Photodissociation of CH2: A test case for the light–heavy–light approximation
- 1 October 1994
- journal article
- research article
- Published by AIP Publishing in The Journal of Chemical Physics
- Vol. 101 (7), 5792-5803
- https://doi.org/10.1063/1.467294
Abstract
The validity of the light–heavy–light (LHL) approximation for calculating absorption spectra and partial photodissociation cross sections has been assessed for photodissociation of H2O, CH2, and CD2 through their first absorption bands. For this purpose, results of full 3D (three‐dimensional) calculations were compared with 3D LHL and 2D LHL results. The 3D LHL approximation works well for calculating absorption spectra and cross sections which are resolved with respect to the final rotational state of the fragment. However, the fragment vibrational distributions calculated using the 2D and 3D LHL approximations are too warm. For the v=0–3 partial cross sections, the LHL approximation gives good results for H2O and reasonable results for CH2, but breaks down when applied to CD2. Using time‐independent perturbation theory, it is found that the colder vibrational fragment distributions obtained using full 3D dynamics arise mostly from the influence of one particular coupling term. This term acts to push the wave packet into the dissociation channel earlier, thereby decreasing the vibrational excitation which results from both oscillators being stretched simultaneously in the molecule’s motion towards the saddle point. The size of this term is proportional to the cosine of the bond angle divided by the mass of the central atom. Therefore, in assessing whether the LHL approximation will be valid for photodissociation of a particular molecule, both the mass of the heavy atom and the equilibrium bond angle should be considered, and CH2 is a ‘‘worse case LHL molecule’’ mostly because its ground state equilibrium bond angle (134°) is larger than that of H2O (104°).Keywords
This publication has 33 references indexed in Scilit:
- Emission spectroscopy of H2O dissociating in the B̃ 1A1 state: Rapid bending motion manifested through excitation of high bending states of H2O (X̃)The Journal of Chemical Physics, 1993
- Nonadiabatic effects in the photodissociation of H2S in the first absorption band: An ab initio studyThe Journal of Chemical Physics, 1993
- Photodissociation of CH2. I. Potential energy surfaces of the dissociation into CH and HThe Journal of Chemical Physics, 1992
- Three-dimensional wavepacket calculation for the photodissociation of water in theChemical Physics Letters, 1992
- An experimental and theoretical study of the bond selected photodissociation of HODThe Journal of Chemical Physics, 1991
- Photodissociation dynamics and emission spectroscopy of H2S in its first absorption band: A time dependent quantum mechanical studyThe Journal of Chemical Physics, 1990
- Photodissociation of vibrationally excited water in the first absorption bandThe Journal of Chemical Physics, 1989
- Spectroscopy and photodissociation dynamics of H2O: Time-dependent viewThe Journal of Chemical Physics, 1989
- Isotope effects in the fragmentation of water: The photodissociation of HOD in the first absorption bandThe Journal of Chemical Physics, 1988
- Photodissociation dynamics of H2O and D2O in the first absorption band: A complete a b i n i t i o treatmentThe Journal of Chemical Physics, 1988