Bond-selective photodissociation of partially deuterated ammonia molecules: Photodissociations of vibrationally excited NHD[sub 2] in the 5ν[sub NH] state and NH[sub 2]D in the 5ν[sub ND] state.

Ultraviolet photodissociation of NHD[sub 2] excited to the fourth overtone state of the NH stretching mode (5ν[sub NH]) and NH[sub 2]D excited to that of the ND stretching mode (5ν[sub ND]) has been investigated by using a crossed laser and molecular beams method. Branching ratio between the NH and ND bond dissociations has been determined by utilizing a (2+1) resonance enhanced multiphoton ionization scheme of H and D atoms. For the photolysis of NHD[sub 2] in the 5ν[sub NH] state, the NH dissociation cross section is 5.1±1.4 times as large as the ND dissociation cross section per bond. On the other hand, for the photolysis of NH[sub 2]D in the 5ν[sub ND] state, the ratio of the NH dissociation cross section per bond to the ND dissociation cross section decreases to 0.68±0.16. In comparison with the branching ratios for the photolysis of vibrationally unexcited NH[sub 2]D and NHD[sub 2] [Koda and Back, Can. J. Chem. 55, 1380 (1977)], the present results indicate that the excitation of the NH stretching mode enhances the NH dissociation with ca. two times larger NH/ND branching ratio, whereas the excitation of the ND stretching mode results in the preferential ND dissociation with ca. 3–4 times larger ND/NH branching ratio than that for the vibrational ground states. The mechanism of the bond-selective enhancement has been discussed in terms of the energetics and dynamics of wave packet. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]