The effect of Wigner singularities on low-temperature vibrational relaxation rates

We have applied the vibrational-close-coupling, rotational-infinite-order- sudden (VCC-RIOS) and vibrational-rotational close coupling (CC) methods to collisions between He and I₂(X ¹Σ_g ⁺) to study the threshold dependence of vibrational deexcitation cross sections and rates on energy and temperature, respectively, for the v = 1→0 and v = 16→15 processes, where v is the vibrational quantum number. The long-range attraction causes the cross sections to be decreasing functions of energy below about 10 cm^-1 and causes the thermally averaged deexcitation rate constants to have a minimum at about 1 K, but the Wigner threshold law holds only within 10^-3 cm^-1 of threshold and the Wigner singularity causes the rate constant to assume a nonzero temperature-independent value only below about 10^-4 K. We also observe low-energy resonances in the CC calculations and these lead to additional structure in the rate constants but do not change the important conclusions.