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

The adiabatic dissociation dynamics of NH2D((A) over tilde) and ND2H((A) over tilde) 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, NH2D, ND2H, and ND3 at 193.3 nm. The rotational energy partitioning about the primary a-axis of the fragments NH2((A) over tilde ,v(2)' = 0) and ND2((A) over tilde ,v(2)' = 0) from NH2D(A) and ND2H((A) over tilde), 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((A) over tilde ,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((A) over tilde ,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((A) over tilde) 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((A) over tilde) is larger for cleavage of an N-H bond from NH2D than by cleavage of an N-D from ND2H.