Infrared Laser Spectroscopy of Jet-Cooled Carbon Clusters

Abstract

Carbon clussers have been of interest for many years due to their astrophysical importance and their role in the chemistry of high temperture. carbon-rich environments.1 Recent experimental and theoretical studies have been highly successful in clucidating the structure and bonding of small carbon, clusters containing an odd number of atoms. The picture which has emerged is that odd clusters of three to nine atoms possess linear $^{1}\Sigma$ ground electronic states with ** bonding. One of the most fascinating characteristics of these odd clusters is their unusual bending montion. $C1$ for example. has long been known to possess an extraordinarity low frequency, high amplitude bending mode, and indeed all of these linear chains are characterized by low bending frequencies. The situation for even numbered carbon clusters containing four to ten atoms is less complete. Ab Initio theory predicts two low energy structural isomers a $^{3}\Sigma$ linear chain and a singlet cyclic $ring.^{1}$ While all of the small odd clusters have been well characterized spectroscopically, only the linear, forms of $c_{4}$ and $C_{5}$ have been detected in the gas $phase,^{2}$ No spectroscopic evidence for the cyclic isomer of any even cluster currently exists. The Berkeley supersonic classes beam apparatus for mid-infrared diode laser $spectroscopy^{5}$ has been applied to the first gas phase characterization of linear $C_{6} C_{7}$ and $C_{6}$. In addition, the bending frequency of C5 has been measured by far infrared laser spectroscopy in our laboratory.4 The bending motion of the longer chains can be characterized indirectly by measuring bending hot bands associated with the asymmetric stretch frequencies. This has been accomplished for $C_{5}. C_{7}$ and most recently. C0 (Ref. 5).