RESONANCE RAMAN SCATTERING OF $CH_{3}L$ IN A SUPERSONIC JET

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1990

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Ohio State University

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This work demonstrates the resonance Raman scattering(RRS) of methyl iodide cooled in neat and seeded supersonic jets. This is the first reported observation of spontaneous RRS in a supersonic jet. Incident excitation is resonant with the predissociative Rydberg B state of $CH_{3}l (201 - 190 nm)$. Depolarization ratio measurements determine the rotational temperature of the molecular beam to be 10K (4\% $CH_{3}1/2$ atm He). The only observed spectral evidence of $(CH_{3}l)_{n}$ species $(n \geq 2)$ in the cooled molecular beam is the appearance of the resonance Raman spectra of $I_{2}$ produced by the photodecomposition of methyl iodide clusters. The nascent $I_{2}$ is born with an extremely hot ($\sim 1000K$) vibrational distribution. As determined by the RRS of $I_{2}$ the Raman excitation profile (REP) of the $I_{2}$ fundamental indicates that the CH3I clusters have an X-B absorption spectrum which is severly broadened as compared to that of monomer. The $I_{2}$ REP is flat over a $125 cm^{-1}$ region $(\sim 201 nm)$ whereas the monomer $CH_{3}I$ absorption band(1) is sharply peaked in this region. The excitation frequency dependence of the $I_{2} RR$ scattering illustrates that the effect of dimerization dramatically decreases the excited state lifetime in the clusters by providing a new efficient dissociation channel leading to the formation of l2. Semiempirical calculations(AM1) predict a CH3l dimer structure with a bent ``head to head'' ground equibrium configuration consistent with efficient l2 production.

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(1) L.D. Ziegler, Y. C. Chung, P.Wang and Y.P.Zhang, J.Chem.Phys. 90,4125(1989)
Author Institution: Department of Chemistry, Northeastern University

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