Knowledge Bank

C-H stretch infrared spectra of selected hydrocarbons containing from one to six carbons have been recorded in a slit supersonic expansion with a difference frequency laser spectrometer. Rotational temperatures of 5 K and sub-Doppler linewidths of $0.0013cm-1$ were observed for small hydrocarbons such as methane, ethane, and cyclopropane. For larger hydrocarbons, particularly linear chains with low frequency bending and torsion modes, the spectra are qualitatively different as a result of intramolecular coupling of rovibrational levels. Despite the extremely low rotational and vibrational temperatures, excess spectral structure is observed as a function of molecular complexity and state density that varies systematically from isolated perturbations to complete loss of resolvable structure. Analysis of the spectra provides information on the number of strongly mixed vibrational levels as well as estimates of the coupling strengths.