Photodissociating trimethylamine at 193 nm to probe dynamics at a conical intersection and to calibrate detection efficiency of radical products

This paper reports crossed laser-molecular beam scattering experiments measuring the photofragment velocities and product branching in the photodissociation of trimethylamine (N(CH3)(3)) at 193 nm. We have observed two primary N-CH3 bond fission channels that ultimately produce different nitrogen-containing species, CH3+N(CH3)(2) ((X) over tilde B-2(1)); CH3+N(CH3)(2)(*)--> CH3+CH2NCH3+H. The data also indicate that a third minor channel may contribute to the dissociation dynamics, CH3+N(CH3)(2)(dagger)--> CH3+NC2H4+H-2. The experiments show that ground state N(CH3)(2) radicals are formed in the photodissociation, in sharp contrast to the exclusive production of NH2 ((A) over tilde (2)A(1)) in a similar molecule, methylamine. We discuss how this results from the differing dynamics through the S-1-S-0 conical intersection in the exit channel in these two dissociating amines. We also use the photodissociation results to calibrate the mass spectrometric sensitivity at the m/e=15 daughter ion for methyl radicals vs N(CH3)(2) and CH2NCH3 products. This provides the only necessary calibration to determine an absolute branching ratio in any system producing a methyl radical in one reaction channel and N(CH3)(2) ((X) over tilde B-2(1)) or CH2NCH3 in other reaction channels. (C) 1999 American Institute of Physics. [S0021-9606(99)01734-1].