Knowledge Bank

Frequency calibration consists of assigning accurate frequencies to a subset of points in a spectrum so that the frequencies at other points can be interpolated or extrapolated. Normally this is done by using the accepted frequencies of standard absorption lines in the spectrum as calibration paints. Computer subroutines have been prepared which find the positions of intensity minima in a spectrum, identify them with lines of known frequency, and determine an average difference in position of a number of lines in a region. These programs have been used to calibrate Fourier Transform-Infrared spectra over a wide frequency range (800 to $5000cm-1$). When fifty to one hundred lines of a band of a molecule such as $N2O,CO2$ or CO are available at high resolution ($0.05cm-1$), the estimated standard deviation of the average difference in position is of the order of $0.0005cm-1$ or better. This precision makes it possible to determine the agreement between calibrations obtained by using different molecules, or line positions obtained from different sources. These programs have also been applied to synthetic spectra to determine the influence of instrumental effects on apparent line positions.