15min:

SHINICHI HIRABAYASHI AND YASUHIRO HIRAHARA, Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.

The infrared spectra of large acetylene clusters formed in pulsed supersonic jets have been measured in the 800-5000 cm^-1 region using step-scan Fourier transform infrared (FTIR) absorption spectroscopy. The ₃ band of C₂H₂ cluster was reproduced well by two Lorentzian profiles peaked at 3234 and 3241 cm^-1. From comparison with the spectral positions of small clusters and two solid phases in the C-H stretching region, the 3234 cm^-1 band was assigned to the solid in the high temperature cubic phase. Another feature is invariant at 3241 cm^-1 under various supersonic jet conditions and is therefore likely attributed to the large clusters with stable structure. The relative intensity for monomer vibration-rotation lines gives the rotational temperature of 12 K at 10% C₂H₂ in Ar. The infrared absorption band of C₂D₂ clusters has also been measured at 2399 cm^-1. The deuterium isotopic shift is in good agreement with the predictions based on the harmonic oscillator model and confirms the assignment of cubic C₂H₂ and C₂D₂. This is the first infrared observation of cubic solid C₂D₂. Although the experimental data do not allow us to estimate the precise size of cubic acetylene clusters, the cluster size is likely to be 10 nm.