Precision measurements of sodium-sodium and sodium-noble gas molecular absorption

Experimental apparatus and measurement technique are described for precision absorption measurements in sodium - noble gas mixtures. The absolute absorption coefficient is measured in the wavelength range from 425 nm to 760 nm with 2% uncertainty and spectral resolution of 0.02 nm. The precision is achieved by using a specially designed absorption cell with an accurately defined absorption path length, low noise CCD detector and double-beam absorption measurement scheme. The experimental set-up and the cell design details are given. Measurements of sodium atomic number density with 5% uncertainty complement absorption coefficient measurements and allow derivation of the reduced absorption coefficients for certain spectral features. The sodium atomic number density is measured using the anomalous dispersion method. The accuracy of this method is improved by employing a least-squares fit to the interference image recorded with CCD detector and the details of this technique are given. The measurements are aimed at stringent testing of theoretical calculations and improving the values of molecular parameters used in calculations.