State-to-state reaction dynamics in crossed supersonic jets:: threshold evidence for non-adiabatic channels in F+H2

The reaction of F+n-H-2 to form HF(v,J)+H is studied in a crossed jet apparatus under single collision conditions, using high-resolution direct absorption spectroscopy to probe the nascent rotational HF distributions. The J-dependent reactive cross-sections into HF(v=3,J) are investigated over a range of center-of-mass collision energies well below the 1.9 kcal mol(-1) barrier for adiabatic chemical reactions with ground state F(P-2(3/2)) atoms. The energy dependent reaction cross-sections decrease much more slowly with E-com than predicted by exact quantum calculations on the adiabatic F(P-2(3/2))+H-2 surface (K. Stark and H. Werner, J. Chem. Phys., 1996, 104, 6515). In addition, product states in the HF(v=3) manifold are observed that are energetically accessible only to the excited spin-orbit state F*(P-1/2)+H-2(j=0,1) channel. These observations strongly suggest that non-adiabatic reactions with spin-orbit excited F-*(P-1/2) contribute significantly in the near threshold region, in good agreement with recent calculations by M. Alexander, H. Werner and D. Manolopoulos (J. Chem. Phys., 1998, 109, 5710). Finally, the feasibility of high-resolution IR laser Dopplerimetry on the nascent products is illustrated on collision free HF(v,J) distributions formed from reactions of F+CH4.