CD₃ Deformation Modes as Preferential Promoters of Methylamine-d₃ to the First Electronic State

The room-temperature photoacoustic Raman and jet-cooled H action spectra, measured in the region of the fundamental CD₃ stretches and the almost isoenergetic overtones or combinations of CD₃ deformations in the methylamine-d₃ (CD₃NH₂) isotopologue, show different relative intensities of the vibrational bands. The observed difference and the vibrational assignment point to favored ultraviolet excitation due to larger Franck-Condon (FC) factors from the deformation modes, leading to more effective N-H bond cleavage in CD₃NH₂ predissociation. The comparable measured two-color reduced-Doppler ion images and total kinetic energy distributions resulting from the predissociation of molecules promoted from vibrationally excited and vibrationless ground states confirmed that the FC factors and not the ensuing dynamics are the main reason for the mode specificity in this molecule.