Photofragmentation dynamics of CH₃I at 248 nm

Abstract

The vibrational distributions of the methyl fragment resulting from the photodissociation of CHzI at 248 nm are obtained by TOF photofragment spectroscopy. Only excitation of the v 2 (umbrella) mode is observed and the distribution peaks strongly at v = 2 for the electronically adiabatic pathway (I zP1/2) but extends up to v = 8 for the minor (~ 30 per cent) non-adiabatic pathway (12P3/~). Angular measurements show that both pathways result from an initial parallel ('~Q0 +-N) transition followed by a curve crossing to produce the ground state I fragment. The coupling of the curves is vibronic in nature and probably involves the v~ methyl rocking mode. The lifetime of the photodissociation is found to be ~ 1 x 10 -13 s.

Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study

“…18 The observed fragmentation behaviours of both neutral and cationic methyl halide species have been rationalised in terms of the excited electronic states accessible upon photoexcitation, and the way in which their respective potential 60 energy surfaces (PESs) correlate with the available dissociation limits. The ground ( 2 E 3/2 ) and first excited ( 2 E 1/2 ) states of CH 3 X + correlate with CH 3 + + X products. In the case of CH 3 Br + , the next lowest lying dissociation limit involves CH 3 + + Br* products, whereas for CH 3 I + , the larger spin-orbit splitting in atomic iodine relative to atomic bromine results in the CH 3 + I + limit lying below that for forming CH 3 + + I* products 19,23,24 .…”

“…The ground ( 2 E 3/2 ) and first excited ( 2 E 1/2 ) states of CH 3 X + correlate with CH 3 + + X products. In the case of CH 3 Br + , the next lowest lying dissociation limit involves CH 3 + + Br* products, whereas for CH 3 I + , the larger spin-orbit splitting in atomic iodine relative to atomic bromine results in the CH 3 + I + limit lying below that for forming CH 3 + + I* products 19,23,24 . Previous photoexcitation studies involving ethyl halide cations, 5 C 2 H 5 X + , have identified the analogous CX bond fission channels and several additional pathways, each of which results in one singly charged species and one or more neutral fragments.…”

“…Starting with the first demonstration of ion 35 imaging by Chandler and Houston in 1987, 8 methyl iodide has also proven to be a popular molecule for demonstrating new experimental imaging methods in chemical dynamics, and has been widely studied by velocity-map imaging (VMI) and related techniques. [10][11][12][13] Unsurprisingly, interest has extended to the 40 photodynamics of other methyl halides (CH 3 X), and there have been several studies on CH 3 Cl, CH 3 Br, and CH 3 I in both their neutral (see 16 and references therein) and cationic forms [17][18][19][20][21][22][23][24] ; these earlier studies provide a valuable point of reference for the present investigations into the photofragmentation of two ethyl 45 halide cations, CH 3 CH 2 Br + and CH 3 CH 2 I + .…”

Excess Energy and Structural Dependence of the Rate of Energy Redistribution during the Photodissociation of Iodotoluenes

“…are the limiting values of the anisotropy parameters for parallel and perpendicular treansitions, respectively, and xjl and xj = (1 -xll) are fractional contributions of 011 and j.L to P, respectively. It has been shown that for our apparatus, due to various apparatus depolarization effects, the limiting values of pll0,15,1 6 are PH1 = 1.63 and Pj.. = -0.8. Therefore the slightly lower values of P observed here at 266 nm may also be due to less than 10% contribution of a perpendicular component due to a transition to the 'QI state.…”

Photodissociation Dynamics of Small Molecules

“…There has been a flurry of recent activity in the study of the photodissociation dynamics of this molecule (186,187,188). Hermann and Leone used the infrared luminescence technique with a circular variable filter to determine the IR emission as a function of frequency when this molecule was photolyzed with lasers at 248 and 266 nm.…”