Showing papers by "Guy Millot published in 1990"

Study of collisional effects on band shapes of the ν1/2ν2 Fermi dyad in CO2 gas with stimulated Raman spectroscopy. I. Rotational and vibrational relaxation in the 2ν2 band

Abstract: The 2ν2 component of the Fermi dyad ν1/2ν2 of CO2 has been studied with high‐resolution stimulated Raman spectroscopy (SRS). The behavior of the band shape has been explored in a large density range: 0.2 to 50 amagat at a temperature of 295 K and 0.5 to 20 amagat at 500 K. Energy corrected sudden (ECS) and modified energy gap (MEG) laws are used to model the relaxation matrix in order to account for the collisional narrowing induced by rotational energy transfers. ECS model allows us to accurately determine the vibrational shift and width as a function of density by fitting the experimental spectra, leading to the determination of the vibrational relaxation coefficients for the 2ν2 mode. Connection is established between the present calculations of the collisionally narrowed SRS spectra based on the diagonalization of the relaxation matrix, which applies for any line overlap, and the usual spectral line shape for weak line coupling. Particular emphasis is put on the situation of strong collapse and on the concomitant predominance of the vibrational dephasing.

Rotationally inelastic rates over a wide temperature range based on an energy corrected sudden–exponential‐power theoretical analysis of Raman line broadening coefficients and Q branch collapse

Abstract: We study the ability of the energy corrected sudden (ECS) scaling law associated with a hybrid exponential‐power (EP) fitting law for the basis rate constants to model the rotational dependence of isotropic Raman linewidths. We determine the temperature dependence of the hybrid law (EP) parameters and give several applications for various molecular collisional systems over a wide temperature range. In particular, we find that the ECS–EP law gives a very good description of both rotational and temperature dependences of the Raman line broadening coefficients of N2 perturbed by H2O contrary to the ECS–P law. For all the collisional systems studied the ECS–EP law is found very suitable to describe the line broadening coefficients. We give a complete set of ECS–EP parameters for these collisional systems which play an important role in coherent anti‐stokes Raman spectroscopy thermometry. We also test the ability of the ECS–EP law to predict accurate collapsed Q branch at high density. An application performed on the Raman Q branch of pure N2 at high density demonstrates that the ECS–EP law properly models the rates of state‐to‐state rotational energy transfer. The overall collisional line shift obtained from the collapsed Q branch is in perfect agreement with low pressure measurements on isolated lines.

Study of collisional effects on band shapes of the ν1/2ν2 Fermi dyad in CO2 gas with stimulated Raman spectroscopy. II. Simultaneous line mixing and Dicke narrowing in the ν1 band

Abstract: An experimental (SRS) and theoretical analysis for the ν1 component of the ν1/2ν2 Fermi dyad of CO2 has been performed for densities lying from 0.01 to 50 amagat at 295 K, and from 0.01 to 20 amagat at 500 K. At subatmospheric pressure, both line mixing and Dicke narrowing take place for this component due to the very weak Q line spacings. A simple method to account for both diffusional narrowing (due to velocity changing collisions) and collisional narrowing (due to energy transfers) on isotropic Raman Q‐branch profile is proposed. This method is based on the transformation of the collapsed Q‐branch profile as a sum of individual Lorentzian plus dispersive components whose parameters are density‐dependent. Such an exact transformation permits to easily introduce the averaging effect of velocity changing collisions on each component, and then on the collapsed Q‐branch itself. In the present study, the Galatry soft collision model is used to define a generalized complete profile for each Lorentzian plus dispersive component. Such a procedure allows us to take into account the velocity changing collision’s effects not only on isolated lines (the well‐known Dicke narrowing) but also on the line couplings resulting from collisionally induced rotational energy transfers. The present analysis permits an accurate description of the observed modifications on the SRS profile of the ν1 band of CO2 (1388 cm−1) as a function of density. The straightforward extension to other spectroscopies (linear and nonlinear) is suggested.

Collisional line broadening and line shifting in N2-CO2 mixture studied by inverse Raman spectroscopy

Abstract: Collisional effects in the Raman Q-branch of N 2 perturbed by CO 2 have been studied by high-resolution stimulated Raman spectroscopy. The Raman spectra recorded in the 0.3–1.0 atm and 295–1000 K pressure and temperature ranges are fitted with a theoretical profile taking into account line broadening, frequency shift and line mixing due to rotational energy transfers. The data at low density are used as basic data for the modeling of rotationally inelastic rates through sets of adjustable parameters. We have used in this study the two main models developed in the last decade and known as modified exponential gap (MEG) and energy corrected sudden (ECS) laws. Experimental spectra recorded at density up to 32 amagat are compared with simulated spectra derived from both models. This constitutes a test for these models which give similar results at low density.

Enhancement of sensitivity in high‐resolution stimulated Raman spectroscopy of gases: Applicaion to the 2ν2 (1285 cm−1) band of CO2

Abstract: The application of a multiple-pass gas cell to quasi-cw stimulated Raman scattering is demonstrated to be a powerful method for significantly increasing the sensitivity. As a consequence, such a device offers the possibility of working in the 0.1–1 Torr pressure range for weak bands. As a striking example, experimental results are given for the 2ν2 band of CO2 at 1285 cm−1. The band is almost completely resolved with a very good signal-to-noise ratio. The Raman frequencies and the collisional broadening coefficients are determined as a function of quantum number J. Moreover, the possibility of auto-stimulated Raman effect on CO2 at moderate pressure via the sharp ν1 band (1388 cm−1) is demonstrated. The first Stokes signal has been measured with a very narrow line width.

Accurate spectroscopic constants of nitrogen determined from stimulated Raman spectra of the fundamental and first hot bands

Abstract: Nitrogen spectra of the Q-branch of the fundamental and the first hot bands were recorde with a high-resolution stimulated Raman spectrometer at atmospheric pressure and ca. 1300 K. The absolute frequencies of the Raman lines were measured with high accuracy, leading to a refinement of spectroscopic constants. A temperature estimation was also performed from the Raman intensities.

High‐resolution non‐linear Raman spectroscopy in gases

Abstract: The resolution in the Raman spectra of gases has been greatly improved by the development of the different methods of non-linear Raman scattering. When two laser beams, one of which has a tunable frequency, are focused in a sample, a stimulated Raman process occurs as soon as the frequency difference between the two lasers is equal to a Raman-active rovibrational or rotational transition frequency. The Raman resonance can be detected in different ways: by coherent anti-Stokes Raman scattering (CARS) or the corresponding Stokes process (CSRS), by a gain in one of the beams (stimulated Raman gain spectroscopy, SRGS) or a loss in the other (inverse Raman spectroscopy, IRS), or even by detection of a photoacoustic signal (photoacoustic Raman spectroscopy, PARS). The selective ionization of the excited molecules by a third ultraviolet laser (ionization-detected stimulated Raman scattering, IDSRS) has considerably increased the sensitivity in special cases. The instrumental resolution is determined by the convoluted line widths of the lasers used for excitation. The narrowest line widths can be achieved with stabilized continuous-wave lasers. Their relatively low power has been compensated for either by intracavity excitation of CARS spectra or by injection locking of dye laser amplifiers which are pumped by pulsed lasers or by flashlamps. Examples of investigations of the structure of rovibrational bands and of line-width measurements as a function of pressure are reviewed.

Measurments of collisional line widths in the stimulated Raman Q-branch of the ν1 band of silane

Abstract: Self-broadened widths of 28SiH4 in the ν1Q-branch have been measured at room temperature (295 K) using high-resolution stimulated Raman Spectroscopy. These collisional widths have been obtained by fitting a super-position of Voigt profiles to the experimental spectra in the pressure range 28–154 Torr. No evidence for line mixing within the tetrahedral components of a Q(J) line has been found. The line broadening coefficients for J up to 13 depend weakly on the rotational quantum number. The mean value is 103.7 × 10−3 cm−1 atm−1.