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International audience; We present a simple approach to predict the main features of optical spectra affected by self-phase modulation (SPM), which is based on regarding the spectrum modification as an interference effect. A two-wave interference model is found sufficient to describe the SPM-broadened spectra of initially transform-limited or up-chirped pulses, whereas a third wave should be included in the model for initially down-chirped pulses. Simple analytical formulae are derived, which accurately predict the positions of the outermost peaks of the spectra.

International audience; We discuss the effects of imparting a temporal sinusoidal phase modulation to a continuous wave on the frequency spectrum. While a practical analytical solution to this problem already exists, we present here a physical interpretation based on interference processes. This simple model will help the students better understand the origin of the oscillatory structure that can be observed in the resulting spectrum and that is characteristic of Bessel functions of the first kind. We illustrate our approach with an example from the field of optics.

International audience; We theoretically study the effects of the temporal intensity profile of the initial pulse on the nonlinear spectral compression process arising from nonlinear propagation in an optical fibre. Various linearly chirped input pulse profiles are considered, and their dynamics is explained with the aid of time-frequency representations. While initially parabolic-shaped pulses show enhanced spectral compression compared to Gaussian pulses, no significant spectral narrowing occurs when initially super-Gaussian pulses are used. Triangular pulses lead to a spectral interference phenomenon similar to the Fresnel bi-prism experiment.

In this paper, laser-induced field-free alignment of CO2 in mixtures with Ar is investigated under dissipative conditions (up to 15 bars) at room temperature. The degree of alignment is temporally monitored by a polarization spectroscopy technique, where a weak probe pulse measures the transient birefringence resulting from the alignment. The data are analyzed with a quantum mechanical density matrix formalism using properly J-dependent and M-dependent state-to-state transfer rates, which was previously successfully tested on pure CO2 and CO2–He mixtures. The same consistency is obtained between experiments and calculations, in particular the decay times of both the transient revivals and the permanent component of the alignment are accurately predicted by the model. Copyright © 2015 John Wiley & Sons, Ltd.

International audience; Experimental and theoretical investigations of coherent anti-Stokes Raman spectroscopy of CH4 have been carried out. Experimental spectra were measured in a heated high-pressure test cell and compared with numerical simulations. Good agreement was obtained for the temperature and the pressure dependence of CARS spectra in the ranges 300-1100 K and 0.1-5.0 MPa. The observed dependencies provide useful guidance for CARS thermometry, allowing quantitative measurements of temperature in high-pressure combustors. Application of multiplex CH4 CARS thermometry for single-shot measurements in a LOX/CH4 combustion at high pressure was demonstrated at supercritical conditions (5.6 MPa).

International audience; The dynamics of the ferric CN complexes of the heme proteins Myoglobin and Hemoglobin I from the clam Lucina pectinata upon Soret band excitation is monitored using infrared and broad band visible pump-probe spectroscopy. The transient response in the UV-vis spectral region does not depend on the heme pocket environment and is very similar to that known for ferrous proteins. The main feature is an instantaneous, broad, short-lived absorption signal that develops into a narrower red-shifted Soret band. Significant transient absorption is also observed in the 360 - 390 nm range. At all probe wavelengths the signal decays to zero with a longest time constant of 3.6 ps. The infrared data on MbCN reveal a bleaching of the C = N stretch vibration of the heme-bound ligand, and the formation of a five-times weaker transient absorption band, 28 cm(-1) lower in energy, within the time resolution of the experiment. The MbC = N stretch vibration provides a direct measure for the return of population to the ligated electronic ( and vibrational) ground state with a 3 - 4 ps time constant. In addition, the CN-stretch frequency is sensitive to the excitation of low frequency heme modes, and yields independent information about vibrational cooling, which occurs on the same timescale.

In this chapter, we will discuss some coherent techniques, namely Raman Induced Polarization Spectroscopy (RIPS) and femtosecond Coherent Raman Anti-Stokes Spectroscopy (fs-CARS). We will demonstrate their ability to be used as non-invasive optical diagnostic tools for temperature, density, or concentration measurements, as well as a means of testing collision induced energy transfer models (in a low field regime), and studying the so-called inhomogeneous lineshape effects that are particularly enhanced in the case of hydrogen. We will also show how molecular alignment achieved in a strong field regime can provide additional information about collisional relaxation processes. In all cases, a precise knowledge of the collisional effects is required, and so a general modeling of collisional relaxations will be detailed.

The shape of the ν1 Raman Q branch of CH4 perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experiments have been made in the 2915–2918 cm−1 spectral region for total pressures from 0.4 to 70 atm and mixtures of ≈5% CH4 with He and Ar. Analysis of the spectra demonstrates that the shape of the Q branch is significantly influenced by line mixing and much narrower than what is predicted by the addition of individual line profiles. For the first time, a model is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the relaxation matrix is constructed, with no adjustable parameter, starting from semiclassical state-to-state rates. Two empirical constants which account for the shift and broadening of the branch due to vibrational effects are introduced and their values are determined from fits of measured spectra. Comparisons between measurements and results computed with and wit...

International audience; We report the experimental study of field-free molecular alignment in CO2 gas mixtures induced by intense femtosecond laser pulses in the presence of collisional processes. We demonstrate that the alignment signals exhibit specific features due to nontrivial collisional propensity rules that tend to preserve the orientation of the rotational angular momentum of the molecules. The analysis is performed with a quantum approach based on the modeling of rotational J- and M-dependent state-to-state transfer rates. The present work paves the way for strong-field spectroscopy of collisional dynamics.

International audience; Time-resolved detection of field-free molec- ular alignment is investigated by phase contrast. The technique based on a 4f imager operating as an inverted Zernike spatial filter makes it possible to discriminate be- tween positive and negative molecular alignment revivals produced in a linear molecule. The measurements are performed in a way that minimizes the contamination of the signal by the plasma generated during the align- ing pulse. The observations are supported by a semi- analytical model, from which the degree of alignment produced at the beam focus is estimated.

Molecular alignment induced by laser fields has been investigated in research laboratories for over two decades. It led to a better understanding of the fundamental processes at play in the interaction of strong laser fields with molecules, and also provided significant contributions to the fields of high harmonic generation, laser spectroscopy, and laser filamentation. In this chapter, we discuss molecular alignment produced under field-free conditions, as resulting from the interaction of a laser pulse of duration shorter than the rotational period of the molecule. The experimental results presented will be confined to the optically probed alignment of linear as well as asymmetric top molecules. Special care will be taken to describe and compare various optical methods that can be employed to characterize laser-induced molecular alignment. Promising applications of optically probed molecular alignment will be also demonstrated.

International audience; This paper presents comparisons between molecular dynamics simulations (MDSs) and the Keilson and Storer (KS) model for collision-induced translational velocity changes in pure H-2 at room temperature from four different points of view. The first considers various autocorrelation functions associated with the velocity. The second and third comparisons are made for the collision kernels and for the time evolutions of some conditional probabilities for changes in the velocity modulus and orientation. Finally, the evolutions, with density, of the half widths of the Q(1) lines of the isotropic Raman (1-0) fundamental band and of the (2-0) overtone quadrupole band are investigated. The results demonstrate that, while the KS approach gives a poor description of detailed velocity-to-velocity changes, it leads to accurate results for the correlation functions and spectral shapes, quantities resulting from large averages over the velocity. On the opposite, collision kernels derived from MDS lead to accurate predictions of all considered quantities. The results open promising perspectives for modeling of the spectral shapes of other systems. They also stress the value of direct calculations of speed-dependent broadening and shifting parameters from the intermolecular potential to avoid their determination from measured spectra and permit fully meaningful tests of the models.

The present work explores the extension of the concept of short-pulse-induced alignment to dissipative environments within quantum mechanical density matrix formalism (Liouville equation) from the weak to the strong field regime. This is illustrated within the example of the CO2 molecule in mixture with Ar and He, at room temperature, for which a steep decrease of the alignment is observed at moderate pressure because of the collisional relaxation. The field-free alignment is measured by a polarization technique where the degree of alignment is monitored in the time domain by measuring the resulting transient birefringence with a probe pulse Raman induced polarization spectroscopy (RIPS) Copyright © 2008 John Wiley & Sons, Ltd.

International audience; CARS laboratory experiments were done in the 2905-2925 cm(-1) range, in the vicinity of the v, band of the methane molecule, for pressures ranging from I to 50 bar, and temperatures up to 1100 K. These experiments were carried out in order to retrieve the pressure evolution of the CH4 spectrum, as well as to confirm its temperature dependance. After a brief recall on the theory used to compute pressure broadening coefficients and relaxation rates, we consider the v(3) and v(4) infrared bands of methane for benchmark calculations purposes. Next, we present recent experimental CARS spectra and calculated ones. Lastly, we discuss flame experiments as well as comparisons of temperature retrieval using N-2 and CH4 as probe molecules.

International audience; Dual-broadband rotational CARS (DB-RCARS) thermometry has been investigated at high temperature and high pressure. Single-shot measurements were performed at 1800 K, in air and nitrogen at pressures up to 1.55 MPa and in oxygen at pressures up to 0.5 MPa. For all conditions, the resonant signal contribution to the spectra clearly dominated over the non-resonant one, implying the high potential for DB-RCARS for temperature and concentration measurements also at the high temperatures and pressures used in the present investigation. The relative standard deviation was generally similar to 2% for single-shot data at pressures from 0.5 to 1.55 MPa. At the investigated temperature, 1800 K, rotational lines from thermally excited vibrational states could be observed in the highly resolved experimental spectra. Using a previously developed weighting procedure applied to these lines, it was demonstrated that the temperature standard deviation could be lowered with as much as 30%.

International audience; We present a study of the spontaneous Raman spectra of (CH4)-C-12 from 1200 to 5500 cm(-1) at various temperatures. This study is of interest from a fundamental as well as from a practical point of view with regards to the temperature diagnostic in hydrocarbon combustion. The present investigation shows that the spontaneous (CH4)-C-12 Raman spectra are very sensitive to temperature and that the complexity of methane spectra is not an obstacle to use methane as a probe molecule in laser-diagnostic techniques. Our study consists in determining the polarisability parameters of methane (CH4)-C-12, unknown at the present time, from spontaneous Raman spectra recorded at pressure less than one atmosphere to minimize the problem of line-mixing. This diagnostic method technique based on spontaneous Raman spectroscopy may be applied to temperature measurements in a uni-element rocket combustion chamber using methane-oxygen propellants.

194317; With the aim of temperature diagnostic, femtosecond time-resolved CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in H2-N2 mixtures. In a first part, a Lorentzian profile is used to model the femtosecond CARS response. A difference between the experimental broadening and the expected one is observed in the collision regime. The observed broadening increases strongly in an inhomogeneous way with respect to the perturber concentration. This is of considerable importance for temperature measurements. In a second part, we show that in the collision regime, this inhomogeneous broadening is due to the speed dependence of the collisional parameters and the memory effects of the radiator speed. A new modelization of the time-resolved CARS response taking into account the speed memory effects is presented and applied to the temperature diagnostic in H2-N2 mixtures. The numerical results are in good agreement with experiments. (c) 2005 American Institute of Physics.

This article reviews the study of Raman line shapes of molecular species involved in reactive media, such flames or engines, at high temperature and high pressure. This study is of interest from a fundamental as well as from a practical point of view with regards to the CARS temperature diagnostic of GH2–LOX combustion systems. We will particularly draw attention to recent investigations by means of Stimulated Raman Spectroscopy (SRS) in H2–H2O mixtures at temperature up to 1800 K. Whereas H2–X systems usually exhibit large inhomogeneous effects, due to the speed dependence of the collisional parameters, the absence of such apparent inhomogeneous signatures in the H2–H2O system allowed us to model the broadening coefficients with simple polynomial laws. These laws permit extrapolations with a narrow confidence interval, as required for temperature measurements. The applications of these results to the temperature diagnostic on the small-scale facility MASCOTTE at ONERA will be described. To cite this article: F. Chaussard et al., C. R. Physique 5 (2004).

International audience; Rotational coherent anti-Stokes Raman spectroscopy (CARS) is a well-established spectroscopic technique for thermometry at pre-combustion temperatures an atmospheric pressure. However, at pressures of several MPa, a previous investigation revealed large discrepancies between experimental data and the theoretical model. A re-evaluation has been made of these data (at room temperature and in the range 1.5-9 MPa) with two improvements to the spectral code. The first is the inclusion of an inter-branch interference effect, which is described in detail in Paper I. The second is the use of experimental S-1-branch Raman line widths measured at 295 K, with a temperature dependence extracted from semi-classical calculations following the Robert-Bonamy formalism. It is shown that these two modifications significantly improve the theoretical model, since both the spectral fits and the accuracy of the evaluated temperatures are considerably improved.

International audience; A summary of the main results obtained by the two groups in the field of H-2 vibrational spectral line signatures for various mixtures. in connection with CARS diagnostics of H-2-O-2 combustion systems, is presented. H-2-X Systems may have specific large inhomogeneous spectral features, due to the dependence of the line broadening and line shifting on the (H-2) radiator speed, particularly at high temperature. Thus, careful attention has to be paid to rigorously analyze such features, both from the experimental point of view (Dijon) and from the theoretical one (Besancon). Applications of the present results to high-pressure H-2/air flame thermometry are also briefly described. They present an approach aiming to include the more recent basic results on coherent Raman line shape in CARS diagnostics. in order to improve the accuracy of temperature measurements.

International audience; In the present paper, we investigate the saturation mechanism of FDFWM (forward degenerate four-wave-mixing) spectra, observed with different polarizations of the input beams, line coupling, and Doppler effect. Our theoretical approach uses a nonperturbative resolution of the density matrix equations, decomposed on the irreducible tensor basis. This formalism greatly simplifies analytical expressions and allows for a correct treatment of magnetic sublevels structure in a three-J level system. Systematic calculations of the FDFWM signal for specific NO lines are performed to characterize intensities and line shapes as a function of saturation. Relative intensities exhibit strong changes in the intermediate regime of saturation further supporting the necessity for using such a complete model. Under saturating conditions, line coupling is shown to distort the spectra, according to the type of transition and the polarization configuration. This effect is observed in the following paper [V. Kruger et al., paper II, Phys. Rev. A 64, 012717 (2001)], where saturated experimental spectra of NO are presented and interpreted for two polarization configurations of interest for diagnostic purposes.

International audience; The paper presents high resolution Raman investigations of the Q(1) line of H-2 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 a remarkable agreement with all the data by using a unique set of parameters. This model should be useful for optical diagnostics at moderate density, as required for combusting media or atmospheric work.

Dans l'objectif du diagnostic de la temperature dans les milieux en combustion, la Diffusion Raman Anti-Stokes Coherente resolue en temps (DRASC - fs) est utilisee pour sonder H 2 dans les melanges H 2 -N 2 a basse pression. Le dispositif DRASC mis en place est decrit en detail. Un nouveau modele de la reponse DRASC temporelle, prenant en compte tous les effets collisionnels specifiques a l'hydrogene (effets dits « de vitesse ») est presentee, ainsi que la comparaison avec l'experience, l'accord se revelant tres satisfaisant.

We present comparisons between measurements and ab initio calculations of the dissipation of the nonadiabatic laser-induced alignment in pure CO2 and CO2-He gas mixtures. The experiments were made for pressures between 2 and 20 bars at 295 K by using short non-resonant linearly polarized laser pulses for alignment and probe. The calculations are carried, free of any adjusted parameter, using refined intermolecular potentials and a requantized Classical Molecular Dynamics Simulations approach presented previously but not yet confronted to experiments. The results demonstrate that the model accurately reproduces the decays with time of both the transient revivals and "permanent" component of the alignment. The significant differences observed between the behaviors resulting from CO2-CO2 and CO2-He collisions are also well predicted by the model.

In this paper, we present measurements and modeling of femtosecond time resolved coherent anti-Stokes Raman spectroscopy (CARS) signal in H(2)-N(2) mixtures at low densities. Three approaches have been used to model the CARS response. The first is the usual sum of Voigt profiles. In the second approach, the speed dependent Voigt profile is used. In the last approach, a model of the temporal CARS signal is developed, which takes into account the velocity changes induced by collisions and the speed dependence of the collisional parameters. The velocity changes are modeled using the Keilson and Storer memory function; the radiator speed dependences of the collisional parameters are determined from their temperature dependences. The results obtained are consistent with previous studies in the frequency domain, showing that the changes of the velocity have important effects for the H(2)/N(2) system in the Dicke narrowing density regime.

This thesis is devoted to the study of dynamics induced by intense femtoseconds lasers pulses. The first studied dynamics deals with molecular alignment of CO2-X mixtures (X=CO2, Ar, N2), in dense gases (up to 20 bar). Up to now, this regime has never been studied experimentally. In the field-free regime (after laser/matter interaction), molecular alignment exhibits two components : a permanent alignment and a transient one. The influence of collisions appears through population transfers between rotational states, which leads to a decrease of these two contributions. Permanent alignment relaxation time is only tied to inelastics collisions whereas transient alignment relaxation time is tied to both inelastics and elastics ones. We show that the determination of the elastic collisions contribution (for which the experimental determination is uneasy), is then possible thanks to the analysis of molecular alignment measurements. This analysis is based on the modelling of inelastics rotational state-to-state transfer rates by ECS-(E)P semi-empirical laws. The elastic contribution of collisions is experimentally determined and happened to be in a good agreement with classically calculated ones. The second studied dynamics is the intensity dependence of the electronic Kerr effect. We pursue the works led by Loriot and al. in 2009 which showed that electronic Kerr index saturated, before nullifying and then presenting a negative contribution when the intensity increases (inversion of the sign for some tens of terawatts by square centimeter). We complete the previous study by performing similar measurements in air at 400 nm (800 nm in the original study)., Le sujet de cette thèse concerne l’étude de dynamiques induites par des impulsions lasers femtosecondes intenses. La première dynamique étudiée porte sur l’alignement de la molécule de CO2, pure ou en mélange avec l’argon ou l’hélium, en phase gazeuse dense (jusqu’à 20 bar), ce régime n’ayant jamais été étudié expérimentalement auparavant. L’alignement moléculaire, quand il est induit par une impulsion laser femtoseconde et intense, présente deux contributions qui apparaissent après passage de l’impulsion : un alignement permanent et un alignement transitoire. L’influence des collisions se manifeste alors par des transferts de population entre états rotationnels qui ont pour conséquence de faire décroître ces deux contributions. Le temps de décroissance de l’alignement permanent est seulement lié aux collisions inélastiques tandis que le temps de décroissance de l’alignement transitoire est lié à la fois aux collisions inélastiques et élastiques. Nous montrons alors que la détermination expérimentale de la contribution des collisions élastiques, expérimentalement difficile d’accès, est possible à partir de l’analyse des traces d’alignement moléculaire. Cette analyse se base sur la modélisation des taux de transfert entre états liés aux collisions inélastiques par des lois semi-empiriques du type ECS-(E)P. La contribution élastique des collisions déterminée est en bon accord avec des valeurs calculées selon un modèle classique. La deuxième dynamique étudiée est la dépendance en éclairement de l’effet Kerr électronique. Nous poursuivons alors les travaux menés par Loriot et al. en 2009 qui ont montré que l’indice Kerr électronique saturait avant de s’annuler puis de présenter une contribution négative lorsqu’on augmente l’éclairement (inversion du signe pour quelques dizaines de térawatts par centimètre carré). Nous avons alors étendu cette étude en observant à une longueur d’onde de 400 nm (800 nm dans l’étude originale) cette inversion du signe de l’indice Kerr dans l’air.