High-resolution multimodulation Fourier-transform spectroscopy.

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Abstract
High-resolution double-modulation Fourier-transform (FT) spectroscopy (FTS) is demonstrated for what is, to our knowledge, the first time. Two high-resolution FT interferograms are simultaneously recorded. The first one is nonselective and contains all the spectral information from the observed source, and the other one is made of the samples that are sensitive to only a specific source modulation. General formulations and practical recording procedures are given for phase- and intensity-modulated spectra. The advantage of selectivity is illustrated by velocity-modulated emission spectra of the Δv = 1 sequence of the Doppler-shifted ArH+ ion. It is also shown that for a source perturbation of small amplitude, only the product of intensity × shift can be retrieved from the selective line shapes obtained in a phase-modulated laser or FT spectra. Thanks to the multimodulation FTS approach, the intensity and the shift of the transitions are measured in a single experiment.