Dynamics of the S(1D2)+HD(j=0) Reaction at Collision Energies Approaching the Cold Regime: A Stringent Test for Theory

We report integral cross sections for the S(D-1(2)) + HD(j = 0) -> DS + H and HS + D reaction channels obtained through crossed-beam experiments reaching collision energies as low as 0.46 meV and from adiabatic time-independent quantum-mechanical calculations. While good overall agreement with experiment at energies above 10 meV is observed, neither the product channel branching ratio nor the low-energy resonancelike features in the HS + D channel can be theoretically reproduced. A nonadiabatic treatment employing highly accurate singlet and triplet potential energy surfaces is clearly needed to resolve the complex nature of the reaction dynamics.