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Time-resolved Raman-infrared double resonance experiments have been conducted on CO2 gas in order to determine collisional relaxation rates of energy levels. These total depopulation constants are measured in a three-level double resonance scheme. A pulsed Raman excitation populates the studied state over a brief time interval (7 ns). The ensuing collisional depopulation of this level is monitored by a continuous probe CO2 laser whose transition originates in the same level. The transient absorption on the probe laser gives the time dependent population behavior. The high selectivity of the lasers allows the study of vibrational and rotational levels. The relaxation rates, of the (1000) vibrational state and, for the first time, of the J=14–34 rotational levels of the (0200) vibrational state, have been measured. The derived broadening coefficients have been compared to those calculated by a semiclassical model. [ABSTRACT FROM AUTHOR]

The fundamental isotropic Raman Q branch of oxygen perturbed by collisions with water vapor has been studied at pressures up to 1.5 atm and for temperatures between 446 and 990 K. The spectra have been recorded by using coherent anti-Stokes Raman spectroscopy (CARS) which has been preferred to stimulated Raman spectroscopy (SRS) in order to obtain more signal and higher sensitivity as the mixture has a small percentage of oxygen. The high resolution CARS spectrometer uses a seeded Nd:YAG laser actively stabilized on an external Fabry–Perot interferometer to prevent any frequency drift during the course of the experiment. The line broadening coefficients have been determined for several rotational quantum numbers (up to N=31 at 990 K). The effect of the splitting into triplets at lower pressure and the effect of interferences between neighboring lines at higher pressure have been taken into account. The influence of Dicke narrowing has also been considered and special care has been taken to avoid Stark broadening. The line broadening coefficients have been calculated according to a semiclassical model. The rotational quantum number and temperature dependencies of the experimental line broadening coefficients have also been studied with another approach based on fitting and scaling laws. Among several laws, the modified exponential energy gap law (MEG), the statistical power-exponential gap law (SPEG), and the energy corrected sudden law with basis rate constants taken as a hybrid exponential-power law (ECS-EP) have given good results. We have used the fitting and scaling laws to extrapolate in temperature the linewidths at 2000 K. [ABSTRACT FROM AUTHOR]

The fundamental isotropic Raman Q branch of oxygen at pressures up to 2 atm and for temperatures between 295 and 1350 K has been recorded using stimulated Raman gain spectroscopy (SRGS) for collisions with oxygen and nitrogen. The line broadening and line shifting coefficients have been determined for several rotational quantum numbers (up to N=55 at 1350 K). The temperature dependence of these coefficients has also been studied for most of the rotational lines. The line parameters (widths and shifts) have been then calculated a priori through a semiclassical model. A good agreement between experimental and theoretical data has been observed. Another theoretical approach based on fitting and scaling law has been used to calculate the line broadening coefficients. It is shown that a modified exponential energy gap model (MEG) and an energy corrected sudden law (ECS) for the state-to-state rotationally inelastic rates, account for the rotational and temperature dependences of the observed linewidths. With regard to the energy corrected sudden law, the best results are obtained when the basis rate constants are modeled with a hybrid exponential-power fitting law (EP). The line broadening and shifting coefficients of the oxygen–nitrogen mixture are very close to those found for pure oxygen. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The influence of CO2 and H2O on the rotational relaxation processes of N2 in ternary mixtures N2–CO2–H2O is investigated. The efficiency of these perturbers is responsible for significant modifications of the state-to-state relaxation rates and broadening coefficients. Flame data are well reproduced by taking into account these modifications. The role of these minor species in the determination of temperatures in premixed flames is analyzed. The present relaxation model allows us to understand why the discrepancy between observed and calculated coherent anti-Stokes Raman spectroscopy (CARS) spectra in flames is sometimes resolved by empirically adding a dephasing component to pure nitrogen linewidths. Moreover, this model improves the accuracy of CARS temperature measurements. [ABSTRACT FROM AUTHOR]

Deals with a study which examined the collisional narrowing induced in the stimulated Raman spectroscopy fundamental Q branch of oxygen by self-collisions and by collisions with water vapor. Information on the modeling of temperature and rotational dependencies of the Raman line broadening coefficients by fitting and scaling laws; Spectra of pure oxygen; Measurements of the stimulated Raman Q branch of pure oxygen.

Long optical similaritons using a Raman fibre amplifier are generated. These pulses, with a highly parabolic profile, are monitored in real time on a high speed oscilloscope. Tunability of both the temporal and spectral widths of the pulses is then investigated. [ABSTRACT FROM AUTHOR]