Vibrational energy distribution of the CH3 radical photodissociated from CH3I

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Abstract
The n→σ* transition in CH3I at 257 nm produces mainly excited I(2P1/2) atoms and the available energy is mainly released as translation. A recent experiment by Y. Lee and co‐workers measured the distribution of the methyl radicals over the bending umbrella vibration, the only mode excited. This dissociation process is attractively simple because it can be expressed in terms of two linear coordinates rC–I, the distance between the carbon and iodine nuclei and rC–H3, the distance between the carbon nucleus and the plane of the three protons. The potential surface chosen for the excited surface dissociating to I(2P1/2) atoms was (atomic units) An essentially exact close coupling calculation using this potential energy together with an appropriate ground state potential fitted the absorption curve very well and the vibrational distribution approximately. Both the experimental and theoretical distributions for λ=266 nm peak at v=2. Calculations for other wavelengths show that the umbrella mode distribution will peak at higher (lower) v when a more (less) energetic photon is absorbed.