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

Abstract

Abstract.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 1 aimed at stringent testing of theoretical calculations and improving the values of molecular parameters used in calculations.

Molecular dispersion spectroscopy for chemical sensing using chirped mid-infrared quantum cascade laser

“…However, due to the extremely small refractive index change associated with the absorption of trace amounts of molecular species, little progress has been made to adopt this approach to routine molecular detection. Interferometric methods such as the "hook" method [4], developed at the early stage of gaseous sample dispersion studies, have been used until now [5]. Other methods inherited from the "hook" technique have also been investigated: Examples include interference fringe shift or slope measurements [6], holographic measurements [7], or applications of detector focal plane arrays and digital image processing [5].…”

Absorption spectra of broadened sodium resonance lines in presence of rare gases

“…We investigated the potentials available in the literature by calculating theoretical values of thermal gas parameters such as diffusion coefficients and index of refraction of sodium matter waves using those potentials and comparing with the measured values [4,5]. Measured satellite positions [1,6,7] were applied to select and modify molecular potentials as well. Molecular potentials of our choice are listed in Table. 1-2 for Na-He and Na-Ar and Na-Xe molecules.…”

“…Experimentally, the absorption coefficients of sodium vapor at 900( § 10¨50)K in presence of various rare gases were measured in the range from 400 nm to 850 nm. With the sodium atomic densities obtained accurately using the anomalous dispersion method [1], density-independent reduced absorption coefficients of Na-Na and Na-Rare gases were obtained simultaneously. The measured reduced absorption coefficients (upper curves) for Na-He, Na-Ar and Na-Xe are compared with the calculated values (lower curves) in Fig.…”

“…The blue wing absorption arises from transitions between the X 2 Σ § molecular state to the B 2 Σ § molecular state and the red wing from transitions between the X 2 Σ § molecular state to the A 2 Π molecular state. Since the ground electronic states of Na and rare gas di-molecules are weakly bound, most absorption occurs from free-bound and free-free transitions and the absorption spectra should be smooth without ro-vibrational structures 1 .…”

“…These methods give a good description of gross features of absorption and emission spectra. However, recent high-precision measurements of absorption coefficients [1] reveal more detailed features such as ro-vibration lines and satellite structures in the far line wings of sodium resonance lines when broadened by the same or different perturbing atoms requiring more precise theoretical descriptions. Accurate quantummechanical methods in molecular spectroscopy are not usually applied to the spectral calculations for high temperature gas partly because the uncertainty of available molecular potentials is comparable to that of the classical and semi-classical methods and partly [11,7] because of the computational limitations resulting from the difficulty of accounting for all the partial waves contributing to the total spectra.…”