DIODE-LASER SPECTROSCOPY OF THE $\nu_{3}$ BAND OF $MoF_{6}$

Abstract

We have used a diode laser to measure the absorption spectrum of the $\nu_{3}$ band of $MoF_{6} (740-750 cm^{-1})$. Our results represent the first step in developing a system for isotope separation of molybdenum using lasers. In our initial experiments the diode laser was operated cw and the $MoF_{6}$ was contained at saturated vapor pressure in a static cell of 70-cm length, cooled to $185-195^{\circ} $K. Under these conditions we observed a very rich spectrum with natural-abundance $MoF_{6}$ and with isotopically-pure samples of $^{92}MoF_{6}, ^{100}MoF_{6}$, and $^{100}MoF_{6}$. The Q-branch $subbandbeads^{1}$ of $^{92}MoF_{6}$ were observed at 749.386, 749.418, 749.444, and $749.462 cm^{-1} (\pm .01 cm^{-1}$ absolute, $\pm .01 cm^{-1}$ relative) and those of $^{100}MoF_{6}$ were observed at 741.241, 741.275, 741.299, and $741.322 cm^{-1} (\pm .003 cm^{-1}$ absolute, $\pm .001^{-1}$ relative). The $HCN \nu_{2}$ ground state ($01^{1}-00_{00}$), hot bands $(02^{20}-01_{10}, 02_{00}-01_{10}$), and ground state lines of $H^{13}CN$ were used as absolute frequency reference to calibrate various spectral scans of the $MoF_{6}$. A 2.5-cm-long germanium etalon provided relative frequency calibrations with a fringe spacing of .0477 $cm^{-1}$. Our recent experiments have employed a gasdynamic expansion to cool a mixture of $MoF_{6}$ and Ar to $75-125^{\circ} $K (10-cm optical depth). The gasdynamic system and the diode laser were repetitively pulsed and signal-averaging techniques were used to record spectra. Typical conditions at the test section following gasdynamic expansion were $75^{\circ} K, 7$ Torr Ar, and and .01 Torr $MoF_{6}$. The absorption cross section for the Q-branch subbandheads of one isotope of molybdenum in $MoF_{6}$ in roughly $6 \times 10^{-17} cm^{2}$ for these conditions and the background absorption cross section due to the sum of the other isotopes is $\leq 1 \times 10^{17} cm^{2}$.