Three-Dimensional Photodissociation Dynamics of Rotational State Selected Methyl Iodide

Abstract

We report three-dimensional quantum mechanical calculations on the photodissociation dynamics of CH3I and CD3I on new ab initio potential energy surfaces. The improved potentials are obtained in the contracted spin−orbit configuration interaction framework by using a larger basis set and more spin-free configurations. The dynamical model includes the C−I stretch, C−H3 umbrella bend, and I−C−H3 bend and allows the overall rotation. The wave packet is propagated in the Chebyshev order domain. The absorption spectrum, product vibrational and rotational distributions, I* quantum yield, and state-resolved angular distributions are calculated for the parent states of |JMK〉 = |000〉 and |111〉, and compared with experiments. The new potential energy surfaces yield a much better agreement with the experimental absorption spectrum, thanks to small potential gradients in the Franck−Condon region. The calculated rovibrational distributions of the methyl fragment are also in good agreement with experimental data. It is shown that the overall rotation has significant effects on the methyl rotational and vibrational distributions as well as the I* yield.

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

“…The probe laser wavelength was set at 333. 45 nm, centered at the Q branch of the 3p z ͑ 2 A 2 ← 2 A 2 ͒ 0 0 0 transition for a two-photon process. In the experiments carried out at the photolysis wavelengths of 286 and 333.45 nm, coincident with REMPI͒ transitions of the methyl fragment ͑the Q branch of the 4p z ͑ 2 A 2 ← 2 A 2 ͒ 0 0 0 transition is located at 286.15 nm͒, the temporal width of the pulses ͑Ϸ10 ns͒ allows us to use the same laser pulse as pump and probe ͑one-laser experiments͒.…”

“…The dissociation coordinate R is the distance between I and the CH 3 ͑or C-X͒ CM, r is the C-X distance, which represents the umbrella bend of the C-H 3 group ͑ 2 ͒, and is the angle between the vectors associated with R and r, and it represents the X-C-I bend ͑ 6 ͒. Modeling the umbrella mode of the C -H 3 group in CH 3 I as a C-X stretch has been applied in various theoretical works, including both three 35,37,44,45 and two 11,36,40,41 dynamical DOFs, and they were able to reproduce successfully most of the experimental data. It has been discussed that the good description provided by the model is due to a large extent to the fact that dissociation occurs mainly along collinear configurations.…”

“…A detailed description of the X 1 A 1 potential surface used has been given elsewhere. 2 For the 3 Q 0 and 1 Q 1 excited electronic states and the nonadiabatic coupling between them, we used the ab initio surfaces calculated by Xie et al, 45 which are an improved version of the previous nine-dimensional surfaces of Amatatsu et al 43 The surfaces and nonadiabatic coupling of Refs. 43 and 45 have the same functional form, only differing in the value of some of the parameters used to fit the ab initio data, as a result of the larger basis set used in the ab initio calculations of Ref.…”

“…43 and 45 have the same functional form, only differing in the value of some of the parameters used to fit the ab initio data, as a result of the larger basis set used in the ab initio calculations of Ref. 45. In order to reduce the dimensionality of the nine-dimensional surfaces, six out of the nine coordinates were fixed at their equilibrium values.…”

“…43 These potential surfaces were used in classical trajectory 42,43 and wave packet calculations. 37,39,44 Improved potential energy surfaces were produced by Xie et al 45 based on contracted SO configuration interaction calculations using a better basis set and more spin-free configurations, in order to better reproduce the absorption spectrum of CH 3 I. These improved potential surfaces were used in reduced dimensionality wave packet calculations of rotationally selected CH 3 I and CD 3 I ͑Ref.…”