Competition between N-H and N-D bond cleavage in the photodissociation of NH₂D and ND₂H

The adiabatic dissociation dynamics of NH₂D(Ã) and ND₂H(Ã) have been probed by time-resolved Fourier transform infrared emission spectroscopy. A product-state spectral pattern recognition technique is employed to separate out the emission features arising from the different photofragmentation channels following the simultaneous excitation of mixtures of the four parent molecules NH₃, NH₂^D, ND₂H, and ND₃ at 193.3 nm. The rotational energy partitioning about the primary a-axis of the fragments NH₂(Ã,v′₂ = 0) and ND₂(Ã,v′₂ = 0) from NH₂D(Ã) and ND₂H(Ã), respectively, is bimodal. We suggest that the origin of this excitation reflects the competition between two distinct dissociation mechanisms that sample two different geometries during the bond cleavage. A larger quantum yield for producing ND₂(Ã,v′₂ = 0) from the photodissociation of ND₂H than ND₃ is attributed to the lower dissociation energy of the N-H as compared with the N-D bond and to the enhanced tunneling efficiency of H atoms over D atoms through the barrier to dissociation. Similarly, the quantum yield for producing the NH₂(Ã,v′₂ = 0) fragment is lower when an N-D bond must be cleaved in comparison to an N-H bond. Photodissociation of ND₂H by cleavage of an N-H bond leads to an ND₂(Ã) fragment with a much larger degree of vibrational excitation (v′₂ = 1,2), accompanied by substantial rotation about the minor b/c-axes, than when an N-D bond is cleaved in the photodissociation of ND₃. The quantum yield for producing NHD(Ã) is larger for cleavage of an N-H bond from NH₂D than by cleavage of an N-D from ND₂H.