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The natural spectral line shape is Lorentzian, but it is greatly influenced by other line broadening sources such as molecular collisions and the Doppler effect, creating an inherent line shape. The inherent line width is also affected by the spectrometer, from interactions due to the irradiation source or the instrument response factor. This complex line shape function is termed the Voigt line shape. It features a peak that is a convolution of many interactions causing various broadening features based upon Lorentzian and Gaussian line shapes, which can interact in varying amounts. The Voigt line shape is the true line shape of most gas phase interactions. In some cases, a Lorentzian line shape can approximate liquid interactions but for most systems which include many different absorbing species the resultant complex line shape can be approximated by the Voigt function.
Unfortunately, the Voigt is not easily solved analytically and therefore must be approximated. The Voigt line shape contributes more area to the wings than a Gaussian, but not to the extent that a Lorentzian does, as seen in the previous figure. If the contribution from the Lorentzian portion of the system (wl) becomes very small compared to 1, then the line shape approaches that of a Gaussian. |
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