Knowledge Bank

The vibrational third overtone spectrum of the acetylenic C-H stretch of propyne was obtained by intracavity photoacoustic spectroscopy with a resolution limited by room temperature Doppler broadening and pressure broadening. Comparing the experimental spectrum with symmetric top simulation, it was found that all the peaks in the P and R branches match quite well within the measurement precision of $0.02cm-1$ when J is larger than 10. However, for J less than 10, extra peaks appeared in the experimental spectrum. In addition, deviation of positions of peaks from simulation was found. A possible explanation is that the transitional upper state is perturbed. It was possible to assign all peaks. But some features of the spectrum have not yet been explained.