Isotope effects in complex scattering lengths for He collisions with molecular hydrogen

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 ìhydrogenî masses less than 1 u or a ìheliumî 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 He and 4 He with H2, HD, HT, and DT should be feasible in the near future.