitions of vibrationally excited states (=2 and higher) of He2- to the He2(X g+1) continuum, since the v=0 transition would have a value 1 eV less. An error of this size is considered to be far outside the error bars for highly accurate correlated ab initio calculations. The electron affinity of the g4 state of the anion (measured relative to a u+3) is computed to be 0.201 and 0.212 eV for different basis sets, compared to an experimental value of 0.175±0.032 eV.