Sensitive sub-Doppler resolution molecular spectroscopy developed by Hall and co-workers [J. Opt. Soc. Am. B 15 (1998) 6] has been applied to the 1.66 µm region using a widely tunable external-cavity diode laser. We have recorded saturated absorption spectra of the 2ν 3 vibrational overtone band of methane with a sensitivity of 9.5 × 10 −11 cm −1 at a time constant of 1.25 s and a spectral linewidth of 320 kHz (full width at half maximun; FWHM) over the tunable range of 1.63 to 1.68 µm.

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We studied the vibration-induced dipole moment of methane by observing Stark-modulation spectra of the 2ν3 band centered at 1.66 μm. The spectral purity of an external cavity semiconductor laser and radiation density in a Fabry–Perot cavity absorption cell are sufficiently high to record sub-Doppler saturated absorption lines with a spectral resolution of 0.8 MHz. First-order Stark shifts of the E-type symmetry components in the P(2), Q(2), Q(4), Q(5), R(2), R(4), and R(5) transitions were measured, and the vibration-induced dipole moments for the v3=2 state were determined. Their values clearly demonstrate the Coriolis-induced dipole term, yielding [5.79(5)+0.440(18)×(2L−3)] mD for the JR level, where L is defined to be −(R+1), 0, and R for the R=J−1, J, and J+1 levels, R and J are rotational and total angular momentum quantum numbers, and numbers in parentheses are 90% confidence intervals in unit of the last digit.