Although methane belongs to the group of nonpolar molecules, Mizushima and Venkateswarlu ( (l) ) nevertheless predicted that it possesses a dipole moment in some of its vibrationally excited states. After their prediction was verified by Uehara et al. (2), Stark effects of the (\nu_{3}(2-4)) and (3 \nu_{1}+\nu_{3}(5)) bands and rotational transitions in the (v_{3}=1(6-8)) and (v_{1}=1(9)) states have been observed. In this letter, we report the magnitude of the vibrationally induced dipole moment for the (v_{3}=2) state as determined using the Stark modulation spectrum of the (2 \nu_{3}) band, which has been extensively studied by absorption spectroscopy carried out with gratings and Fourier transform spectrometers ( (10-13) ).
Near-infrared radiation is generated by a laboratory-built external cavity semiconductor laser employing a Fabry-Perot type semiconductor laser which has an anti-reflection coating on one of its facets and a Littrow-type grating ( 600 lines (/ \mathrm{mm}), braze wavelength: (1.6 \mu \mathrm{m}) ) as a wavelength-selective mirror. Lasing wavelength is tuned by rotating the grating about a point specifically selected to suppress mode-hoppings (14-16). The laser has a tuning range from 1.59 to (1.67 \mu \mathrm{m}), a spectral linewidth of less than (1 \mathrm{MHz}), and an output power greater than (10 \mathrm{~mW}). We made a Stark absorption cell using a Pyrex glass tube with CaF windows and a Stark electrode using a pair of partially aluminized polished glass plates ( (500 \times 30 \times 5 \mathrm{~mm}^{3}) ) which are separated by eight stainless steel spacers (thickness (=100 \mu \mathrm{m}) ) placed on the unaluminized region. One electrode is connected to a DC voltage supply (Fluke 4 15 B) and a (60-\mathrm{kHz}) sinusoidal oscillator through a tuned amplifier, while the other electrode is grounded. A Stark field of up to (30 \mathrm{kV} \mathrm{V}_{\mathrm{p} p} / \mathrm{cm}) can be applied at a bias of (100 \mathrm{kV} / \mathrm{cm}) at a sample pressure of 2.3 Torr without inducing electrical breakdown. The laser radiation is linearly polarized perpendicular to the Stark field. The radiation transmitted through the cell is detected by an InGaAs pin photodiode (Hamamatsu G5851), after which the received signal is demodulated synchronously with the ac Stark modulation by a lock-in amplifier (NF LI-575) with a 1.25 msec time constant. The wavenumber of the laser radiation is measured by a wavemeter (Anritsu MF9630A).
Figure 1 shows the observed video absorption spectrum of the (P(2)) transition, which consists of the (E) (\rightarrow E) and (F_{2} \rightarrow F_{1}) tetrahedral components that could not be resolved under a Doppler-limited resolution of (280 \mathrm{MHz}) (HWHM). Also shown is the observed Stark modulation spectrum of this transition. Note that in contrast, it consists only of the (E \rightarrow E) component, as determined by the symmetric lineshape which indicates the first-order Stark shift of the (E) symmetric species, whereas the (A_{1}, A_{2}, F_{1}), and (F_{2}) symmetric species usually show the second-order Stark shift. The wavenumber of the (E \rightarrow E) component was precisely determined using this spectrum, being (5983.1837 \pm 0.0005 \mathrm{~cm}^{-1}), which is in good agreement with the value derived from an analysis of the overlapped lineshape recorded with a Fourier transform spectrometer (13).
Although up to the maximum available electric field the observed Stark modulation spectrum does not exhibit either resolved (M)-elements or an appreciable change in its lineshape, the magnitude of the dipole moment can still be estimated if the intensities from both spectra are used. Namely, when the radiation frequency is fixed at the zero-field reasonance of the (E \rightarrow E) component, the absorption intensity in the presence of the Stark field is given by