Knowledge Bank

Line shift and line broadening coefficients $\delta \nu$ and $\gamma$ have been measured for 21 rotational lines of the P and R branch of the $\nu 1+3\nu 3$ band of acetylene at 789 nm using a tunable cw semiconductor laserdiode system. The absorption cell was of the multi-pass Herriot-type with an effective absorption length of 35 m. Up to now, self-broadening and broadening by $N2$, Ar and air perturbers have been investigated. Taking air-broadening as an example we obtained $\gamma =0.176(12)cm-1$/bar and $\delta \nu =0.012(2)cm-1$/bar for the P(4) line. With increasing value of J the values of $\gamma$ decrease, e.g. to $0.147(2)cm-1$/bar for R(13) whereas $\delta \nu$ increases to $0.016(2)cm-1$/bar. A similar behaviour has been observed for the case of other foreign gas perturbers and for self-broadening. The present investigation might be of interest for the reliable detection of small amounts of acetylene in air of atmospheric pressure. In our system we could detect a minimal concentration of 1 pbar acetylene in a $N2$ atmosphere of 15 mbar pressure.