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

• Published in: Chemical physics Vol. 93; no. 2; pp. 293 - 306
• Main Authors: Ashfold, M.N.R., Bennett, C.L., Dixon, R.N.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.1985
• ISSN: 0301-0104
• DOI: 10.1016/0301-0104(85)80026-4

Electronically excited ND 3(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 ND 3(A¯) molecules carrying one quantum of out-of-plane bending vibration ν′ 2 are least susceptible to predissociation, is confirmed. ND 3(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 ND 3(A¯) molecules in the region of the D 2N—D dissociation coordinate. At short D 2N—D separations this surface exhibits a barrier. The presence of a conical intersection (involving the ND 3 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.