Fourier transform vibrational spectroscopy of pure carbon and silicon-carbon clusters

Abstract

Fourier transform infrared studies of pure carbon and silicon-carbon clusters produced by vacuum ultraviolet (VUV) photolysis and by the newly developed method of laser evaporation have resulted in the identification of new vibrational information for the C$\sb4,$ SiC$\sb4$ and C$\sb9$ clusters. For the first time, the far-infrared bending vibration of C$\sb4$ has been observed at a frequency of 172.4 cm$\sp{-1}$ and confirmed by $\sp{13}$C isotopic data in agreement with predictions of theoretical ab initio calculations for the linear geometry. Along with the earlier observation of the antisymmetric stretching mode at 1543.4 cm$\sp{-1},$ the characterization of the infrared active fundamentals of C$\sb4$ under the strict linear geometry is now complete. With the exception of C$\sb3,$ C$\sb4$ remains the only pure carbon cluster to be detected in the far-infrared by direct observation. An analysis of the products of the VUV photolysis of a mixture of silane (SiH$\sb4)$ and 1,3-butadiene $\rm (C\sb4H\sb6)$ has resulted in the first identification of a vibration of SiC$\sb4$ at 2080.1 cm$\sp{-1}$ assigned to the $\nu\sb1$ stretching mode. Prior to this, only rotational transitions for this cluster had been observed. SiC$\sb4$ is one of the few molecules to be identified in the circumstellar shell of an evolved carbon star, and the detection of the first vibrational frequency may facilitate its further detection in astronomical sources. A new technique employing laser evaporation of a graphite rod, designed specifically for the detection of the vibrational spectrum of C$\sb9,$ has resulted in the confirmation of an absorption at 1998.0 cm$\sp{-1}$ assigned to the $\nu\sb6(\sigma\sb{u})$ stretching fundamental. Another band at 1601.0 cm$\sp{-1}$ is tentatively assigned to the $\nu\sb7(\sigma\sb{u})$ vibration of the linear C$\sb9$ cluster. Laser evaporation has many advantages over high temperature evaporation and it is expected that this method may be beneficial in the observation of vibrational spectra of other molecular species, such as the pure silicon clusters, Si$\sb{n}.$ The observation of these three seemingly unrelated molecules provides an understanding of the pure carbon and mixed silicon-carbon clusters when put into context with theoretical predictions and experimental observations of the variety of structures that can form from these two elements.