Predissociation dynamics of Ã-state ammonia probed by two-photon excitation spectroscopy

TLDR: In this paper, the potential energy surface for ND3(A¯) molecules in the region of the D2N-D dissociation coordinate was investigated and it was shown that at short separations this surface exhibits a barrier.

Abstract: Electronically excited ND3(A¯) molecules have been prepared by laser two-photon excitation on theA¯1A″2—X¯1′1 transition and monitored via their resulting short-lived emission. The earlier observation of Douglas that ND3(A¯) molecules carrying one quantum of out-of-plane bending vibration ν′2 are least susceptible to predissociation, is confirmed. ND3(A¯) predissociation rates are found to be both vibronic and rovibronic level dependent. Both observations may be understood by considering the likely form of the potential energy surface for ND3(A¯) molecules in the region of the D2N—D dissociation coordinate. At short D2N—D separations this surface exhibits a barrier. The presence of a conical intersection (involving the ND3 ground state surface) further out along the dissociation coordinate has a crucial influence on the magnitude of this barrier. The envisaged form of theA¯-state potential energy surface also provides a qualitative rationale for all previous experimental findings concerning electronic branching ratios and energy disposal amongst the primary photofragments arising in the photodissociation ofA¯-state ammonia.