Knowledge Bank

Using the stabilized diode laser spectrometer in Cologne we have precisely determined pressure-induced line broadening and line shift coefficients of six rovibrational transitions in the $v2$ band of $NH3$. Foreign gas effects due to $O2$, $N2$, $H2O$, CO, He, and Ar have been investigated and the self-effects as well. The high accuracy in the frequency position of the laser was achieved by stabilizing the diode laser on an interference maximum of a Fabry-Perot interferometer of variable optical length. The laser frequency was swept by tuning the cavity length of the etalon: the cavity length was monitored by a stabilized HeNe laser. A simultaneous recording of an $NH3$ spectrum with a reference cell at a fixed pressure has been introduced for an absolute calibration. The line positions relative to reference lines have been measured to a precision of 300 kHz. A good signal-to-noise ratio was achieved by using a Herriott-type multi-reflection cell with a total path length of 46.86 m. This allowed to use very small $NH3$ partial pressures of the order of a few hundred $\mu$bar in the measurements of foreign gas effects to minimize the influence of $NH3$ self-effects. It turned out that contamination of the gas sample with $H2O$ imposed a serious upon the precise determination of the pressure effects, because of the large shift and broadening caused by $H2O$. To remove any residual water contamination, the sample cell was connected to a cold finger submerged in a Dewar vessel filled with dry ice. The results of the present study are thus thought to be the most precise data of this kind now available.