Knowledge Bank

Direct absorption of an F-center laser beam in a pulsed free jet is used to measure the acetylenic C - H stretching spectra of propyne and acetylene. The use of a pulsed nozzle offers low temperatures (7 K to 50 K) and low background pressures $(10-5Torrrange)$ with modest pumping requirements. Resolution at - 100 MHz is better than Doppler limited. The jet-cooled spectra are compared to high resolution FTIR spectra of gas phase samples. At the lowest temperatures only the $K=0$ and $K=1$ levels, required by conservation of nuclear spin, are observed. The resolved splitting between these levels provides a value for the difference between ground state and vibrationally excited state rotational constants,$ A^{\prime} - A^{\prime\prime}$.