Coriolis-dependent Stark effect of the 2ν3 band of methane observed by saturated absorption spectroscopy

Abstract

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.

Comb coherence-transfer and cavity ring-down saturation spectroscopy around 1.65 μm: kHz-accurate frequencies of transitions in the 2ν3 band of 12CH4

“…65,66 To the best of our knowledge, the first measurements of sub-Doppler saturated absorption (Lamb dips) in the 2n 3 band of methane were reported in the 1990s by the group of Sasada of Keio University in Yokohama (Japan). [68][69][70] Optical frequency combs were not available at that time and precise frequency differences between 2n 3 transitions were obtained as the optical beat frequency between two external-cavity diode lasers locked at 1 MHz-wide saturated absorption dips of the methane lines. A number of frequency differences mostly between Q-branch transitions of the 2n 3 band were reported with a precision better than 40 kHz.…”

Permanent Dipole Moments of Methane-Type Molecules: Calculation of the ΔJ= 0 Matrix Elements in thel= 1 Vibrational State

“…The first two mechanisms were theoretically predicted by Mizushima and Venkateswarlu The molecule-fixed PDM m a can be expanded in the usual way (2-5, 8, 13, 14, 17) into a series of terms which are the (2), and by Watson (3), Fox (4), and Aliev (5), and were experimentally verified by Uehara et al (6) and Ozier (7) products of the dimensionless normal coordinate operator q ta , the dimensionless total angular momentum operator J a , and more than 20 years ago. Very recently the last mechanism, proposed by Aliev et al (8), was confirmed in sub-Doppler the dimensionless vibrational angular momentum operator l ta Å (q tb p tg 0 q tg p tb ), where a Å x, y, z are the S 4 axes, p ta resolution spectroscopy of the 2n 3 band of methane (9).…”

“…Fox ( 26 ) attributed the similarity to the facts that vibrational state. This set of the PDMs has been used in the anharmonicity separates the degenerate vibrational state the previous experimental works ( 9,25 ) for the analysis of the Stark effects.…”

Highly Sensitive Cavity-Enhanced Sub-Doppler Spectroscopy of a Molecular Overtone Band with a 1.66 µm Tunable Diode Laser

“…Another frequency modulation is applied to NICE-OHMS by the EOM at 478.5 MHz, which is equal to the FSR of the FP cell, and the modulation index is 0.092, which is limited by the small gain of the rf-amplifier used. A sawtooth voltage is applied to the PZT to sweep the frequency of the cavity mode, while the laser frequency follows it through the fre-sitivity of the previous spectrometers [6][7][8][9][10][11][12][13] does not induce any noise in the FM signal. They observed overtone transitions of C 2 HD, C 2 H 2 , and CO 2 with a stable 1.064 µm Nd : YAG laser 16) and obtained a spectral resolution of 70 kHz.…”

“…7-10) Following them, Suzumura and co-workers observed the saturation spectrum of the 2ν 3 band of methane in the 1.66 µm region, 11) and conducted sub-Doppler resolution spectroscopy. 12,13) The technique [6][7][8][9][10] is also sensitive because of its large absorption length. However, even a small frequency difference between the laser and the cavity mode is converted into a large amplitude noise of the radiation intensity transmitted through the FP cell because the high-finesse cavity works as a sensitive frequency discriminator.…”