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We present the first quantum mechanical model of the collisional dissipation of the alignment of a gas of symmetric-top molecules (ethane) impulsively induced by a linearly polarized non-resonant laser field. The approach is based on use of the Bloch model and of the Markov and secular approximations in which the effects of collisions are taken into account through the state-to-state rates associated with exchanges among the various rotational states. These rates are constructed using the Energy Corrected Sudden (ECS) approximation with (a few) input parameters obtained independently from fits of the pressure-broadening coefficients of ethane absorption lines. Based on knowledge of the laser pulse characteristics and on these rates, the time-dependent equation driving the evolution of the density matrix during and after the laser pulse is solved and the time dependence of the so-called "alignment factor" is computed. Comparisons with measurements, free of any adjusted parameter, show that the proposed approach leads to good agreement with measurements. The analysis of the ECS state-to-state collisional rates demonstrates that, as in the case of linear molecules, collision-induced changes of the rotational angular momentum orientation are slower than those of its magnitude. This explains why the collisional decay of the permanent component of the alignment is significantly slower than that of the amplitudes of the transient revivals in both experimental and computed results. It is also shown that, since intermolecular forces within C2H6 colliding pairs weakly depend on rotations of the molecules around their C–C bond, the dissipation mechanism of the alignment in pure ethane is close to that involved in linear molecules. [ABSTRACT FROM AUTHOR]

We present the first theoretical study of collisional dissipation of the alignment of a symmetric-top molecule (ethane gas) impulsively induced by a linearly polarized non-resonant laser field. For this, Classical Molecular Dynamics Simulations (CMDSs) are carried out for an ensemble of C2H6 molecules based on knowledge of the laser-pulse characteristics and on an input intermolecular potential. These provide, for a given gas pressure and initial temperature, the orientations of all molecules at all times from which the alignment factor is directly obtained. Comparisons with measurements show that these CMDSs well predict the permanent alignment induced by the laser pulse and its decay with time but, as expected, fail in generating alignment revivals. However, it is shown that introducing a simple requantization procedure in the CMDS "creates" these revivals and that their predicted dissipation decay agrees very well with measured values. The calculations also confirm that, as for linear molecules, the permanent alignment of ethane decays more slowly than the transient revivals. The influence of the intermolecular potential is studied as well as that of the degree of freedom associated with the molecular rotation around the symmetry axis. This reveals that ethane practically behaves as a linear molecule because the intermolecular potential is only weakly sensitive to rotation around the C–C axis. [ABSTRACT FROM AUTHOR]

The consideration of the higher-order Kerr effect (HOKE) drastically improves the quantitative agreement between measured and simulated harmonic yield as well as intensity and electron density in laser filaments generated by pulses below a few hundreds of fs. In longer pulses, the plasma defocusing plays a much more important role.

Ab initio calculations of the shapes of pure CO2 infrared and Raman bands under (pressure) conditions for which line-mixing effects are important have been performed using requantized classical molecular dynamics simulations. This approach provides the autocorrelation functions of the dipole vector and isotropic polarizability whose Fourier-Laplace transforms yield the corresponding spectra. For that, the classical equations of dynamics are solved for each molecule among several millions treated as linear rigid rotors and interacting through an anisotropic intermolecular potential. Two of the approximations used in the previous studies have been corrected, allowing the consideration of line-mixing effects without use of any adjusted parameters. The comparisons between calculated and experimental spectra under various conditions of pressure and temperature demonstrate the quality of the theoretical model. This opens promising perspectives for first principle ab initio predictions of line-mixing effects in absorption and scattering spectra of various systems involving linear molecules. [ABSTRACT FROM AUTHOR]

Concentration measurement in molecular gas mixtures using a snapshot spatial imaging technique is reported. The approach consists of measuring the birefringence of the molecular sample when field-free alignment takes place, each molecular component producing a signal with an amplitude depending on the molecular density. The concentration measurement is obtained on a single-shot basis by probing the time-varying birefringence through femtosecond time-resolved optical polarigraphy (FTOP). The relevance of the method is assessed in air. [ABSTRACT FROM AUTHOR]

In this paper, we present measurements and modeling of femtosecond time resolved coherent anti-Stokes Raman spectroscopy (CARS) signal in H2–N2 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 H2/N2 system in the Dicke narrowing density regime. [ABSTRACT FROM AUTHOR]

Following the scheme recently proposed by Remacle and Levine [Phys. Rev. A 73, 033820 (2006)], we investigate the concrete implementation of a classical full adder on two electronic states (X 1A1 and C 1B2) of the SO2 molecule by optical pump-probe laser pulses using intuitive and counterintuitive (stimulated Raman adiabatic passage) excitation schemes. The resources needed for providing the inputs and reading out are discussed, as well as the conditions for achieving robustness in both the intuitive and counterintuitive pump-dump sequences. The fidelity of the scheme is analyzed with respect to experimental noise and two kinds of perturbations: The coupling to the neighboring rovibrational states and a finite rotational temperature that leads to a mixture for the initial state. It is shown that the logic processing of a full addition cycle can be realistically experimentally implemented on a picosecond time scale while the readout takes a few nanoseconds. [ABSTRACT FROM AUTHOR]

Femtosecond Raman-induced polarization spectroscopy (RIPS) was conducted at low pressure (250 mb at 295 K and 400 mb at 373 K) in ethylene. The temporal signal, resulting from the beating between pure rotational coherences, was measured with a heterodyne detection. The temporal traces were converted to the frequency domain using a Fourier transformation and then analyzed thanks to the D2hTDS software (http://www.u-bourgogne.fr/LPUB/shTDS.html) dedicated to X2Y4 molecules with D2h symmetry. The effective Hamiltonian was expanded up to order 2, allowing the determination of five parameters with an rms of 0.017 cm-1. Special care was taken in the precise modeling of intensities, taking into account all instrumental effects. Relative intensities were fitted (with an rms of 7.2%) and two polarizability operators were determined. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Studies the collisional relaxation among vibrational levels of the Fermi dyad of carbon dioxide mixed with Ar and He at room temperature with a double resonance experiment. Five-level model for the mixtures; Rotational relaxation; Depopulation rates; Rotational energy transfer.

The generalized energy corrected sudden model is used to calculate the Q branch of the anisotropic coherent anti-Stokes Raman scattering spectra corresponding to the ν1 and 2ν2 transitions of the Fermi dyad of CO2. The corresponding experimental spectra have been recorded for pressures leading to a consistent overlapping between the rotational components. This leads to a new experimental evaluation of the relaxation time of the second-order tensor associated with the rotational angular momentum. The value is in excellent agreement with that already obtained from the study of the infrared absorption Q branch, the Rayleigh scattering, and the viscomagnetic effect. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High resolution stimulated Raman spectra (SRS) of the ν1/2ν2 Fermi dyad of CO2 have been recorded in the 0.4–22. amagat density range at 700 K and in the 0.6–16 amagat density range at 900 K. The data have been successfully analyzed with a theoretical model taking into account both rotational line mixing through the energy corrected sudden-polynomial (ECS-P) law for the relaxation matrix and vibrational line broadening γv and line shifting δv. The two vibrational parameters have been accurately determined from fit to the experimental spectra. These new high temperature data together with previous data obtained at 295 and 500 K [B. Lavorel et al., J. Chem. Phys. 93, 2176 (1990); B. Lavorel et al., J. Chem. Phys. 93, 2185 (1990)] entirely determine the temperature dependence in the 295–900 K range of Raman band shapes of ν1 and 2ν2 in a large pressure range. These results will be very useful in CARS thermometry. A few coherent anti-Stokes Raman spectra (CARS) recorded in the 1100–1300 K domain are consistent with extrapolations of the 295–900 K data. [ABSTRACT FROM AUTHOR]

Self-, argon-, and helium-broadening coefficients have been measured for 13 lines in the 2ν2 Raman Q branch of CH4 using stimulated inverse Raman spectroscopy. The linewidths clearly show the symmetry-state dependence characteristic of pressure broadening, and inelastic processes in general, involving spherical-top molecules. Pressure-induced line shifts have also been measured for these features in pure methane. The pressure-shift coefficients do not display the symmetry-state dependence found for the linewidths. By applying the Rosenkranz perturbation treatment to a pair of collisionally mixed lines, we have been able to obtain an estimate of individual state-to-state contributions to the overall linewidth. [ABSTRACT FROM AUTHOR]

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 dispersive component. Such a procedure allows us to take into account the velocity changing collision’s effects not only on isolated lines (the well-known Dicke narrowing) but also on the line couplings resulting from collisionally induced rotational energy transfers. The present analysis permits an accurate description of the observed modifications on the SRS profile of the ν1 band of CO2 (1388 cm-1) as a function of density. The straightforward extension to other spectroscopies (linear and nonlinear) is suggested. [ABSTRACT FROM AUTHOR]

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 concomitant predominance of the vibrational dephasing. [ABSTRACT FROM AUTHOR]

Direct connection between collisional Raman cross sections and state-to-state rotational ones permits, within the sudden approximation, to evidence drastic decouplings from isotropic to anisotropic lines. These decouplings are consistent with the results obtained by using the strong collision model. Convenient energy corrections to the sudden approximation are used for N2 to calculate the resulting effects on the isotropic and anisotropic Q-branch profiles in a large density range (several hundred bars). The comparison of the calculations with experimental coherent anti-Stokes Raman scattering (CARS) and stimulated Raman (SRS) spectra of nitrogen exhibits a good agreement. Such effects could have applications in optical diagnostics. [ABSTRACT FROM AUTHOR]

The influence of CO2 and H2O on the rotational relaxation processes of N2 in ternary mixtures N2–CO2–H2O is investigated. The efficiency of these perturbers is responsible for significant modifications of the state-to-state relaxation rates and broadening coefficients. Flame data are well reproduced by taking into account these modifications. The role of these minor species in the determination of temperatures in premixed flames is analyzed. The present relaxation model allows us to understand why the discrepancy between observed and calculated coherent anti-Stokes Raman spectroscopy (CARS) spectra in flames is sometimes resolved by empirically adding a dephasing component to pure nitrogen linewidths. Moreover, this model improves the accuracy of CARS temperature measurements. [ABSTRACT FROM AUTHOR]

Accurate values of linewidth and line shift in the isotropic vibrational Raman spectrum of compressed liquid N2 have been obtained by using inverse Raman spectroscopy. Experiments have been performed for eight isotherms, from the normal boiling point to the critical point temperatures of N2, the number density varying between the value on the coexistence line, and the maximum value of 2.1×1022 cm-3. Minima of the linewidth have been observed above 86 K, showing the increasing influence of vibration–rotation coupling (motional narrowing) competing with the broadening due to pure vibrational dephasing. Moreover, for the first time, maxima of the red line shifts have been found, exhibiting increasing values as temperature increases. As far as the linewidth is concerned, the comparison with two existing models shows that negative interference of the attractive and repulsive forces must be taken into account for obtaining reasonable agreement between the experimental and theoretical parameters. The experimental density dependence of the linewidth can be described well enough with the Schweizer–Chandler model by taking only into account a soft-core, hard-sphere approximation for the calculation of the radial distribution functions. In the case of the frequency shifts, there is only qualitative agreement between the experimental and theoretical density dependences for the effective repulsive contributions calculated within the framework of the Schweizer–Chandler model. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The pure rotational Raman linewidths of the symmetric‐top molecule C2H6 are determined from a time‐resolved femtosecond experiment at room temperature and moderate pressure in a pump‐probe scheme. After excitation by a strong 100 fs‐duration nonresonant near infrared pump pulse, the molecular sample exhibits periodic post‐pulse alignment revivals that produce an anisotropic angular distribution of the molecular axis. Such molecular motion induces in turn a time‐dependent optical birefringence, which can be probed using a weak time‐delayed probe pulse. The Fourier transform of the temporal signal, recorded over a wide time delay range (several hundred picoseconds) for high frequency resolution, is directly related to the rotational Raman spectrum of ethane molecules. The Raman lines are analyzed with a frequency lineshape taking into account the collisional relaxation and the apparatus function. The linewidths are measured for rotational quantum numbers J=3 to J=30 in the pressure range 0.69–1.06 bar. A decrease of the collisional broadening coefficient is observed as J increases. These first measurements of pure rotational Raman linewidths of ethane are compared with previously reported infrared and far‐infrared values. [ABSTRACT FROM AUTHOR]

The field-free molecular alignment of symmetric-top molecules, ethane, induced by intense non-resonant linearly polarized femtosecond laser pulses is investigated experimentally in the presence of collisional relaxation. The dissipation dynamics of field-free molecular alignment are measured by the balanced detection of ultrafast molecular birefringence of ethane gas samples at high pressures. By separating the molecular alignment into the permanent alignment and the transient alignment, the decay time-constants of both components are quantified at the same pressure. It is observed that the permanent alignment always decays slower compared to the transient alignment within the measured pressure range. This demonstrates that the propensity of molecules to conserve the orientation of angular momentum during collisions, previously observed for linear species, is also applicable to symmetric-top molecules. The results of this work provide valuable information for further theoretical understanding of collisional relaxation within nonlinear polyatomic molecules, which are expected to present interesting and nontrivial features due to an extra rotational degree of freedom. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Time-resolved detection of field-free molecular alignment is investigated by phase contrast. The technique based on a 4 f imager operating as an inverted Zernike spatial filter makes it possible to discriminate between 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 aligning pulse. The observations are supported by a semi-analytical model, from which the degree of alignment produced at the beam focus is estimated. [ABSTRACT FROM AUTHOR]

Abstract. The measurement of relative intensities in CO[sub 2] combination bands spectrum is performed using wavelength modulation spectroscopy (WMS) and a DFB (distributed feedback) diode laser operating at 1.6 mu m. The diode laser is stabilized with an external Fabry-Perot interferometer and absorption spectroscopy is performed in a multipass gas cell. A spectrum containing spectral lines of both [sup 13] CO[sub 2] and [sup 12] CO[sub 2] isotopic species is recorded. The variation of laser power during frequency scanning and the line shape are taken into account to accurately extract line intensities from experimental data. The isotopic concentration ratio is deduced from the intensity ratio. Both ratios am measured with an accuracy of about 0.5% in pure CO[sub 2]. [ABSTRACT FROM AUTHOR]

Deals with a study which examined the collisional narrowing induced in the stimulated Raman spectroscopy fundamental Q branch of oxygen by self-collisions and by collisions with water vapor. Information on the modeling of temperature and rotational dependencies of the Raman line broadening coefficients by fitting and scaling laws; Spectra of pure oxygen; Measurements of the stimulated Raman Q branch of pure oxygen.

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. [ABSTRACT FROM AUTHOR]

Recently, we have demonstrated the ability of the Raman-induced polarization spectroscopy (RIPS) technique to accurately determine concentration or polarizability anisotropy ratio in low-pressure binary molecular mixtures [E. Hertz, B. Lavorel, O. Faucher, and R. Chaux, J. Chem. Phys. 113, 6629 (2000)]. It has been also pointed out that macroscopic interference, occurring when two revivals associated to different molecules time overlap, can be used to achieve measurements with picosecond time resolution. The applicability of the technique is intrinsically limited to a concentration range where the signals of both molecules are of the same magnitude. In this paper, a two-pump pulse sequence with different intensities is used to overcome this limitation. The relative molecular responses are weighted by the relative laser pump intensities to give comparable signals. Furthermore, by tuning the time delay between the two-pump pulses, macroscopic interference can be produced regardless of the accidental coincidences between the two molecular temporal responses. The study is performed in a CO[sub 2]–N[sub 2]O gas mixture and the concentration is measured with and without macroscopic interference. Applications of the method in the field of noninvasive diagnostics of combustion media are envisaged. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quantum control over molecular alignment rephasing is experimentally investigated in gaseous CO[sub 2]. The control process is achieved by illuminating the medium with a pair of pump-pulses separated in time by approximately an integer value of T[sub 0]=1/8B[sub 0], where B[sub 0] is the rotational constant. Through a Raman-type process, each pulse alone produces rotational coherence leading to a periodic orientational anisotropy. It is the combination of the two pulses that yields to quantum interference, resulting in a modification of this anisotropy probed by a third delayed pulse. The effect is accurately analyzed for different time delays between the two pulses. A theoretical analysis supplies a clear understanding of the role played by the different rotational motions involved in the overall process. The relative orientation of the electric field vector for the two pulses is discussed in terms of an additional control parameter. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We show that recently discovered rotational echoes of molecules provide an efficient tool for studying collisional molecular dynamics in high-pressure gases. Our study demonstrates that rotational echoes enable the observation of extremely fast collisional dissipation, at timescales of the order of a few picoseconds, and possibly shorter. The decay of the rotational alignment echoes in CO2 gas and CO2-He mixture up to 50 bar was studied experimentally, delivering collision rates that are in good agreement with the theoretical expectations. The suggested measurement protocol may be used in other high-density media, and potentially in liquids. [ABSTRACT FROM AUTHOR]