The photodissociation of CH3I in the red edge of the A-band: Comparison between slice imaging experiments and multisurface wave packet calculations

Abstract

The photodissociation of methyl iodide at different wavelengths in the red edge of the A-band ͑286-333 nm͒ has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the methyl fragment in the vibrational ground state ͑ =0͒. The kinetic energy distributions ͑KED͒ of the produced CH 3 ͑ =0͒ fragments show a vibrational structure, both in the I͑ 2 P 3/2 ͒ and I ‫ء‬ ͑ 2 P 1/2 ͒ channels, due to the contribution to the overall process of initial vibrational excitation in the 3 ͑C-I͒ mode of the parent CH 3 I. The structures observed in the KEDs shift toward upper vibrational excited levels of CH 3 I when the photolysis wavelength is increased. The I͑ 2 P 3/2 ͒ / I ‫ء‬ ͑ 2 P 1/2 ͒ branching ratios, photofragment anisotropies, and the contribution of vibrational excitation of the parent CH 3 I are explained in terms of the contribution of the three excited surfaces involved in the photodissociation process, 3 Q 0 , 1 Q 1 , and 3 Q 1 , as well as the probability of nonadiabatic curve crossing 1 Q 1 ← 3 Q 0 . The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and nonadiabatic couplings, employing a reduced dimensionality ͑pseudotriatomic͒ model. A general qualitative good agreement has been found between theory and experiment, the most important discrepancies being in the I͑ 2 P 3/2 ͒ / ͓I͑ 2 P 3/2 ͒ +I ‫ء‬ ͑ 2 P 1/2 ͔͒ branching ratios. Inaccuracies of the available potential energy surfaces are the main reason for the discrepancies.

Imaging the radical channel in acetaldehyde photodissociation: Competing mechanisms at energies close to the triplet exit barrier

“…The experimental setup has been described in detail previously 42 and only a brief account of the standard procedure will be given here. The whole experiment runs at a repetition rate of 10 Hz.…”

A velocity-map imaging study of methyl non-resonant multiphoton ionization from the photodissociation of CH 3 I in the A-band

“…The main characteristics of the experimental set-up have been described in detail previously [23,24]. The whole experiment runs at a repetition rate of 10 Hz.…”

“…We note that for all the CH 3 + images measured, the two-colour signal represents five times the probe signal while the pump signal is almost inexistent. Independent velocity-radius calibration of the apparatus is done by measuring resonantly multiphoton ionized CH 3 (ν = 0) fragments produced after the photodissociation of CH 3 I at 333.45 nm (one colour pump-probe experiment) at different repeller potentials, taking advantage of the well-known kinetic energy release of the I( 2 P 3/2 ) yielding channel at this photolysis wavelength [23].…”

“…Thus, in the model the 3 Q 1 ←X 1 A 1 transition is neglected, and only the 3 Q 0 ←X 1 A 1 and 1 Q 1 ←X 1 A 1 transitions are considered. A detailed description of the representation of the potential-energy surfaces of the different electronic states involved in the photodissociation process and of the transition dipole moment functions for the 3 Q 0 ←X 1 A 1 and 1 Q 1 ←X 1 A 1 transitions has been given elsewhere [23,25,27,[36][37][38][39]. Photodissociation of CH 3 I at 281 nm has been simulated by means of a wave packet method.…”

“…Later on, the study was extended to the red [4,22,23] and blue edges [24] using imaging techniques in conjunction with (2 + 1) REMPI detection of the CH 3 and I( 2 P 3/2 ) and I*( 2 P 1/2 ) photofragments at different excitation wavelengths. Two channels were identified corresponding to the formation of the CH 3 (X 2 A 2 ) radical in correlation with the I*( 2 P 1/2 ) and I( 2 P 3/2 ) atoms.…”