Investigation of single-factor calibration of the wave-number scale in Fourier-transform spectroscopy

Abstract

We have used the fundamental and frequency-doubled output of a single-frequency tunable laser locked to a precisely known transition in molecular iodine to provide calibration of a Fourier-transform spectrometer (FTS) in the visible and near-ultraviolet regions and to investigate the limiting uncertainty involved in calibrating spectra by using a single multiplicative correction to the entire optical frequency scale. An integrating sphere was used to introduce the laser light as a pseudoincoherent source and provide uniform illumination of the FTS field of view. The sphere also served to combine the laser beams with light from a series of mercury electrodeless discharge lamps containing argon carrier gas at selected pressures. Four strong lines in the spectrum of 198Hg were measured with these lamps to obtain precise wavelengths and argon-pressure-shift coefficients. These lines, emitted from lamps with argon pressures in the range 33 Pa (0.25 Torr) to 1330 Pa (10 Torr), are suitable for future calibration of FT spectra without need for the laser source. The limiting relative uncertainty component in the reported wavelengths is 6.19×10−9, as estimated from observed deviation of the frequency ratios of the calibration lasers from the exact value of 2. The adequacy of a single multiplicative correction factor for the absolute calibration of an individual FT spectrum is supported by our data, at the level of better than a part in 108.