By use of a cw dye laser, the resonance-fluorescence method is used to make absolute density measurements of sodium vapor from 10<sup>2</sup> atoms/cm<sup>3</sup> at -28°C to 10<sup>11</sup> atoms/cm<sup>3</sup> at 144°C. The results are compared with other measurements at higher temperature by a thermodynamic analysis. A new vapor-pressure curve is derived, which is slightly different from previous curves based on the higher-temperature measurements. The application of laser resonance fluorescence to other species is also discussed. In particular, a scheme for detecting sodium-quark atoms, if they exist, with a sensitivity of <i>n</i><sub><i>q</i></sub>/<i>n</i><sub><i>B</i></sub>~10<sup>-15</sup>, is proposed.
W. M. Fairbank, Jr., T. W. Hänsch, and A. L. Schawlow, "Absolute measurement of very low sodium-vapor densities using laser resonance fluorescence," J. Opt. Soc. Am. 65, 199-204 (1975)