Extensive measurements of vibration-induced permanent electric dipole moments of methane

Abstract

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.35]{fig.eps} \end{wrapfigure}

A methane molecule ($\chem{CH_4}$) has a permanent electric dipole moment (PEDM) in the excited state of the triply-degenerate vibrational modes\footnote{M. Mizushima and P. Venkateswarlu, \it{J. Chem. Phys.} \bf{21}\rm{, 705 (1953)}.}$^{,}$\footnote{K. Uehara, K. Sakurai and K. Shimoda, \it{J. Phys. Soc. Jpn.} \bf{26}\rm{, 1018 (1969)}.}. The rotational dependence of the PEDM was reported in the 2$\nub{3}$ band\footnote{H. Sasada, K. Suzumura and C. Ishibashi, \it{J. Chem. Phys.} \bf{105}\rm{, 9027 (1996)}.}. However, in the $\nub{3}$ band, it was only determined on the $P(7) E$ transition which fortunately lies in the tunable range of a 3.4 $\mu$m He-Ne laser.

We have developed a mid-infrared broadband sub-Doppler resolution spectrometer consisting of a difference-frequency-generation source and an optical frequency comb linked to International Atomic Time. This spectrometer enables us to measure the Stark effects of 20 transitions in the $\nub{3}$ band of methane from 87.7 to 92.8 THz (2927$\sim$3095 $\wn$). The observed linewidth is 0.5 MHz, and the frequency scale is absolutely calibrated. The figure depicts the Stark modulation spectrum of the $P(4) E$ transition. The applied DC electric field was 3.5 kV/cm. We determined Stark coefficients with a relative uncertainty of 1 \%. Our goal is to reveal the rotational dependence of the PEDM. For this end, we yield molecular constants which reproduce the transition frequencies by a least-square method and determine the mixing of the wave functions.