Identification of crystalline structures in jet-cooled acetylene large clusters studied by two-dimensional...

We investigated the crystalline structures of jet-cooled acetylene (C₂H₂) large clusters by laser spectroscopy and chemometrics. The CH stretching vibrations of the C₂H₂ large clusters were observed by infrared (IR) cavity ringdown spectroscopy. The IR spectra of C₂H₂ clusters were measured under the conditions of various concentrations of C₂H₂/He mixture gas for supersonic jets. Upon increasing the gas concentration from 1% to 10%, we observed a rapid intensity enhancement for a band in the IR spectra. The strong dependence of the intensity on the gas concentration indicates that the band was assigned to CH stretching vibrations of the large clusters. An analysis of the IR spectra by two-dimensional correlation spectroscopy revealed that the IR absorption due to the C₂H₂ large cluster is decomposed into two CH stretching vibrations. The vibrational frequencies of the two bands are almost equivalent to the IR absorption of the pure- and poly-crystalline orthorhombic structures in the aerosol particles. The characteristic temperature behavior of the IR spectra implies the existence of the other large cluster, which is discussed in terms of the phase transition of a bulk crystal.