Equivalent widths of absorption lines

Definition of equivalent width. The source function, e for a gas or atomic vapour which is not excited by an 

If the incident radiation comes from a tungsten lamp or high-pressure xenon arc, 1( (0) will be essentially constant in the region of an absorption line. The transmitted intensity will then have a dip centred on and the spectral line is observed in absorption, as shown in Fig.10.3. The equivalent width, W, of the absorption line is defined as the width of a rectangular strip of height 1 (0) which has the same area as that of the absorption line. This equality is represented by the two shaded areas in Fig.10.3 and leads to the expression  

In older references (e.g. Mitchell and Zemansky 1966) the equivalent width is often referred to as the total absorption since the total power absorbed from the beam is given by Thus.it is not essential to resolve the 

The equivalent width is given in terms of the absorption coefficient by substituting equation (10.15) into equation (10.17), giving 

Referring to equation (10.7) we see that the equivalent width of an absorption line is determined by the product B ,N-L, or alternatively f.,N-L when the oscillator strength is in-troduced. Thus the measurement of equivalent widths is an important method for determining either oscillator strengths or atomic densities and further details of these techniques are given in the sections which follow. 

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