LINE SHAPE CHANGES WITH PRESSURE REDUCTION
Across a spectral line, the absorption coefficient, (alpha) as a function of wavenumber (nu), will depend on pressure, P, approximately as
where C1 and C2 are constants characteristic of the absorbing gas. At the center of the line, alpha will be inversely proportional to pressure. As the pressure goes down, the height of the line goes up. In the wings of the line, alpha will be directly proportional to pressure. As the pressure goes down, the absorbance goes down, shifting the absorption out of the wings and into the line center.
The change of line profile with pressure is illustrated in Figure 1. A full width of 0.2 cm-1 at points halfway up the line is typical of lines at normal pressure and temperature. The total area under a single line depends on the transition probability of the vibration-rotation change. This transition probability is an unchanging physical constant. It is this fact that makes the reference spectra an absolute calibration of the quantitative measurements of gases.
The extent to which a composite absorption feature changes with pressure depends on the number of lines it contains and on the spacing of those lines. When there are hundreds of lines in a band, as is the case with most of the larger pollutant molecules, the absorption band will not develop a fine line structure and a pressure dependence until the pressure is reduced to a few hundredths of an atmosphere. When there are about 10 lines per wavenumber, as may be the case for some of the lighter molecules, the spectrum will be continuous at one atmosphere, but it will break into individual lines at about one-fourth of an atmosphere. This is illustrated in Figures 2 and 3 which show seven lines spaced about 0.1 cm-1 apart. At one atmosphere, figure 2, the seven lines form one absorption feature which will remain as one feature, even to an instrument of very high spectral resolution.. At one-fourth of an atmosphere, figure 3, the seven lines have separated. They will still look like one absorption line to an instrument operating at one cm-1, but they will be seen as separated by an instrument of very high resolution.