Direct evidence for nonadiabatic dynamics in atom+polyatom reactions:Crossed-jet laser studies of F+D_2O- DF+OD

摘要

Quantum-state-resolved reactive-scattering dynamics of F+D_2O->DF+OD have been studied at E_(c.m.)=5(1) kcal/mol in low-density crossed supersonic jets,exploiting pulsed discharge sources of F atom and laser-induced fluorescence to detect the nascent OD product under single-collision conditions.The product OD is formed exclusively in the v_(OD)=0 state with only modest rotational excitation (=0.50(1) kcal/mol),consistent with the relatively weak coupling of the 18.1(1) kcal/mol reaction exothermicity into "spectator" bond degrees of freedom.The majority of OD products [68(1)%] are found in the ground (~2PI_(3/2)~(+-)) spin-orbit state,which adiabatically correlates with reaction over the lowest and only energetically accessible barrier (DELTA E~(not=)approx =4 kcal/mol).However,32(1)% of molecules are produced in the excited spin-orbit state (~2PI_(1/2)~(+-)),although from a purely adiabatic perspective,this requires passage over a DELTA E~(not=)approx=25 kcal/mol barrier energetically inaccessible at these collision energies.This provides unambiguous evidence for nonadiabatic surface hopping in F+D_2O atom abstraction reactions,indicating that reactive-scattering dynamics even in simple atom+polyatom systems is not always isolated on the ground electronic surface.Additionally,the nascent OD rotational states are well fitted by a two-temperature Boltzmann distribution,suggesting correlated branching of the reaction products into the DF(v=2,3) vibrational manifold.