H. Berger

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...

High-resolution stimulated Raman spectroscopy of O2

Abstract The S S and O O branches of the fundamental vibrational band of molecular oxygen in its electronic ground state have been resolved for the first time in Raman spectroscopy. The spectra have been observed, at room temperature and low pressure, with a stimulated Raman scattering (S.R.S.) spectrometer including a multipass cell. From these accurate Raman data combined with microwave data, we have improved the values of the vibrational, rotational, spin-spin, and spin-rotation interactions constants in the v = 1 vibrational state. Moreover, Raman Q branches of the first and second hot-bands have been recorded, allowing us to determine a set of molecular parameters for the v = 2 and v = 3 states, useful for Coherent Anti-Stokes Raman Spectroscopy (C.A.R.S.) diagnostics in combustion media.

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...

Stimulated Raman spectroscopy of the Q branch of nitrogen at high pressure: collisional narrowing and shifting in the 150–6800 bar range at room temperature

The Raman Q branch of N2 has been recorded at room temperature in the pressure range 150–6800 bar, which corresponds to densities from 135 to 800 amagat. In this domain, the Raman Q branch profile is mainly determined by the well-known collisional narrowing. The experimental data have been obtained by means of a high resolution stimulated Raman spectrometer. The linewidth and line shift of the band have been accurately measured as functions of the density, and their density dependences have been fitted by polynomials. The minimum of the linewidth and the maximum of the red shift have been clearly observed at respectively 735·8 and 532·3 amagat. The lineshape was found to be Lorentzian above 288 amagat with a linewidth inversely proportional to the density up to 670 amagat. This led to the determination of the rotational relaxation constant nτE = 0·160 ns amagat. Above 670 amagat, a nonlinear increase in the linewidth due to the increasing influence of the vibrational contribution has been observed. Fittin...

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 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.

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...

High-resolution stimulated Raman spectroscopy of methane 13CD4 in the pentad region

Abstract We present the first Raman spectrum of 13CD4 recorded at room temperature in the pentad region by inverse Raman spectroscopy, thus including the ν1 (A1), 2ν2 (A1), 2ν4 (A1), and ν2 + ν4(F1 + F2) Q branches. It is noteworthy that the overtone bands 2ν2 and 2ν4 are observed for the first time in a methane-like molecule by a coherent Raman process. The wide frequency range investigated, covering 45 cm−1 in three parts, contains more than 300 lines with uncertainty less than 10−3 cm−1 in most cases. These Raman data are combined with high-resolution infrared data in a weighted least-squares fit of the vibration-rotation constants of the pentad, thanks to a relevant partially reduced effective Hamiltonian including all the third-order interaction terms between the upper states of the five vibrational bands. These interactions within the pentad induce strong perturbations in the isotropic Raman spectrum. In particular the ν1 band exhibits a complex structure, which results essentially from the second-order Coriolis interaction between the v1 = 1 (A1) state and the v2 = v4 = 1 (F1) state. The analysis has been realized up to J = 18 in such a way that almost all of the Raman transitions were assigned up to J = 17. These observed Raman transitions are reproduced with an overall weighted standard deviation of 1.86 × 10−3 cm−1, which is the order of magnitude of the accuracy of the experimental data.

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.