The method of doing dual-beam single-detector wavelength-modulation spectroscopy originally proposed by Bonfiglioli and Trench is demonstrated experimentally, using a tunable laser. This system monitors both the reference and the sample beams continuously in phase quadrature and measures the derivative and the direct spectra simultaneously. When this system was used in conjunction with an electro-optically tuned cw dye laser, a sensitivity of (1/T) (dT/dλ) = 3 × 10−6 Å−1 was experimentally achieved for modulation depths from 1 to 50 Å and can be extended down to 0.1 Å. The derivative spectrum corresponding to the extremely weak fifth overtone of the C–H stretching vibration in the electronic ground state of benzene has been observed in a path length as short as 1 cm.
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