A new model for the treatment of optical saturation for measuring Ne*(3P0) metastable atoms density down to 1010 m-3 by Cavity Ring-Down Spectroscopy

Cavity ring-down spectroscopy with a cw diode laser at 653 nm is used for measuring the density of Ne*(3P0) metastable atoms in low pressure Ne-Ar glow discharges. Non-exponential ring-down (RD) signals are observed due to the optical saturation, which decreases along the RD time with the decay of the intra-cavity laser power (ICLP). Gaussian absorption line profiles, with Lamb-dips at the line center, are obtained for different power levels from the time-varying rates of the RD decays. The amplitude of the deduced line profile decreases with the rise of the ICLP, thus of the optical saturation. As the laser saturation model described in books of W. Demtröder cannot account for the velocity-changing (VC) and the metastability-exchanging (MC) collisions between Ne*(3P0) and ground state atoms, a new model for the treatment of the saturated absorption line profiles is developed here. The necessary use of the power broadened width of the absorption line for obtaining the spectral power density, hence the saturation parameter S, is demonstrated in this model. This model demonstrates the necessary use of the power-broadened width of the absorption line to obtain the spectral power density and hence the saturation parameter S. It predicts very well the line profiles obtained with 224 µW at pressures between 0.1 and 3.2 Torr and provides the density of Ne*(3P0) atoms down to a few 1010 m-3. It is found that VC and MC collisions significantly contribute to the refilling of the Bennet hole produced in the velocity distribution of Ne*(3P0) atoms by the optical pumping. Consequently, Ne*(3P0) atoms inside the laser beam volume are depleted, for all velocity classes. The refilling of the depleted laser volume by diffusion of Ne*(3P0) atoms from the volume surrounding the laser beam and its consequence on the optical saturation is treated in an Appendix.