Near ultraviolet photolysis of ammonia and methylamine studied by H Rydberg atom photofragment translational spectroscopy

Abstract

H(D) Rydberg atom photofragment translational spectroscopy has been used to provide new insights into the primary photochemistry of methylamine, ammonia and various of their respective isotopomers following excitation at wavelengths in the near ultraviolet (UV). The bimodal appearance of the total kinetic energy release (TKER) spectra associated with H atom production in the near UV photolysis of methylamine is consistent 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 state potential energy surface and then evolving into the region of the conical intersection connecting the ~ A state and ground-state surfaces. ‘Dynamical’ energy disposal is associated with those molecules which pass directly through this conical intersection en route to the ground-state (H + CH3NH( ~ X)) asymptote, whilst the ‘statistical’ contribution is attributed to those molecules that ‘miss’ the conical intersection on the rst traversal and only make the ~ A! ~ X transfer on a later 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 respective ~ A states. A similar model which associates ‘dynamical’ energy disposal with those molecules that pass through the ~ A= ~ X conical intersection during bond extension, and ‘statistical’ kinetic energy release with those that transfer during N{H(D) bond compression, appears to provide a qualitative explanation for the way the observed H and/or D atom yields and their associated TKER spectra vary with excitation wavelength (0 0 versus 2 1 band excitation) and isotopic composition.