LINE WIDTHS AND RESOLUTION REQUIREMENTS


The fine structure in the spectra of gaseous molecules is of great value in identification, in discrimination against interferences and in quantitative measurements. High enough resolution should be used to take advantage of this fine structure. At atmospheric pressure and room temperature the widths of individual spectral lines are about 0.2 cm-1 (full width, halfway up the line). To see all spectral detail therefore requires resolving power of about 0.1 cm-1. Most commercial spectrometers, however, are limited to 0.5 cm-1 or 1.0 cm-1. This is reasonable, because as resolving power increases, the noise in the spectrum also increases. Trade-offs are in order.


High resolving power is exchanged for a more favorable signal-to-noise ratio. In choosing resolving power, one should also consider what types of molecules are to be measured. Only the smaller and more symmetrical molecules show arrays of individual lines that would benefit from higher resolution. The heavier molecules have so many spectral lines that their overlap produces smooth bands with occasional “spikes”. High resolution brings no benefits to the study of these heavier molecules. For measurement of a variety of molecular gases, a good compromise choice of resolution is 0.5 cm-1.


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