J. Bonamy

Rotationally inelastic rates for N2–N2 system from a scaling theoretical analysis of the stimulated Raman Q branch

Self‐broadened nitrogen isotropic Q(J) Raman linewidths have been inverted to obtain effective rotation–translation (R–T) state‐to‐state rate constants using the energy corrected sudden (ECS) formalism. These rate constants are discussed as a function of the rotational levels J and temperature T. Collisional Q(J) line shifts have been investigated by high‐resolution inverse Raman spectroscopy (IRS) over a wide temperature range. Semiclassical calculations lead to a clear understanding of their J and T dependence. This exhaustive study of both diagonal and off‐diagonal relaxation matrix elements has allowed us to calculate the collisionally narrowed Q branch at high pressure. New measurements of N2 Q branch at high pressure have been performed by IRS. The good agreement of ECS profiles with IRS data, for various pressures and temperatures, underlines the consistency of the present R–T ECS scaling analysis.

A test of different rotational Raman linewidth models: Accuracy of rotational coherent anti-Stokes Raman scattering thermometry in nitrogen from 295 to 1850 K

Rotational Raman linewidths calculated from three different models have been used in temperature measurements by rotational coherent anti-Stokes Raman scattering (CARS)—a semiclassical ab initio model, the modified exponential energy gap model (MEG), and the energy corrected sudden scaling law (ECS). Experimental rotational CARS spectra were generated, using the dual-broadband approach, in pure nitrogen at atmospheric pressure in a heat pipe in the temperature range from 295 to 1850 K. Below 1500 K, the temperatures evaluated using the ECS linewidths agreed with the heat-pipe temperatures to within 20 K. Above 1500 K, the errors in the evaluated temperatures increased steeply for all linewidth models, reaching errors of several hundreds of Kelvins at 1850 K. This behavior of the evaluated temperature is probably caused by the uncertainty in the values of the rotational Raman linewidths for high rotational states at high temperatures. This work therefore illustrates that rotational CARS can be used for experimentally studying Raman linewidths and in particular their dependence on temperature and rotational quantum number. The influence of different experimental parameters on the evaluated temperatures is discussed, and the spectral synthesis program is presented. The Journal of Chemical Physics is copyrighted by The American Institute of Physics. (Less)

Simplified models for the temperature dependence of linewidths at elevated temperatures and applications to CO broadened by Ar and N2

Abstract The broadening coefficients for i.r. lines of CO perturbed by Ar are calculated in the temperature range 300–3500 K using the formalism previously developed by two of us (D.R. and J.B.). The results are compared with high-resolution spectroscopic measurements of shock-heated CO-Ar gas mixtures. A simplified model is proposed to describe the temperature dependence of the linewidths. The resulting model is applied to CO broadened by N 2 and the results are critically discussed.

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

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...

Collisional effects in the stimulated Raman Q branch of O2 and O2–N2

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 r...

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

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 di...

Line broadening, line shifting, and line coupling effects on N2-H2O stimulated Raman spectra

In order to understand the influence of H2O on the stimulated Raman Q‐branch spectra of nitrogen in combusting media, an exhaustive theoretical and experimental study has been carried out. Starting from a semiclassical model, particularly convenient at high temperature, the Q‐line broadening and shifting coefficients have been calculated over a wide temperature range and for a large number of lines. Stimulated Raman Spectra (SRS) measurements have allowed us to test these calculated line broadening coefficients and thus establish the high accuracy of semiclassical values. The theoretical broadening coefficients have been inverted to deduce state‐to‐state rotational relaxation rates by using two types of fitting laws. A partial test of the resulting Q‐branch profiles has been realized at moderate pressures leading to a discrimination between these two laws. Furthermore, the effect of rotational energy transfers on collisionally narrowed profiles at higher densities has been simulated and compared with the ...

Collisional effects on spectral line shape from the Doppler to the collisional regime: A rigorous test of a unified model

The paper presents high resolution Raman investigations of the Q(1) line of H2 in Ar mixture from low density (Doppler regime) to high density (collisional regime) analyzed with a unique line shape profile. Measurements are performed by stimulated Raman gain spectroscopy between 300 and 1000 K in a wide density range (from 0.2 to 11 amagat). All the observed spectral features are accurately described by a unified model recently proposed by two of the authors. This model accounts for a velocity-memory process, not restricted to the usual hard and soft limits. It also includes correlation between velocity- and phase-changing collisions. An exhaustive analysis of various possible mechanisms on the line shape is achieved. These mechanisms are the Dicke narrowing, the radiator speed dependence of the collisional broadening and shifting parameters, the collisionally induced speed-class exchange and the nonimpact effect. The present test shows the high consistency of the unified model, since it allows one to get...

Speed-dependent line profile: A test of a unified model from the Doppler to the collisional regime for molecule–molecule collisions

A speed-dependent line profile combining soft and hard fully correlated Dicke-narrowing collisions was recently successfully tested on Ar-broadened H2 spectra in a wide density and temperature range. A further test for mixtures of H2 in nitrogen molecules (instead of Ar atoms) is presented. This test is also based on high resolution Raman investigation of the isotropic Q(1) line of H2 from low to high density at various temperatures. The same consistency of the speed-dependent line profile as for H2–Ar is obtained for H2–N2 through a remarkable agreement with all the data by using a unique set of four parameters (the collisional width and shift, the kinetic frequency, and a characteristic velocity memory parameter). The present study is a preliminary step for the hydrogen CARS thermometry in H2–air flames at high pressure.

Semiclassical calculations of collision line broadening in Raman spectra of N2 and CO mixtures.

We present a detailed theoretical study of pressure-broadened Raman line shapes in binary mixtures of nitrogen and carbon monoxide. The semiclassical Robert-Bonamy theory was used to calculate self-broadened Q-branch linewidths of N(2) and CO, and Lennard-Jones (LJ) potential energy surface parameters were fixed by comparing our results with extensive experimental linewidth data. For the case of N(2), the ab initio PES8 potential energy surface was investigated, however, the anisotropic repulsive part had to be reduced to ensure a good agreement with experimental linewidths. The agreement between calculations and experiments was remarkably good, both for self-broadened N(2) and CO Q-branch linewidths. Yet, our calculations were not able to predict the experimentally observed difference between Q- and S-branch linewidths of self-broadened N(2). The central results of this work are the Q-branch linewidths of N(2)-CO and CO-N(2), which have been calculated through an extrapolation of the parameters of the potential energy surfaces used for self-broadened linewidths by common combination rules.