COMPETING BOND FISSION AND MOLECULAR ELIMINATION CHANNELS IN THE PHOTODISSOCIATION OF CH3NH2 AT 222 NM

This paper presents the first experimental investigation under collisi onless conditions of the competing photodissociation channels of methy lamine excited in the first ultraviolet absorption band. Measurement o f the nascent photofragments' velocity distributions and preliminary m easurements of some photofragments' angular distributions evidence fou r significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H-2 elimination. The data, taken on photofragments from bo th methylamine and methylamine-d(2), elucidate the mechanism for each competing reaction. Measurement of the emission spectrum of methylamin e excited at 222 nm gives complementary information, evidencing a prog ression in the amino wag (or inversion) and combination bands with one quantum in the methyl (umbrella) deformation or with two quanta in th e amino torsion vibration. The emission spectrum reflects the forces i n the Franck-Condon region which move the molecule toward a ciscoid ge ometry. The photofragment kinetic energy distributions measured for CH 3ND2 show that hydrogen elimination occurs via a four-center transitio n state to produce HD and partitions considerable energy to relative p roduct translation. The reaction coordinates for N-H and C-N fission a re analyzed in comparison to that for ammonia dissociation from the (A ) over tilde state and with reference to a initio calculations of cuts along the excited state potential energy surface of methylamine which show these reactions traverse a small barrier in the excited state fr om a Rydberg/valence avoided crossing and then encounter a conical int ersection in the exit channel. The measured kinetic energy distributio n of the C-N bond fission photofragments indicates that the NH2 (ND2) product is formed in the (A) over tilde (2)A(1) state; the C-N fission reactive trajectories thus remain on the upper adiabat as they traver se the conical intersection. The mechanism for C-H bond fission is les s clear; most of the kinetic energy distribution indicates the reactio n evolves on a potential energy surface with no barrier to the reverse reaction, consistent with dissociation along the excited state surfac e or upon internal conversion to the ground state, but some of the dis tribution reflects more substantial partitioning to relative translati on, indicating that some molecules may dissociate via a repulsive trip let surface. In general, the photofragment angular distributions were anisotropic, but the measured beta approximate to -0.4 +/- 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation. We end with analyzing why in methylamine three other primary dissociation channels effectively compete with N-H fission while in CH3OH and CH3SH primarily O-H and S-H fission, respe ctively, dominate.