5. High Resolution Saturated Absorption Studies of Methane and Some Methyl-Halides J. L. HALL and J. A. MAGYAR With 6 Figures It is clear that lasers have opened new vistas in spectroscopy. Indeed the high laser intensity and monochromaticity are leading to a virtual explosion in the field of molecular spectroscopy. For example in the visible region, selective laser excitation [5.1] of a single upper level gives simple spectral patterns in fluorescence that contain information only about the accessible lower states. By contrast, in absorption spec- troscopy, one such pattern is contributed by each lower state in a vibra- tion-rotation manifold of lower states. Without laser selective excitation, the apparent chaotic overlapping of lines requires prodigious effort to unravel, even for diatomic molecules such as 12. In the infrared however, the pre-laser situation was more tenable. The basic point is that infrared transitions are between vibration- rotation levels of the same electronic state and therefore the transition can produce at most a very small change in the average internuclear distance. This fact in turn leads to the approximate selection rule that the vibrational and rotational quantum numbers can change by only 0 and __+ 1. Thus the spectrum is collapsed from a virtual continuum as in the case of the visible spectrum of 12, to a clear and unambiguous infrared line spectrum. For molecules as complex as the methyl-halides, however, infrared spectrographs of the appropriate resolution (--~100,000) have been available only quite recently with the development of large high-resolution diffraction grating instruments by HENRY et al. [5.2] and by PLYLER et al. [5.3], and with the techniques for Fourier-transform spectroscopy of CONNES [5.4] and his colleagues. Thus, for example, it is only very recently that a definitive analysis of the methyl bromide v1 spectrum has been available [5.5]. In counterpoint to the above-discussed developments in "classical" spectroscopy has been the development of several techniques that can provide resolution far superior to the usual Doppler limit of the classical methods. Although we will only here make use of saturation spectroscopy, several additional sub-Doppler techniques besides saturation are con- sidered in other parts of this volume. Nor is there a shortage of molecules and lasers that can interact with them. For example, the 3.39 Ilm He-Ne laser is just in the midst of the CHa-stretching band: we will list approxi-