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 + CH 3 NH 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 theà state potential energy surface and then evolving into the region of the conical intersection connecting thẽ 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 + CH 3 NH(X)) asymptote, whilst the 'statistical' contribution is attributed to those molecules that 'miss' the conical intersection on the first traversal and only make theà →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 respectivẽ A states. A similar model which associates 'dynamical' energy disposal with those molecules that pass through theÃ/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 0 band excitation) and isotopic composition.

Reactive quenching of OD A 2Σ+ by H2: Translational energy distributions for H- and D-atom product channels

“…38 This model assumes complete randomization of vibrational energy in the OD-H 2 reactant pair and that the probability of forming product pairs with a particular translational energy P(E T ) is proportional to the total density of states. The density of vibrational states, N (E v ), is evaluated from a direct count of the HOD or H 2 O product vibrational states including anharmonicity.…”

Formation of Vibrationally Excited Methyl Radicals Following State-Specific Excitation of Methylamine

“…The photochemistry of methylamine after excitation in its first UV absorption band has been the subject of significant experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretical [18][19][20][21][22][23][24] attention. Methylamine is the simplest alkyl-substituted analog of ammonia, which provides an almost textbook example of the influence of exit-channel conical intersections on photodissociation dynamics.…”

On the representation of coupled adiabatic potential energy surfaces using quasi-diabatic Hamiltonians: A distributed origins expansion approach

“…This H d represents a significant improvement of that determined in P1 and P2 where the domain was determined largely through chemical intuition. Section III describes the ab initio treatment of, and the H d used to describe, the 1,2 1 A states of NH 3 which have been used in our previous work on quasi-diabatic representations, and provide an archetypical system for nonadiabatic dynamics involving conical intersections, [32][33][34] of considerable current experimental [35][36][37] and theoretical [38][39][40][41][42][43] interest. Section IV describes the performance of H d .…”