Saturation and Power Broadening

When a gas in an absorption cell is first irradiated at a transition frequency there is a transient imbalance in the population of the two levels as molecules absorb the radiation and the upper level becomes over-populated. Thermal relaxation processes may not be sufficiently fast to repopulate the lower level. Subsequent power absorption will become less than expected from LTE considerations. This effect will be greater at the line centre where the absorption coefficient is greatest and can lead to complete saturation of the transition. The rate of absorption of radiation is then dictated solely by the rate of relaxation of molecules from the upper to the lower state. This is more noticeable at low pressures where the thermalisation processes are less rapid and the line widths narrower. 

The result of this power broadening or saturation is to reduce the absorption in the line centre compared with the absorption in the wings of the line. This in turn leads to loss of analytical signal intensity and an apparent broadening of the absorption line profile. The resulting effect on the line shape function can be described by an equation due to Karplus and Schwinger, for low powers and incomplete saturation (ref 3, p. 50)  

The term P/J s m is the power density influencing the molecule. It is identical to the intensity / and is related to the electric field strength P/Vm  

By setting v = Vq in Equation 1.39 it can be seen that the effect of the power broadening is to reduce the value of S and hence ao at the line centre. The influence on ao is not great although the Q of the cavity does amplify the effect. Relatively little attention appears to have been given to this amplification in the literature, and so in the next section we derive expressions for the conditions under which it may occur. 

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