Dynamics of theS(D21)+HD(j=0

Abstract

We report integral cross sections for the Sð 1 D 2 Þ þ HDðj ¼ 0Þ ! DS þ H and HS þ 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 þ 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. DOI: 10.1103/PhysRevLett.109.133201 PACS numbers: 34.50.Lf, 31.15.xj, 31.50.Àx, 37.20.+j For theory to furnish a good description of elementary gas-phase reaction dynamics requires the use of a highly accurate potential energy surface (PES) describing the passage from reagents to products. Recent progress in the determination of PESs by ab initio methods [1] has allowed quantum-mechanical (QM) or quasiclassical trajectory treatments of the reaction dynamics to reproduce the integral and differential cross sections (ICSs and DCSs) obtained in high-resolution crossed-beam experiments for prototypical four-atom [3,4]. This has followed the exquisite agreement between theory and experiment found for the benchmark three-atom 7,8] reactions for which resonance features have been fully rationalized. However, all these studies have been performed at medium to high collision energies to be able to surmount the classical energy barriers, between 70 and 300 meV, which characterize these reactions. The lowest collision energy attained so far is E T ¼ 6 meV in the case of the F þ H 2 reaction for which substantial tunneling through the barrier occurs [8]. An important question arises: Will the accuracy of electronic structure calculations (currently recognized to be within the 10-40 meV range) be sufficient to reproduce experimental studies of reactive processes occurring at very low collision energies? When only a few partial waves, characterized by a given value of total angular momentum J which is conserved throughout the collision, contribute to the dynamics, individual quantum effects become apparent and the slightest inaccuracy of the PES can lead to dramatic differences in the theoretical results. Multisurface effects arising from the open-shell nature of the reactants may also start to play a dominant role as recently outlined for the Fð 2 P 1=2 Þ þ H 2 ðj ¼ 0Þ reaction in which a pronounced resonance peak is predicted in the ICSs around E T ¼ 0:2-0:3 meV [9].Despite the tremendous advances in the generation of cold atomic and molecular species by Stark or Zeeman deceleration, buffer-gas, or laser cooling [10,11], very few experiments are able to approach the cold energy regime at temperatures below T ¼ 1 K. One can cite the recent buffer-gas cooling study of the Li þ CaH ! LiH þ Ca reaction at T ¼ 1 K [12] but experiments of this type provide rate coefficients, not cro...

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