Coriolis-dependent Stark effect of the 2v₃ band of methane observed by saturated absorption spectroscopy

We studied the vibration-induced dipole moment of methane by observing Stark-modulation spectra of the 2v₃ 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), g(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)J mD for the J_R 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.