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Une superposition linéaire d'états rotationnels est produite dans le niveau vibronique de base de la molécule d'azote (N2) par une impulsion laser ultra-brève non résonnante. Un contrôle précis de ce paquet d'ondes, à travers la modulation de phase spectrale du champ électrique excitateur, est démontré expérimentalement. L'excitation relative entre modes Raman associés à des niveaux de parité donnée est réalisée. L'évolution temporelle du paquet d'ondes rotationnel mis en forme est observée par une technique de spectroscopie de polarisation. Les résultats expérimentaux sont confortés par une analyse théorique menée dans l'espace des temps et des fréquences.

Measurements of concentrations in gas mixtures of three and four molecular components are presented. They rely on a femtosecond time-resolved pump–probe technique based on Raman-induced polarization spectroscopy. The rotational time response of the molecular gas mixture is measured as a function of the pump–probe time delay. No selective frequency tunability is needed as the molecular rotational spectra are excited within the laser bandwidth. The results obtained from experiments performed at room temperature in N2O–CO2–N2 and N2O–CO2–O2–N2 mixtures are presented and the accuracy of the method is discussed. Copyright © 2002 John Wiley & Sons, Ltd.

We report a theoretical and experimental investigation of the dynamical Stark effect in a tetrahedral molecule, silane (SiH4). We use a tetrahedral formalism and Floquet theory to calculate the absorption spectra for the molecule dressed by an intense nonresonant pulsed laser. Experimentally, the dynamical Stark effect is observed for transitions of the ν2/ν4vibrational polyad of SiH4by means of nanosecond diode laser absorption spectroscopy and a Nd:YAG laser excitation.

The SiH2D2asymmetric top has nine vibrational modes, five of them forming a pentad strongly perturbed by Coriolis interactions. High-levelab initiocalculations of SiH2D2have been performed which yield numerous spectroscopic parameters related to the harmonic and anharmonic force fields. The bending pentad comprising ν4(A1), ν7(B1), ν5(A2), ν9(B2), and ν3(A1) has been studied by high-resolution Fourier transform spectroscopy; the region 600–1050 cm−1has been investigated with a resolution of ca. 4 × 10−3cm−1. Raman BOXCARS spectroscopy has been used for the infrared inactive ν5band. The Raman apparatus function was 0.0054 cm−1. Assignments of about 4000 transitions including all bands have been made, mostly employing ground state combination differences techniques, and a global fit has been performed. The fundamentals ν4(681.624 cm−1), ν7(742.640 cm−1), ν5(842.381 cm−1), ν9(859.750 cm−1), and ν3(942.741 cm−1) are strongly coupled by A-, B-, and C-type Coriolis interactions, andab initiopredictions of these interaction parameters were used to set up a network of interactions that was refined by the experimental data. The global standard deviation for the entire body of data is 7.1 × 10−4cm−1. Satisfactory synthetic spectra which are very sensitive to relative signs of dipole moment derivatives and Coriolis interaction constants were obtained with the guidance ofab initiocalculations. Finally, fair to good agreement of experimental andab initiocalculated molecular parameters was obtained. For the first time, a complete analysis of the pentad of SiH2D2in the 10–16 μm region has been carried out. A full set of rovibrational parameters is given for these five interacting levels, including first and second order Coriolis interaction constants.

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. Fitting laws usually employed to model the relaxation matrix and thus to account for the collisional narrowing have been tested against our experimental spectra. The Schweizer-Chandler model has been also applied to reproduce the density dependences of the Raman linewidth and line shift at high density.

We present the first Raman spectrum of cyclopropane, obtained at nearly Doppler-limited resolution by inverse Raman spectroscopy, in the region containing the Q branches of the totally symmetric fundamental band ν1and the harmonic band 2ν2. Many additional vibrational states occur in this region, and their anharmonic and Coriolis interactions with ν12ν2make the appearance of the spectrum rather complex. An analysis of the spectrum taking into account the most important interactions is described and a preliminary set of spectroscopic constants is reported for the interacting states ν1, 2ν2, ν7+ ν9+ ν14, and ν4+ ν5+ ν7.

The study of the isotropiv Raman Q-branch of a hot band Π← Πallows one to establish a direct connection between the vibration-rotation angular momentum coupling and the resulting spectra. Due to the l-doubling, the Q-branch is split into two subbranches characterized by either even or odd rotational quantum number j. The vibrational bending reduces the rotational transfer rates inside each of these subbranches by a factor of about two and induces an inter-subbranch coupling. The expected propensity rule towards conservation of the parity index for high rotational levels is well observed. Calculated spectra are in excellent agreement with CARS experiments for the v1+ v2← v2band of CO2.

High-resolution stimulated inverse Raman spectroscopy has been applied to the study of collisional broadening, shifting, and line mixing for theOO(J,N= 5) triplet line of the fundamental vibrational band of molecular oxygen. Accurate line broadening coefficients for the individualJcomponents within the triplet have been measured for the first time and show a significantJdependence. The line broadening coefficients are larger than those previously obtained for unresolved pure rotational Raman lines. The additional broadening is expected to result from electronic spin relaxation. The pressure-induced line shift has been obtained for this line and compared to the value obtained for the fundamentalQbranch. By applying the Rosenkranz perturbation treatment to the collisionally mixed components of the triplet line, we have been able to obtain an estimate of the coupling parameters.

The stimulated Raman spectrum of 12CD4was recorded at high resolution, in the spectral range 1963 to 2260 cm−1. Intensities of the rovibrational transitions were estimated by fitting the line profiles. The recorded elementary spectrum was 1 cm−1wide; all recordings were then intensity calibrated. Afterward, the relative Raman intensities were analyzed by using a polarizability model developed previously [A. Boutahar and M. Loëte, Can. J. Phys.69,26-35 (1991); J. P. Champion, M. Loëte, and G. Pierre, in"Spectroscopy of the Earth′s Atmosphere and Interstellar Molecules" (K. Narahari Rao and A. Weber, Eds.), pp. 388-397, Academic Press, Boston, 1992]. The investigated region has five interacting bands (ν1, ν3, 2ν2, ν2+ ν4, and 2ν4) and they were analyzed simultaneously. Experimental data enabled retrieval of six polarizability parameters. The agreement between calculated and measured intensities is within about 12%. To confirm the results, some calculations were made of other spectra, recorded using spontaneous or stimulated Raman techniques.

The stimulated Raman spectrum of 28SiH4has been recorded in the ν1/ν3region between 2182 and 2190 cm−1. The intensities of the Raman transitions were modeled using a dyad scheme for the two interacting bands with a first-order polarizability expansion. The involved polarizability parameters have been adjusted to fit about 800 measured intensities of transitions reaching upper states up to J= 18. The resulting standard deviation is about 10%.

We have determined the temperature from SRS spectra of N2-N2, N2-CO2, O2-O2, and CO2-CO2 recorded in wide pressure and temperature ranges. The fitting procedure takes simultaneously into account the Dicke effect and motional narrowing. We have quantified the accuracy of the MEG and ECS-P models for rotational relaxation. The temperature extracted from each model is compared with thermocouple measurements. The influence of vibrational broadening and shifting is discussed in detail.

High-resolution stimulated Raman spectroscopy was applied to the study of collisional broadening and shifting for rovibrational anisotropic Raman lines of the Fermi dyad of molecular carbon dioxide. The O(J) lines of the ν1 band and the S(J) lines of the 2ν2 band were recorded at 295 K. The pressure-induced line shifts were obtained and compared with the overall shift of the high-density Raman Q-branch. A rotational quantum number dependence of the rovibrational line broadening coefficients was observed. The experimental line broadening coefficients were used in order to check the ability of two theoretical methods (random phase approximation and sum rule) for calculating the line broadening coefficients in anisotropic Raman scattering. © 1998 John Wiley & Sons, Ltd.

The recent measurement of negative higher-order Kerr effect (HOKE) terms in gases has given rise to a controversial debate, fed by its impact on short laser pulse propagation. By comparing the experimentally measured yield of the third and fifth harmonics, with both an analytical and a full comprehensive numerical propagation model, we confirm the absolute and relative values of the reported HOKE indices.

New high-temperature measurements of collisional linewidths of the Raman Q branch of nitrogen have been performed at 1700–2400 K with stimulated Raman spectroscopy in a tungsten filament. We fitted these data together with previous data obtained in the 295–1500-K range to redetermine the parameters of the relaxation models used in coherent anti-Stokes Raman spectroscopy thermometry. The improvement in the accuracy of the temperature measurement has been checked. Semiclassical calculations of linewidths in the 1700–2400-K range agree with the experimental data and have been extended to even higher temperatures.

Dans l'objectif du diagnostic de la température dans les milieux en combustion, la Diffusion Raman Anti-Stokes Cohérente résolue en temps (DRASC – fs) est utilisée pour sonder H2dans les mélanges H2-N2à basse pression. Le dispositif DRASC mis en place est décrit en détail. Un nouveau modèle de la réponse DRASC temporelle, prenant en compte tous les effets collisionnels spécifiques à l'hydrogène (effets dits ”de vitesse") est présentée, ainsi que la comparaison avec l'expérience, l'accord se révélant très satisfaisant.

Par le biais d'une méthode originale, nous avons déterminé expérimentalement la probabilité d'ionisation de l'azote (N2) soumis à un champ laser non résonnant et ultracourt (100 fs). La technique de défocalisation croisée mis en œuvre dans ce travail est sensible à l'alignement moléculaire post-impulsion et à l'ionisation. L'analyse du signal expérimental permet d'extraire une probabilité d'ionisation calibrée par une mesure d'alignement.

Manipulating at will the propagation dynamics of high power laser pulses is a long-standing dream whose accomplishment would lead to the control of fascinating physical phenomena emerging from laser-matter interaction. The present work represents a significant step towards such a control by manipulating the nonlinear optical response of the gas medium. This is accomplished by shaping an intense laser pulse experiencing filamentation at the subcycle level with a relatively weak (...) third-harmonic radiation. The control results from quantum interference between a single- and a two-color (mixing the fundamental frequency with its third-harmonic) ionization channel. This mechanism, which depends on the relative phase between the two electric fields, is responsible for wide refractive index modifications in relation with significant enhancement or suppression of the ionization rate. As a first application, we demonstrate the production and control of an axially modulated plasma channel. [ABSTRACT FROM AUTHOR]

Field-free molecular alignment has been used in order to track the collisional relaxation occurring in a molecular gas. CO2 molecules were initially irradiated by a short linearly polarized laser pulse resulting in the increase of their rotational energy. The evolution of the subsequent ultrafast relaxation process was optically probed after irradiating the sample with a second, weaker, short pulse leading to the alignment of the preheated molecules. Using classical molecular dynamic simulations, we were able to quantitatively reproduce the experimental shapes and amplitudes of the recorded revival transients for a time interval extending from 25 to 500 ps until thermalization of the gas sample is reached. [ABSTRACT FROM AUTHOR]

The analysis of negative birefringence optically induced in major air components [Loriot et al., Opt. Express 17, 13429 (2009); Loriot et al., Opt. Express 18, 3011 (2010)] is revisited in light of the recently reported plasma grating-induced phase-shift effect predicted for strong-field pump-probe experiments [Wahlstrand and Milchberg, Opt. Lett. 36, 3822 (2011)]. The nonlinear birefringence induced by a short and intense laser pulse in argon is measured by femtosecond time-resolved polarimetry. The experiments are performed with degenerate colors, where the pump and probe beam share the same spectrum, or with two different colors and nonoverlapping spectra. The interpretation of the experimental results is substantiated using a numerical 3D+1 model accounting for nonlinear propagation effects, cross-beam geometry of the interacting laser pulses, and detection technique. The model also includes the ionization rate of argon and high-order Kerr indices introduced by Loriot et al., enabling one to assess the contribution of both terms to the observed effect. The results show that the ionization-induced phase shift has a minor contribution compared to the high-order Kerr effect formerly introduced, the latter allowing a reasonably good reproduction of the experimental data for the present conditions. [ABSTRACT FROM AUTHOR]

We experimentally investigate odd-order harmonic generation in molecular gases produced by circularly polarized laser fields. While forbidden in isotropic medium, this effect is allowed by symmetry breaking resulting from nonadiabatic laser-induced molecular alignment. The demonstration is provided by generating the third harmonic in CO2 molecules. Attractive properties and challenging potential applications are discussed in the context of higher-order harmonic generation. [ABSTRACT FROM AUTHOR]

A method based on optical interferences for measuring optical nonlinearities is presented. In a proof-of-principle experiment, the technique is applied to the experimental determination of the intensity dependence of the photoionization process. It is shown that it can also be used to control and optimize the nonlinear process itself at constant input energy. The presented strategy leads to enhancements that can reach several orders of magnitude for highly nonlinear processes. [ABSTRACT FROM AUTHOR]

We report the experimental study of field-free molecular alignment in C02 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. [ABSTRACT FROM AUTHOR]

Permanent planar alignment of gas-phase linear molecules is achieved by a pair of delayed perpendicularly polarized short laser pulses. The experiment is performed in a supersonic jet, ensuring a relatively high number density of molecules with moderately low rotational temperature. The effect is optically probed on a femtosecond time scale by the use of a third short pulse, enabling a time-resolved birefringence detection performed successively in two perpendicular planes of the laboratory frame. The technique allows for an unambiguous estimation of the molecular planar delocalization produced within the polarization plane of the pulse pair after the turn-off of the field. The measurements are supported by numerical simulations which lead to the quantification of the observed effect and provide more physical insights into the phenomenon. [ABSTRACT FROM AUTHOR]

In this article, we derive the general exact solution of the modulation instability gain. The solution described here is valid for 1-D, 2-D, and 3-D cases considering any temporal response function of the medium and with possible higher order Kerr nonlinearities. In particular, we show that the gain induced by modulation instability is initial condition dependent, while the usual calculations do not lead to such a dependence. Applications for current and high-interest nonlinear propagation problems, such as 1-D optical fiber propagation with delayed Raman response and 2-D filamentation in gases, are investigated in detail. More specifically, we demonstrate that the 2-D model of filamentation based on the balance between higher order Kerr terms leads to a modulation instability window. The impact of both self-steepening and space-time defocusing effects is also highlighted. Finally, we discuss the influence of the finite-time response of the different order electronic Kerr effects on the growth of the expected modulation instability bands. [ABSTRACT FROM AUTHOR]

We present an observation of the rotational Doppler shift in the frequency of optical harmonic generated in fast rotating molecules. Conservation of energy and angular momentum in the light-molecule interaction suggests four different kinds of shifts depending on the mutual handedness of the circularly polarized fundamental and harmonic fields, as well as the handedness of the molecular rotation. All four types of the frequency shifts were observed in our experiments on third-harmonic generation in a gas of fast spinning O2 molecules. [ABSTRACT FROM AUTHOR]

We investigate experimentally and numerically the field-free orientation of the symmetric-top molecule of methyl iodide at high temperatures using a terahertz radiation generated by a plasma induced by a two-color laser beam. The degree of orientation is measured from the free-induction decay emitted by the sample. The observed experimental signal is reproduced with good accuracy by numerical simulations. [ABSTRACT FROM AUTHOR]

We report the observation of fractional echoes in a double-pulse excited nonlinear system. Unlike standard echoes, which appear periodically at delays which are integer multiples of the delay between the two exciting pulses, the fractional echoes appear at rational fractions of this delay. We discuss the mechanism leading to this phenomenon, and provide experimental demonstration of fractional echoes by measuring third harmonic generation in a thermal gas of CO2 molecules excited by a pair of femtosecond laser pulses. [ABSTRACT FROM AUTHOR]