Competition between N-H and N-D bond cleavage in thephotodissociation of NH2D and ND2H
The adiabatic dissociation dynamics of NH{sub 2}D({tilde A}) and ND{sub 2}H({tilde A}) 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 NH{sub 3}, NH{sub 2}D, ND{sub 2}H, and ND{sub 3} at 193.3 nm. The rotational energy partitioning about the primary a-axis of the fragments NH{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) and ND{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) from NH{sub 2}D({tilde A}) and ND{sub 2}H({tilde A}), 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 ND{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) from the photodissociation of ND{sub 2}H than ND{sub 3} 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 NH{sub 2}({tilde A}, {nu}{sub 2}{prime} = 0) fragment is lower when an N-D bond must be cleaved in comparison to an N-H bond. Photodissociation of ND{sub 2}H by cleavage of an N-H bond leads to an ND{sub 2}({tilde A}) fragment with a much larger degree of vibrational excitation ({nu}{sub 2}{prime} = 1,2), accompanied by substantial rotation about the minor b/c-axes, than when an N-D bond is cleaved in the photodissociation of ND{sub 3}. The quantum yield for producing NHD({tilde A}) is larger for cleavage of an N-H bond from NH{sub 2}D than by cleavage of an N-D from ND{sub 2}H.
- Research Organization:
- COLLABORATION - U.Colorado
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 928317
- Report Number(s):
- LBNL-52067; R&D Project: 401001; BnR: KC0301020; TRN: US200815%%836
- Journal Information:
- Journal of Physical Chemistry A, Vol. 104, Issue 45; Related Information: Journal Publication Date: 2000
- Country of Publication:
- United States
- Language:
- English
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