NEAR-ULTRAVIOLET PHOTOLYSIS OF AMMONIA AND METHYLAMINE STUDIED BY H RYDBERG ATOM PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY

H(D) Rydberg atom photofragment translational spectroscopy has been us ed to provide new insights into the primary photochemistry of methylam ine, ammonia and various of their respective isotopomers following exc itation at wavelengths in the near ultraviolet (UV). The bimodal appea rance of the total kinetic energy release (TKER) spectra associated wi th H atom production in the near UV photolysis of methylamine is consi stent with there being both 'dynamical' (high TKER) and 'statistical' (slower) contributions to the total H + CH3NH dissociation yield. Both contributions arise as a result of one H atom tunnelling through (or passing over) an earlier barrier in the N-H dissociation coordinate of the (A) over tilde state potential energy surface and then evolving i nto the region of the conical intersection connecting the (A) over til de state and ground-state surfaces. 'Dynamical' energy disposal is ass ociated with those molecules which pass directly through this conical intersection en route to the ground-state (H + CH3NH((X) over tilde)) asymptote, whilst the 'statistical' contribution is attributed to thos e molecules that 'miss' the conical intersection on the first traversa l and only make the (A) over tilde --> (X) over tilde transfer on a la ter encounter. This interpretation has inspired further consideration of the form of the TKER spectra derived from TOF measurements of the H /D atom products arising in the dissociation of various isotopomers of ammonia following excitation to the 0(0) and 2(1) levels of their res pective (A) over tilde states. A similar model which associates 'dynam ical' energy disposal with those molecules that pass through the (A) o ver tilde/(X) over tilde conical intersection during bond extension, a nd 'statistical' kinetic energy release with those that transfer durin g N-H(D) bond compression, appears to provide a qualitative explanatio n for the way the observed H and/or D atom yields and their associated TKER spectra vary with excitation wavelength (0(0) versus 2(0)(1) ban d excitation) and isotopic composition.