We report a limit on the fractional temporal variation of the proton-to-electron mass ratio as ( ) ( ) . From a growing number of theoretical papers, three reviews might be mentioned [6][7][8]. In the large majority of laboratory experiments two

Parity violation (PV) effects in chiral molecules have so far never been experimentally observed. To take up this challenge, a consortium of physicists, chemists, theoreticians, and spectroscopists has been established and aims at measuring PV energy differences between two enantiomers by using high-resolution laser spectroscopy. In this article, we present our common strategy to reach this goal, the progress accomplished in the diverse areas, and point out directions for future PV observations. The work of André Collet on bromochlorofluoromethane (1) enantiomers, their synthesis, and their chiral recognition by cryptophanes made feasible the first generation of experiments presented in this article.

To date no experiment has reached the level of sensitivity required to observe weak nuclear force induced parity violation (PV) energy differences in chiral molecules. In this paper, we present the approach, adopted at Laboratoire de Physique des Lasers (LPL), to measure frequency differences in the vibrational spectrum of enantiomers. We review different spectroscopic methods developed at LPL leading to the highest resolutions, as well as 20 years of CO 2 laser stabilization work enabling such precise measurements. After a first attempt to observe PV vibrational frequency shifts using sub-Doppler saturated absorption spectroscopy in a cell, we are currently aiming at an experiment based on Doppler-free two-photon Ramsey interferometry on a supersonic beam. We report on our latest progress towards observing PV with chiral organo-metallic complexes containing a heavy rhenium atom.

Originating from the weak interaction, parity violation in chiral molecules has been considered as a possible origin of the biohomochirality. It was predicted in 1974 but has never been observed so far. Parity violation should lead to a very tiny frequency difference in the rovibrational spectra of the enantiomers of a chiral molecule. We have proposed to observe this predicted frequency difference using the two photon Ramsey fringes technique on a supersonic beam. Promising candidates for this experiment are chiral oxorhenium complexes, which present a large effect, can be synthesized in large quantity and enantiopure form, and can be seeded in a molecular beam. As a first step towards our objective, a detailed spectroscopic study of methyltrioxorhenium (MTO) has been undertaken. It is an ideal test molecule as the achiral parent molecule of chiral candidates for the parity violation experiment. For the 187 Re MTO isotopologue, a combined analysis of Fourier transform microwave and infrared spectra as well as ultra-high resolution CO 2 laser absorption spectra enabled the assignment of 28 rotational lines and 71 rovibrational lines, some of them with a resolved hyperfine structure. A set of spectroscopic parameters in the ground and first excited state, including hyperfine structure constants, was obtained for the ν as antisymmetric Re=O stretching mode of this molecule. This result validates the experimental approach to be followed once a chiral derivative of MTO will be synthesized, and shows the benefit of the combination of several spectroscopic techniques in different spectral regions, with different set-ups and resolutions. First high resolution spectra of jet-cooled MTO, obtained on the setup being developed for the observation of molecular parity violation, are shown, which constitutes a major step towards the targeted objective. † Supplementary information available in the ancillary file: Measured and fitted transition frequencies as well as spectroscopic constants.1

: We demonstrate a new simple technique to measure IR frequencies near 30 THz using a femtosecond (fs) laser optical comb and sum-frequency generation. The optical frequency is directly compared to the distance between two modes of the fs laser, and the resulting beat note is used to control this distance which depends only on the repetition rate fr of the fs laser. The absolute frequency of a CO2 laser stabilized onto an SF6 two-photon line has been measured for the first time. This line is an attractive alternative to the usual saturated absorption OsO4 resonances used for the stabilization of CO2 lasers. First results demonstrate a fractional Allan deviation of 3×10-14 at 1 s.

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Index Terms--Carbon dioxide lasers, Frequency stability, SF 6 , Optical frequency standard. I. INTRODUCTIONThe carbon dioxide laser has given to the 30 THz spectral region particular significance in frequency metrology. The current standard at 30THz is provided by the CO 2 laser locked onto a saturated absorption resonance of OsO 4 in a cell, the reference signal having full width half maximum (FWHM) of 20 kHz [1,2]. The same area is particularly rich in molecular spectra and many problems in this area, together with questions of fundamental physics, have been investigated using the related saturation spectroscopy [3,4]. Typical linewidths are 1-100 kHz. This paper presents continuing work on a two-photon Ramsey fringe experiment on a supersonic beam of SF 6 with the objectives, now essentially realized, of resolving the entire complex hyperfine structure over some 50 kHz and establishing absolute frequencies at the 1Hz level. Both aspects represent huge advances over cell saturation techniques. Two experimental developments have led to the recent advances. First, the distance between the absorption zones has been increased to 1m, so that the fringe periodicity is now 200 Hz for pure SF 6 . Data is routinely recorded with a signal-to-noise ratio (SNR) of 20 in a bandwidth of 1 Hz. The implied limit on the resolution of two components within the hyperfine structure of the SF 6 spectrum is only 10Hz. This is indeed found when fringe patterns are fitted. Second the entire system can now be directly related to the frequency comb of a femtosecond laser, itself referenced to a Hydrogen maser. The maser is compared to a Caesium fountain and to the GPS system. This gives a long-term stability and frequency reproducibility limited by the performance of the current electronics, various optical links and, possibly, the reference maser. The fs technology is necessarily operated in a new area of precision, comparable with the best currently in use [5,6]. Figure 1 gives a block outline of the entire experiment.Two related series of experiments are presented. First the central Ramsey fringe is employed as the reference point for a molecular clock; the clock frequency is measured, and its performance established, relative to the H maser. Second, and separately, Ramsey fringes are measured on an absolute frequency scale provided by the H maser. This gives a second measurement of the central fringe and the entire spectrum of Ramsey fringes over 50 kHz is put onto an absolute frequency scale.This work is partly directed at a new frequency standard. Since a two-photon transition is used the absolute frequency depends on the optical power. Given the absolute scale, this small effect can be investigated and results are reported.

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