Bond-selective photodissociation of partially deuterated ammonia molecules: Photodissociations of vibrationally excited NHD_2 in the 5upsilon_(NH) state and NH_2D in the 5upsilon_(ND) state

摘要

Ultraviolet photodissociation of NHD_2 excited to the fourth overtone state of the NH stretching mode (5upsilon_(NH)) and NH_2D excited to that of the ND stretching mode (5upsilon_(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_2 in the 5upsilon_(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_2D in the 5upsilon_(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_2D and NHD_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.