High-Resolution Nonlinear Raman Spectroscopy in Gases

The applicability of Raman spectroscopy to the investigation of gases has been greatly improved by the development of the different methods of nonlinear Raman scattering. When two laser beams, one of which has a tunable frequency, are brought to a common focus in a sample, a stimulated Raman process occurs, as soon as the frequency difference between the two lasers is equal to aRaman active rovibrational or rotational transition frequency of the sarnple, and the corresponding state is popuJated above equilibrium. The Raman resonance can be detected in different ways: by coherent anti-Stokes Raman scattering (CARS) or the corresponding Stokes process (CSRS), by again in one of the beams (stimulated Raman gain spectroscopy, SRGS) or a loss in the other one (inverse Raman spectroscopy, IRS), or even by detection of a photoacoustic signal (photoacoustic Raman spectroscopy,PARS). The selective ionisation of the excited molecules by a third ultraviolet laser pulse (ionisation detected stimulated Raman scattering, IDSRS) has considerably increased the sensitivity in special cases. The instrumental resolution of the se techniques is determined by the convoluted linewidths of the lasers used for excitation. This is of special importance for the investigation of high resolution rotation-vibrational spectra of gases

Variation avec la pression de l'intensité observée en diffusion Raman antistokes cohérente

For the computation of the variation versus pressure of the intensity of the coherent light emitted by a gas in a non-linear process, Doppler-Fizeau width must sometimes be taken into account through gas kinetic theory, not by convolution of linewidth.Dans le calcul de la variation avec la pression de l'intensité de la lumière cohérente émise par un gaz, on ne peut pas toujours tenir compte de l'élargissement Doppler-Fizeau par convolution avec les autres élargissements ; il peut être nécessaire d'utiliser la théorie cinétique des gaz

HIGH RESOLUTION RAMAN SPECTROSCOPY USING THE CARS METHOD

Author Institution: Laboratoire de Spectronomie Moleculaire, Universite de DijonHigh resolution Raman spectra are obtained by CARS spectroscopy. The sources used are a pulsed ruby laser and a pulsed dye laser; the spectral width of these lasers are $2 \times 10^{-3}$ and $3 \times 10^{-3} \;cm^{-1}$, so that the expected resolution is of the order of $4 \times 10^{-3}\;cm^{-1}$. We shall show the spectrum of the $Q^{+}$ branch of $\nu_{3}$ of $CH_{4}$ for a gas under a pressure of 50 Torr

INTENSITY OF THE LINES FOR EXTERNALLY STIMULATED RAMAN EFFECT (CARS) IN LOW PRESSURE GASES

$^{1}$ P. D. Maker and R. W. Terhune, Phys. Rev. 137, A801 (1965). $^{2}$ R. Y. Shen and N. Bloembergen, Phys. Rev. 137, 6A (1965).Author Institution: Laboratoire de Spectronomie Mol\'{e}culaire, Universit\'{e} de Dijon, 6 Bd Gabriel, 21000It has often been written that the observed intensity in externally stimulated Raman effect is proportional to the square of the $pressure.^{1,2}$ Precise measurements show, however, that at low pressure the intensity is proportional to the pressure. This result may be used for Stokes as well as for anti-Stokes lines. A theoretical explanation is given

Study of Collisional Broadening in Oxygen Lines by High-Resolution Coherent Anti-Stokes Raman-Spectroscopy

0008-420

HIGH RESOLUTION COHERENT STOKES RAMAN SPECTROSCOPY OF THE ν1\nu_{1} BAND OF METHANE

Author Institution: Laboratoire de Spectronomie Mole'culaire, Universite' de Dijon, 6Bd Gabriel, 21000We use here coherent Stokes Raman effect in high resolution spectroscopic experiments and we give the first results obtained with low pressure of methane. Unlike coherent anti-Stokee resonance, the frequency mixing $2\omega_{1} - \omega_{2}$ will be equal to $\omega_{3}$; indeed for technological reasons, the second laser frequency is $\omega_{2} = \omega_{AS}$. The experiment is realized with Q-switched high power lasers allowing to work at low pressures and thus to have no collision line broadening. We have optimized the lasers to obtain very weak linewidths limited only by the pulse duration. We used a ruby laser giving 20 MW pulses in 20 ns, transversal and longitudinal single mode, with spectral linewidths of about 0.003\ $cm^{-1}$ and a dye laser with Hansch design, transversely pumped by the second harmonic of the ruby laser. This second laser gives 100 kw pulses in 18 ns, with one or two axial modes, each 0.004\ $cm^{-1}$ wide, with Rhodamine 6G. Tuning is accomplished linearly in wavenumbers by means of stepping motors rotating the grating and the intra-cavity Fabry-P\’{e}rot. We have obtained the $\nu_{1}$ band of methane spectrum at 40 Torr pressure and with a resolution of $0.005 cm^{-1}$. The mixing measures are done comparatively with a 10 atm. Argon reference cell and by averaging on several pulses. Other developments are presented: possibility of work with gaz pressures well below Torr or spectroscopy of weak lines of gases

Développement d'un système d'enlèvement de vernis par laser sur une machine de soudage automatique

National audienc