Triplet Excitons in Small Helium Clusters

An electron traveling through liquid helium with sufficient kinetic energy can create a low-lying triplet exciton via inelastic scattering. Accompanying repulsion between the exciton and nearby atoms results in bubble formation. That is not all, however. Repulsion compresses an “incipient He2* exciton”, pushing it into a region where an He2* moiety commences evolution toward its potential energy minimum. The above picture follows from ab initio calculations of the two lowest adiabatic potential energy surfaces for collinear three-atom systems and dynamics studies launched on the lowest adiabat that calculate said surface on the fly. The timescale for launching trajectories toward the He2* moiety is significantly shorter than the timescale for pushing helium away from the exciton in large systems, making results with three atoms relevant to liquid helium. This explains how He2* might be created in the aftermath of electron-impact excitation of He*. Interplay between the lowest adiabats is discussed, underscoring the importance of nonadiabatic processes in such systems. Results with eight-atom systems further illustrate the critical role of nonadiabatic transitions.