State-to-state reaction dynamics in crossed supersonic jets : threshold evidence for non-adiabatic channels in F + H 2

The reaction of to form HF(v,J) ] H is studied in a crossed jet apparatus under F ] n-H 2 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(2P 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 surface (K. F(2P 3@2 ) ] H 2 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 spinÈorbit state channel. These observations strongly suggest that F*(P 1@2 ) ] H 2 ( j \ 0,1) non-adiabatic reactions with spinÈorbit excited contribute signiÐcantly in the near F*(P 1@2 ) 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 ] CH 4