RA05 15 min9:56
PRESSURE BROADENING AND SHIFT OF SOME NH3 LINES MEASURED BY DIODE LASER SPECTROSCOPY.

KOICHI M. T. YAMADA, M. FABIAN, National Institute for Advanced Interdisciplinary Research, Tsukuba 305, Japan; R. SCHIEDER AND G. WINNEWISSER, I. Physikalisches Institut, Universität zu Köln, D-50937 Köln, Germany.

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 nu2 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 étalon; 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 µbar 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 problem 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.