J. Legrand

The dipole moments of methyl bromide and methyl iodide

Abstract The dipole moments of CH 3 79 Br, CH 3 I and CD 3 I have been accurately measured by Doppler-free microwave Stark spectroscopy. The results are: μ(CH 3 79 Br) = 1.8203(4) D; μ(CH 3 I) = 1.6406(4) D and μ(CD 3 I) = 1.6507(4) D. The dipole moments of the methyl halides CH 3 X (X = F, Cl, Br, I) are shown to be smaller than those of the deuterated species by a factor which seems independent of X in first approximation.

Foreign gas relaxation of the J=0→1 transition of HC15N. A study of the temperature dependence by coherent transients

The temperature dependence of the foreign gas relaxation of hydrogen cyanide, one of the best candidate for radioastronomical observations in planetary atmospheres, is investigated in the 135–300 K range. The measurement process exploits the delayed nutation phenomenon which leads to the determination of the population relaxation. This coherent transient technique avoids any deconvolution of the Doppler effect and a novel pulse sequence which alleviates the difficulties related to the finite saturation actually achieved is proposed. The great sensitivity of the method requires active gas pressures (≲10−4 Torr) much lower than in linewidth techniques, and thus results in an extension by about 60 K of the lower bound of attained temperatures. The millimeter Stark spectrometer used, driven by a microprocessor system, is temperature controlled by a cold gaseous nitrogen flow; its careful design allows a strong reduction of thermal transpiration effects, and thus leads to proper pressure measurements for worki...

Microwave spectrometer for saturated absorption experiments.

A spectrometer has been built to perform Doppler-free saturated absorption experiments in the millimeter range (30-300 GHz); a plane-cylindrical resonator between Stark plates has been used. With that device, inverted Lamb-dips have been observed at 115 GHz with a width 25 times below the Doppler width. However, the essential feature of this apparatus is to allow the application of Stark field typically of 2500 V/cm, leading to such specific uses as the Stark tuned Lamb-dip, level-crossing, and mode-crossing experiments. Typical examples are given and other applications are proposed.

Study of the ν6 band of CH3Br by infrared laser sideband and submillimeter‐wave spectroscopy

About 136 transitions in the ν6 band of CH3Br have been measured using an infrared laser sideband spectrometer. The sidebands were generated by mixing the 10 μm radiation of a CO2 laser with the radiation of a tunable high power microwave source. Frequencies of transitions with J≤71 and K≤8 are reported. The accuracy of the measurements is estimated to be better than 10 MHz. Some high J rotational transitions in the v6=1 state have also been measured with a submillimeter‐wave spectrometer. The new measurements have been combined with previous results to derive vibration–rotation parameters for the v6=1 state.

Level crossing in microwave spectroscopy of CH3F

Abstract Stark tuned level crossing resonances have been detected in CH 3 F through non-linear microwave absorption. In our experiments, the level crossing width is about ten times smaller than the Doppler width.

Experimental observation of noise deamplification in a period-doubling laser

Abstract Noise deamplification has been demonstrated in a CO 2 laser with modulated losses when it is driven near period-doubling bifurcations. Noise squeezing of the quadrature component up to 8 dB has been obtained on the first ( T −2 T ) bifurcation while the noise for the in-phase component is increased by 19 dB. A similar phase sensitive behaviour was also observed on the second (2 T −4 T ) bifurcation. Critical exponents for the signal amplification of 2.71 and 2.93 respectively were also obtained near these period-doubling bifurcations.

Microwave echoes in inhomogeneous stark fields

Abstract By observing microwave echoes of a polar gas at very low pressure in inhomogeneous Stark fields, effects due to molecular motion have been exhibited. A naive calculation neglecting molecular collisions is proposed and discussed. Other applications are also briefly presented.

Tunable Sideband Laser Spectroscopy of Atoms and Molecules

Lasers provide useful tools for spectroscopy as far as these systems have some reasonable tunability and their usefulness relies in large part on that characteristic of the lasers. In the medium infrared range (λ = 5–10 μm), molecular lasers have provided a large amount of spectroscopic datas but special techniques were required to overcome their very limited tunability. For instance the typical tuning range of a CO2 laser line is 50-100 MHz while the separation between successive lines is of the order of 50 GHz and the available part of the spectrum is only 10-3 of the total range. One method that has been extensively used to take advantage of these lasers in spite of their rather restricted tunability is to tune the molecular (or ionic) absorption lines in resonance with the laser by Stark or Zeeman shifting the energy levels. This method is very helpful for the spectroscopic investigation of light molecules that must be paramagnetic or should have a large permenent electric dipole. Another way to compensate for the lack of tunability of molecular lasers is to add a tunable microwave (or radiofrequency) photon to the almost fixed frequency photon emitted by the laser by making the molecule undergo a two-photon process resonant with a molecular transition. This technique was introduced by Freund and Oka who could obtain high resolution spectroscopic datas on NH3 for instance and later extended to other molecules1.