The spontaneous resonance Raman scattering of CH3I in a supersonic jet

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Abstract
The resonance Raman scattering of methyl iodide cooled in neat and seeded supersonic jets has been observed. The incident radiation is resonant with the X→B Rydberg transition (∼200 nm). To our knowledge this is the first reported observation of the resonance Raman scattering of jet-cooled species. Depolarization ratio measurements determine the rotational temperature of the molecular beam to be 10 K (4% CH3I/2 atm He). The only observed spectral evidence of (CH3I)n species (n≥2) in the cooled molecular beam is the appearance of the resonance Raman spectra of I2 produced by the photodecomposition of methyl iodide clusters. The nascent I2 is born with an extremely hot (∼1000 K) vibrational distribution. The excitation profile of the I2 vibrational resonance Raman scattering reveals that the absorption band of the clusters are much broader than that of the monomer. Semiempirical calculations (AM1) predict a CH3I dimer with a bent ‘‘heat-to-head’’ ground-state equilibrium structure which is a favorable orientation for the production of I2.