We examine the effect of theoretically varying the collision-system reduced mass in collisions of He with vibrationally excited molecular hydrogen and observe zero-energy resonances for select atomic “hydrogen” masses less than 1 u or a “helium” mass of $1.95$ u. Complex scattering lengths, state-to-state vibrational quenching cross sections, and a low-energy elastic scattering resonance are all studied as a function of collision-system reduced mass. Experimental observations of these phenomena in the cold and ultracold regimes for collisions of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ with ${\mathrm{H}}_2$, HD, HT, and DT should be feasible in the near future.

• Received 30 March 2009

DOI:https://doi.org/10.1103/PhysRevA.81.014701