Reaction pathway and energy disposal of the CaH product in the reaction of Ca(4s4p 1P1)+CH4→CaH(X 2Σ+)+CH3

The reaction pathway for Ca(4s4p P-1(1))+CH4-->CaH(X (2)Sigma(+))+CH3 has been investigated by using the pump-probe technique in combination with potential energy surface (PES) calculations. The nascent product distributions of CaH have been found with a Boltzmann rotational temperature of 988+/-66 and 864+/-75 K for the v=0 and 1 levels, respectively, and a Boltzmann vibrational temperature of 1960+/-80 K. The rotational and vibrational energy partitions in CaH have been estimated to be 555+/-22 and 995+/-10 cm-1, respectively. According to the PES calculations, the pathway is found to favor an insertion mechanism. Ca(4 P-1(1)) approaches CH4 in C-2v or C-s symmetry and then the collision complex may undergo a series of surface transitions to the ground state surface with which the products correlate. The findings of low rotation and high vibration for CaH may be interpreted from two aspects. First, the Ca-C bond distance of the intermediate around the surface crossing region is 2.7-2.8 Angstrom, close to the equilibrium bond distance 2.349 Angstrom. The strong coupling of the moieties renders the energy transfer sufficient from CaH into the CH3 radical. Second, after the 2A(')-1 A(') surface transition, the HCaCH3 intermediate with a small excess energy may be energetically stabilized. The long-lived collision complex may have enough time for energy randomization prior to flying apart. (C) 2003 American Institute of Physics.