State-selective predissociation dynamics of methylamines: The vibronic and H∕D effects on the conical intersection dynamics

Abstract

The photodissociation dynamics of methylamines ͑CH 3 NH 2 and CD 3 ND 2 ͒ on the first electronically excited state has been investigated using the velocity map ion imaging technique probing the H or D fragment. Two distinct velocity components are found in the H͑D͒ translational energy distribution, implying the existence of two different reaction pathways for the bond dissociation. The high H͑D͒ velocity component with the small internal energy of the radical fragment is ascribed to the N-H͑D͒ fragmentation via the coupling of S 1 to the upper-lying S 2 repulsive potential energy surface along the N-H͑D͒ bond elongation axis. Dissociation on the ground S 0 state prepared via the nonadiabatic dynamics at the conical intersection should be responsible for the slow H͑D͒ fragment. Several S 1 vibronic states of methylamines including the zero-point level and n 9 states ͑n =1, 2, or 3͒ are exclusively chosen in order to explore the effect of the initial quantum content on the chemical reaction dynamics. The branching ratio of the fast and slow components is found to be sensitive to the initial vibronic state for the N-H bond dissociation of CH 3 NH 2 , whereas it is little affected in the N-D dissociation event of CD 3 ND 2. The fast component is found to be more dominant in the translational distribution of D from CD 3 ND 2 than it is in that of H from CH 3 NH 2. The experimental result is discussed with a plausible mechanism of the conical intersection dynamics.

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

“…More recently, Ahn et al have also exploited the vibronic structure to state-selectively dissociate CH3NH2 (CH3ND2) via the 0 0 0 , 9 0 1 , 9 0 2 (and 9 0 3 ) bands, measuring H or D-atom kinetic energy distributions using velocity map ion imaging (VMI). 10 While VMI generally has lower resolution than the HRA-PTS technique, similar bimodal kinetic energy distributions were observed. Bar and co-workers have performed numerous vibrationally-mediated photodissociation studies, demonstrating mode-specific dissociation dynamics, 8,9,[11][12][13][14][15] while electronic action spectroscopy studies of CD3NH2 photodissociation showed that increasing the photon energy led to increasing yields of both H and D atom fragments.…”

“…Once on S0, the majority of these returning trajectories will subsequently dissociate to H + CH3NH, giving rise to the slow fragments that comprise 50-80% of the H-atoms yield. 10 As discussed by Ashfold and co-workers, the sampling of phase space is somewhat curtailed as in the fully statistical limit C-N cleavage would dominate, but this remains a minor pathway.…”

“…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.…”

A new method for determining absorption cross sections out of initially excited vibrational states

“…1͑a͒, the UV laser was tuned to 236.295 nm, corresponding to excitation of the vibrationless ground state to the 9 Ј ͑NH 2 wag͒ on the à state, 12 and inducing resonant two-photon ionization ͑R2PI͒. When the ILSRS transferred population out of the vibrationless ground state, it led to a reduction in the detected molecular ion signal, allowing measurement of the Raman signature of methylamine.…”

“…Because of its importance, methylamine and its deuterated isotopologues have been a target for numerous experimental and theoretical studies exploring their dissociation, [9][10][11][12][13][14][15][16][17] and structural and spectroscopic [18][19][20][21][22][23][24][25][26] properties. Most of the previous dissociation studies have been performed without characterizing the initial quantum state of the methylamine on the ground, X , or the first electronic state, Ã .…”