ULTRAHIGH-RESOLUTION SATURATION SPECTROSCOPY USING SLOW MOLECULES IN AN EXTERNAL CELL

We present the narrowest molecular lines so far recorded in the 10 mum spectral region. A linewidth of 80 Hz (HWHM) has been obtained for the P(39)A1(3)(-) line of OsO4, by selecting slow molecules (T(eff) = 0.6 K) in saturation spectroscopy at low laser fields (30 nW) and low pressures (2 x 10(-6) Torr). In these conditions, the contribution of the fast molecules is greatly reduced because of the finite size of the beam. This method, applied previously to methane at 3.39 mum, is used for the first time in an external cell and improves by a factor 8 the best resolution of our spectrometer. Heterodyne detection and double frequency modulation have been necessary to extract a signal at a contrast of only 10(-6). The physical ideas concerning this regime are described and a detailed analysis of the line shape is given.