Non-Resonant Probing of the Methyl Fragment at 213 nm following 266nm Photolysis of Methyl Iodide

The velocity map imaging of the methyl radical formed by 266 nm photolysis of methyl iodide using 213 nm non-resonant multi-photon ionization (NRMPI) method. Comparison of the NRMPI method with the well-known (2+1) resonance multi-photon ionization (REMPI) method at 333.45 nm, which selectively probes Q-branch of band-origin transition of the methyl radical, indicates that the NRMPI method yields a significantly higher I/I* branching ratio in comparison to the REMPI method, even though the velocity map images of both the methods are qualitatively similar. The higher I/I* branching ratio obtained in the NRMPI method is attributed to the non-resonant ionization of higher quanta states of the umbrella bending mode along with higher rotational states of the methyl fragment in the CH3+I dissociation channel. Thus, results obtained in the present work signify that a 213 nm excitation source, which is easily available as the fifth harmonic of Nd:YAG laser, can be used as an alternative and efficient probe to investigate photo-dissociation dynamics of polyatomic molecules.