LASER FREQUENCY SPECTROSCOPY: A REVIEW

Abstract

The extension of frequency measurements from the microwave to the ultra-violet has paved the way for the current revolution in ultra-high-resolution, high-accuracy spectroscopy. The extension of frequency measurements to the visible brought about the 1983 redefinition of the meter, fixing the value of the speed of ligtht and making any frequency measured source a length standard. Lasers stablized to sub-Doppler spectroscopic features have been found to be the most useful and accurate of these reference standards. In the far-infrared (FIR) (from 10 to 200 wavenumbers) rotational transitions of CO, HF, and $CH_{3}OH$ are three of the most commonly used standards. Far-infrared (FIR) spectrometers have been built using the difference frequencies from a pair of frequency-measured fluorescence-stabilized $CO_{2}$ lasers. This spectrometer provides frequency-based, tunable source of FIR radiation for measuring FIR spectra. In the infrared, $CO_{2}$ and $N_{2}O$ lasers stabilized to saturated fluorescence features in their respective lasing gases or to $OsO_{4}$ are excellent standards. We have built an ultra-high-resolution low-loss infrared laser oscillating on more than 400 lines of $CO_{2}$ or $N_{2}O$, provides a radiation source for the extension of frequency measurement to high J regular band lines and lines of other bands of $CO_{2}$, and $N_{2}O$. These measurements yield new molecular constants and a new set of standard frequencies and wavelengths in the infrared. The CO laser oscillating in the 2000 wavenumber region is an excellent source for it can be stabilized on many other molecular transitions. Recently, it has been made to oscillate on the 0-1 band; thus, the laser can be stabilized on CO itself. This is one of the most stable new sources in this spectral region. The accidental coincidence of methane with the 3.39 micron He-Ne laser was one of the first highly stable lasers. It is now well measured and has served as an 88 Terahertz standard for many other measurements. In the visible, a half a dozen iodine lines have been frequency measured and serve as wavelength and frequency standards in the visible. The best known of these is the iodine line used to lock the red helium-neon laser. The current ""world record"" frequency is that of the mesurement of the Lyman alpha line of atomic hydrogen yielding a definitive value of the Rydberg constant. The frequencies of many of these molecules is available in readily accessible locations, including the Internet. A summary of these will be presented.