Rotation lines of hydrogen chloride for wavenumber calibration of far infrared spectrometers.

Contours are calculated for the pairs of H(35)Cl and H(37)Cl rotation lines for a range of spectral slitwidths between 0.10 cm(-1) and 3.0 cm(-1). The peak positions are compared with the frequencies of the H(35)Cl lines with a view to predicting the displacement of each peak due to overlap of the H(37)Cl lines. Experimental measurements of the J = 12 and 13 lines show that the predictions are correct. It is suggested that the calculated peaks be used as calibration data for instruments, although it is pointed out that if the wavenumber error of the instrument exceeds one-twentieth of the spectral slitwidth, the displacements can be ignored.

Precise measurements with a compact vacuum infrared spectrometer.

A 3-m vacuum grating IR spectrometer with digital recording is described. Resolution better than 0.025 cm(-1) at 3000 cm(-1) is demonstrated, and details are given of a fast deconvolution procedure by which the resolution is enhanced to near 0.010 cm(-1) . The grating drive incorporates a Merton nut, which facilitates the measurement of highly precise wave numbers. Lines in the 12 degrees 1 band of N(2)O near 4630 cm(-1) have been measured and agree within an average of +/-0.0004 cm(-1) with interferometric values.