D. Robert

Overlapping effects and motional narrowing in molecular band shapes: Application to the Q branch of HDa)

A new formulation of the theory of strong overlapping effects in molecular line broadening is presented in terms of the intermolecular potential. This formulation takes into account the vibrational degrees of freedom and includes in an acceptable form the close collisions. A detailed analysis of such effects is carried out for the motional narrowing arising in the Q branch of HD at sufficiently high densities. A very good consistency is obtained between the experimental data and the present calculation at room and low temperatures. The nonadditivity effects in infrared absorption and anisotropic Raman overlapping lines treated in the general frame of this paper may also be easily calculated when required.

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.

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...

Collisionally induced population transfer effect in infrared absorption spectra. I. A line‐by‐line coupling theory from resonances to the far wings

The line‐by‐line coupling for a pressure broadened rovibrational band is formulated in the far‐wing limit. The present quasistatic theory assumes that the wave frequency is displaced from the line centers by an amount that is large compared with both the reciprocal duration of a typcial binary collision and frequency separation between strongly coupled lines. This theory generalizes that of Rosenkranz [J. Chem. Phys. 83, 6139 (1985)] where the Fano’s relaxation operator was reduced to a scalar parameter through a band average. The present approach permits computation of far‐wing absorption more specifically tailored to individual lines. Such a line‐by‐line approach is needed for rovibrational bands where some far lines contribute significantly to the total absorption. In order to obtain a qualitative picture of the line coupling as a function of the frequency displacement, calculations for collisions of CO2 with Ar have been performed for some lines. The results are compared with the corresponding variati...

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...

An analytical model for collisional effects on spectral line shape from the Doppler to the collision regime

A spectral line shape model accounting for both the collisional confinement narrowing of the Doppler distribution and the inhomogeneous effects due to the radiator speed dependence of the collisional broadening and shifting parameters is proposed. The velocity changes are assumed to be induced as well by hard as soft collisions. Doppler-collision correlations and speed-class exchanges are taken into account. A comparison with the previous models used for the Doppler regime and for the collisional one is done. The present model also applies for the intermediate regime where the two above mechanisms are simultaneously efficient. The spectral line shape characteristics are exemplified through the H2–Ar and C2H2–Xe prototype molecular systems.

IOS and ECS line coupling calculation for the CO-He system - Influence on the vibration-rotation band shapes

Line coupling coefficients resulting from rotational excitation of CO perturbed by He are computed within the infinite order sudden approximation (IOSA) and within the energy corrected sudden approximation (ECSA). The influence of this line coupling on the 1–0 CO–He vibration–rotation band shape is then computed for the case of weakly overlapping lines in the 292–78 K temperature range. The IOS and ECS results differ only at 78 K by a weak amount at high frequencies. Comparison with an additive superposition of lorentzian lines shows strong modifications in the troughs between the lines. These calculated modifications are in excellent quantitative agreement with recent experimental data for all the temperatures considered. The applicability of previous approaches to CO–He system, based on either the strong collision model or exponential energy gap law, is also discussed.

Finite duration of collisions and vibrational dephasing effects on the Ar broadened HF infrared line shapes: Asymmetric profiles

The existence of an asymmetry for the HF rovibrational absorption isolated lines induced by Ar pressure is clearly observed. This confirms the expected behavior based on a previous theoretical analysis of the finite duration effect on the molecular line shapes in the core region at moderate densities. The observed profile is well represented by the addition to the usual Lorentzian line shape of a dispersion component. This component is proportional to a characteristic parameter, linear with respect to the density of perturber and explicitly connected to the intermolecular potential. The a priori calculation of this parameter from an empirical potential leads to a super‐Lorentzian shape in the low frequency side in agreement with the experiment. Moreover, the expected increase of the asymmetry when going from the fundamental to the harmonic band is well observed. Such an increase results from the modification of the vibrational dephasing process due to molecular anharmonicities.

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 ...