On the formation of (anionic) excited helium dimers in helium droplets.

Metastable atomic and molecular helium anions exhibiting high-spin quartet configurations can be produced in helium droplets via electron impact. Their lifetimes allow detection in mass spectrometric experiments. Formation of atomic helium anions comprises collision-induced excitation of ground state helium and concomitant electron capture. Yet the formation of molecular helium anions in helium droplets has been an unresolved issue. In this work, we explore the interaction of excited helium atoms exhibiting high-spin triplet configurations with ground state helium using the equation-of-motion coupled-cluster method. Transition barriers in the energetically lowest He*-He and He*(-)-He interaction potentials prevent molecule formation at the extremely low temperatures present in helium droplets. In contrast, some excited states allow a barrier-free formation of molecular helium (anions). Moreover, we show that the necessary excitation energies pinpoint (higher) resonances in recently recorded mass spectra and emend the assignment of those resonances that have previously been assigned to electron-impact ionization of ground state helium necessitating subsequent double-electron capture. Embedding molecules or molecular clusters in helium droplets is a predestined experimental technique for the study of phenomena at very low temperatures. Profound knowledge about active processes in the helium environment is required for a proper assessment of experimental data.