A. Goncharov

Molecular interferometry experiments

Abstract We have performed interferometry experiments with de Broglie waves of the I2 molecule, with an interferometer comprising four laser waves as molecular beam splitters. We have investigated how this interferometer can be tuned by communicating a different velocity to the molecules along each arm. A comparison with the neutron interferometer is outlined.

Frequency measurements of hyperfine splittings in ground rovibronic states of I2 by stimulated resonant Raman spectroscopy

Abstract:Measurements of hyperfine splittings in the ground electronic state of have been performed by stimulated Raman spectroscopy. An argon laser emitting at 514.5 nm, drives the coherence between hyperfine levels of the J”=13 or J”=15 rotational levels of the ground vibronic state, via resonant excitation of the hyperfine transitions of the optical resonances (43-0) P(13) or R(15). We study the influence of the various experimental parameters on the line shape: the beam geometry, the laser modulation spectrum, the laser power, the molecular frequency shifts. We show that only beam aberrations can give rise to a significant asymmetry of the line shape, which contributes to an error in the determination of the resonance frequency. From a theoretical expression of the line shape taking into account the beam geometry, a detailed study of this error is performed. The theoretical predictions and the experimental results are in very good agreement. From the measurements, improved sets of hyperfine interaction constants for the molecule have been calculated for J”=13 and J”=15. These constants are identical for both levels, except for quadrupole coupling constant eqQ which exhibits a J-dependence, which we attribute to the centrifugal distortion of the molecule.

HCOOH high-resolution spectroscopy in the 9.18 μm region

Abstract We report on highly accurate absolute frequency measurement against a femtosecond frequency comb of six saturated absorption lines of formic acid (HCOOH) with an accuracy of 1xa0kHz. We also report the frequency measurement of 17 other lines with an accuracy of 2xa0kHz. Those lines are in quasi coincidence with the 9R(36) to 9R(42) CO 2 laser emission lines and are probed either by a CO 2 or a widely tunable quantum cascade laser phase locked to a master CO 2 laser. The stability of HCOOH stabilized lasers is characterized by a fractional Allan deviation of 3.1xa0×xa010 −12 τ −1/2 . They give suitable frequency references for H 2 + Doppler-free two-photon spectroscopy.

High-frequency modulation transfer technique for ultra-high resolution spectroscopy of I/sub 2/

Linewidths only limited by the transit time and by collisions are obtained in stimulated Raman and Rayleigh spectroscopy by a second-harmonic detection in high-frequency modulation transfer. The distortion of the line shape due to a residual amplitude modulation is shown to vanish for a third- or a fourth-harmonic detection with a proper choice of the modulation index and the detection phase.

Resolving power and sensitivity in modulation transfer stimulated resonant Raman spectroscopy

We show that the lineshape and size of the detected signal in ultra-high-resolution stimulated Raman spectroscopy computed from a general expression of the modulated signal are in good agreement with the experimental results. Using the theoretical expression of the signal, we analyze the resolving power and the sensitivity of our Raman spectrometer. We show that a second harmonic detection in high-frequency transfer modulation generates a lineshape without modulation broadening. A 2.6-kHz-wide resonance has been obtained with iodine, mainly limited by both the transit time and collisions.

Frequency Measurement of the Iodine-Stabilized Ar+ Laser at 501 nm

We report the first absolute frequency measurement of 127 I2 hyperfine components of the R(26)62-0 transition at 501.7 nm with an optical clockwork based on a femtosecond laser frequency comb generator

Narrow Lines in Molecular Iodine at 501.7 NM

We report here experimental results obtained in saturated absorption spectroscopy of iodine in a low pressure cell at 501.7 nm. We obtained lines of 30 kHz wide (HWHM) for the R(26)62-0 transition, which are, to our knowledge, the narrowest resonances ever observed in iodine spectroscopy in cell

Absolute frequency measurements for hyperfine structure determination of the R(26) 62-0 transition at 501.7 nm in molecular iodine

The absolute frequencies of the hyperfine components of the R(26) 62-0 transition in molecular iodine at 501.7 nm are measured for the first time with an optical clockwork based on a femtosecond laser frequency comb generator. The set-up is composed of an Ar+ laser locked to a hyperfine component of the R(26) 62-0 transition detected in a continuously pumped low-pressure cell (0.33 Pa). The detected resonances show a linewidth of 45 kHz (half-width at half-maximum). The uncertainty of the frequency measurement is estimated to be 250 Hz.

Precise molecular spectroscopy using a stable and tuneable frequency comb

Accurate molecular spectroscopy in the mid-infrared (mid-IR) region allows precision measurements with applications in fundamental physics. We present our on-going work towards measuring absolute vibrational frequencies of various polyatomic species — in particular methanol — around 10 μm, at an unprecedented level of accuracy, using a both ultra-stable and widely tuneable near-infrared frequency comb. We have recently been able to lock mid-IR radiation to a frequency comb stabilized to a 1.54 μm near-IR reference [1, 2]. This reference, generated at the French national metrology institute (LNE-SYRTE), is monitored against atomic frequency standards [3] and transferred to LPL via a 43-km long optical fibre [4] (see Fig. 1). This provides the ultimate frequency accuracy (potentially the 3x10−16 of the Cs fountain clock) and stability (∼10−15 after 1s of integration).

Quantum cascade laser spectrometer for frequency metrology and high accuracy molecular spectroscopy around 10 μm

Summary form only given. Quantum cascade lasers (QCLs) are an emerging technology [1] suitable for high-resolution spectrocopy and frequency metrology [2] with an incomparable frequency tunability in the mid-infrared range. We are currently developing a new compact, widely tunable QCL-based spectrometer. Such an instrument will broaden the scope of our experimental setups dedicated to molecular spectroscopy-based precision measurements.Eventually, we want QCLs to reach the state-of-the-art metrological stability and accuracy of our existing stabilized CO2 lasers (~10 Hz width, 0.1 Hz frequency instability for 100 s integration time). As recently demonstrated with the CO2 laser, we will lock the QCL to a frequency comb itself referenced via an optical fibre to the atomic foutain clocks in Paris. Stabilizing the laser this way not only provides us with the ultimate frequency accuracy and stability, it also frees us from having to lock the QCL to a molecular transition, making it possible to have a stabilized QCL at any desired wavelength. The use of QCLs will allow the study of any species showing absorption between 3 and 25 μm [1]. We are currently concentrating on QCLs at ~10 μm, which allows us to test them against our CO2 laser. Our first characterization of a free-running continuous-wave mode near-room-temperature distributed-feedback 10.3 μm QCL looks promising. The beat-note with our CO2 laser shows a record ~200 kHz linewidth (see Figure 1). The low level of amplitude and frequency noise, measured using an NH3 linear absorption line as frequency discriminator, should enable spectroscopy with unprecedented levels of precision. Narrowing of the QCL linewidth was achieved by straightforwardly phase-locking the beat-note between the QCL and CO2 laser on a radiofrequency reference. The great stability of the CO2 laser was transferred to the QCL resulting in an expected linewidth of a few tens of hertz.In the future, frequency locking to absolute frequency references consisting of a sub-Doppler molecular lines or to ultra-stable Fabry-Perot cavities, and finally phase-locking to a frequency comb will be investigated. This work will result in a major technological leap that will benefit two of our main projects respectively dedicated to a precise determination of the Boltzmann constant by laser spectroscopy of a molecular gas [3] and to the first observation of parity violation in chiral molecules by ultra-high resolution molecular jet-spectroscopy [4].