A high-resolution, far ir lamellar grating interferometer that operates in either a single-beam or a double-beam differencing mode of operation is described. The instrument covers a frequency range of 10 cm−1 to 125 cm−1 (1000 μ to 80 μ). To illustrate the general performance of the instrument the pure rotational spectrum of water vapor between 15 cm−1 and 115 cm−1 is presented. It is estimated that the absorption line centers of strong isolated lines are measured to within ±0.008 cm−1. To illustrate the resolution of the instrument, low wavenumber portions of the pure rotational spectrum of DCl are shown. The Cl35–Cl37 isotope splitting of the J = 2 → 3 transition (32.3 cm−1) is clearly resolved. The calculated separation of these two lines is 0.094 cm−1.
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