Vibrational structures of predissociating methylamines (CH3NH2 and CH3ND2) in (A)over-tilde states:: Free internal rotation of CH3 with respect to NH2

Resonantly-enhanced one-color two-photon (1+1) ionization spectra of jet-cooled methylamines (CH3NH2 and CH3ND2) reveal the vibrational structures of these molecules in predissociative A states. Rotational fine structure is clearly resolved for CH3ND2 at the origin and first wagging vibrational level in the excited state. The spectral linewidth becomes homogeneously broadened to give only vibrationally resolved spectral features for the higher vibrational energy levels of CH3ND2 (A). From the spectral analysis of the A-X transition of CH3ND2, it is found that the methyl moiety rotates nearly freely about the C-N axis with respect to the amino group in the A state, indicating that the removal of an electron from the nonbonding orbital of N is responsible for the free internal rotation. Vibrational levels are only barely resolved in the A-X excitation spectrum of CH3NH2 due to severe homogeneous line-broadening, indicating ultrashort lifetimes of similar to0.4 ps for predissociating CH3NH2 molecules in the A state. Spectral interpretation of the A-X excitation spectrum of CH3NH2 is carried out by the comparison with that of CH3ND2, giving the confirmative vibrational assignment of methylamines in A states for the first time. The dramatic difference of CH3NH2 and CH3ND2 in their lifetimes in A states suggests that the major dissociation channel of the excited methylamine may be the N-H (or D) bond dissociation. (C) 2002 American Institute of Physics.