Interactions and dynamics of metastable triplet-state helium dimer with Ne and Ar

The helium dimer in its metastable triplet electronic state, He2(aΣu+3), is a promising candidate for direct laser cooling and precision measurements. Interactions and collisions involving He2(aΣu+3) remain unexplored in ultracold and cold energy domains. Here, we present the first theoretical investigation of intermolecular interactions and collision dynamics of He2(aΣu+3) + Ne van der Waals complex. For the lowest electronic state, X3A', we employ a spin-restricted coupled cluster method with single, double, and noniterative triple excitations. We construct a three-dimensional vibrationally averaged potential energy surface. The resulting interaction potential exhibits an extremely shallow well with a depth of 4.612 cm-1. Using this potential energy surface, we perform full close-coupling calculations, including fine-structure levels of He2(aΣu+3), to investigate collisional excitation and quenching induced by Ne. We analyze the low-energy scattering dynamics and model resonant features associated with quasi-bound states supported by the interaction potential. A propensity rule for fine-structure conserving transitions is established. We additionally consider He2(aΣu+3)+Ar collisions and compare their energy dependence and resonance structure with those of He2 + Ne. The He2 + Ar system exhibits a richer resonance structure, highlighting the sensitivity of low-energy scattering to interaction potential and reduced mass. Calculated PES and scattering properties provide a reliable benchmark for the description of the He2 + Ne van der Waals complex and may be useful for the interpretation of future experimental spectra and collision measurements.