Knowledge Bank

At very low pressures the resolving power of saturated absorption spectroscopy in the infrared depends mostly on the diameter of the light beam. By increasing this beam diameter, it has been possible to reduce the linewidth below 6 KHz in the case of the Coriolis component $F2(2)$ of the line P(7) of the $\nu 3$ band of $CH4$ which can be studied by the He-Ne laser at 3.39 $\mu$m. At this level of resolution $(\sim 1010)$ the magnetic hyperfine structure of the line is clearly resolved. Besides the lines that would correspond to a linear spectrum, one finds Doppler-generated level crossings that are characteristic features of saturated absorption spectra in which two transitions share a common level. Interpretation of the complete spectrum enables one to verify the hyperfine structure of the ground state that one can calculate from the magnetic resonance experiments and to have access to the knowledge of the hyperfine structure of the excited state. It is shown how the existence of this hyperfine structure of the line accounts for an intensity-dependent shift of the apparent line center at lower resolution. Residual broadening factors are discussed and the way towards still higher resolution is presented.