Integrated Absorption and Line Strength

If the Einstein coefficients are known, the oscillator strength can be derived and the absorption coefficient and the dispersion are obtained, or, vice versa, from measured oscillator strengths the Einstein coefficients can be determined. 

The mean absorption cross section for the absorption within the natural linewidth of a transition with oscillator strength / is about // A /4 and depends therefore only on the wavelength X and the oscillator strength. 

The mean absorption cross section of the Na D-line at A = 589nm is according to (2.73) about CT,fe = 9 x 10 cm (the measured value is 10 X 10 ° cm ). At a vapour pressure of 10 mb the atom density is Ni = 2.5 X 10 ° cm The absorption coefficient is then = 2 cm. This means that the intensity Iq of a laser beam, tuned to the centre wavelength of the Na transition has deaeased already to (1 /e)Io after 0.5 cm absorption path length. 

---