Raman-induced Polarization Spectroscopy Research Articles

Field‐free molecular alignment of CO2 mixtures in presence of collisional relaxation

AbstractThe 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.

Measurement and calculation of the Q‐branch spectrum of nitrogen using inverse Raman spectroscopy and cw Raman‐induced polarization spectroscopy

AbstractThe techniques of inverse Raman spectroscopy, Raman‐induced polarization spectroscopy (RIPS), and optical heterodyne RIPS (OHD‐RIPS) are compared by probing the Q‐branch of the nitrogen molecule. The signal is measured employing either a photomultiplier tube (low background level–RIPS) or a photodetector (high background level–IRS and OHD‐RIPS). The measurements are performed using atmospheric mixtures of N2Ar with concentrations varying from 0 to 79% N2. This strategy permits estimation of detection limits using the different techniques. Pump and probe energy levels are varied independently to study signal dependence on laser irradiance. A theoretical treatment is presented on the basis of the Raman susceptibility equations, which permits the calculation of spectra for all three techniques. Calculated Q‐branch spectra are compared with the measured spectra for the interactions of a linearly polarized probe beam with a linearly or circularly polarized pump beam. The polarizer angle in the detection path for OHD‐RIPS has a dramatic effect on the shape of the spectrum. The calculated and experimental OHD‐RIPS spectra are in good agreement over the entire range of investigated polarizer angles. Detection limits using these techniques are analyzed to suggest their applicability for measuring other species of importance in combustion and plasma systems. Copyright © 2007 John Wiley & Sons, Ltd.

Rotational Raman spectroscopy of ethylene using a femtosecond time-resolved pump-probe technique

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.

Femtosecond Raman time-resolved molecular spectroscopy

The applicability of several femtosecond time resolved non-linear coherent techniques such as Raman induced polarization spectroscopy (RIPS), degenerate four-wave mixing (DFWM) and coherent anti-Stokes Raman spectroscopy (CARS) for molecular spectroscopy is presented. All methods rely on the initial coherent excitation of molecular states producing wavepackets, whose time evolution is then measured. In the case of RIPS and DFWM only pure rotational transitions are involved, whereas in CARS vibrational states can be excited. First the methodology of concentration and temperature measurements using RIPS in gas mixtures involving N 2, CO 2, O 2, and N 2O is shown. In addition some applications are given for the two closely related techniques DFWM and CARS. DFWM is suitable to extract the rotational constants of molecules to a high accuracy as is demonstrated by measurements on CO 2 and pyrimidine, which is a biological building block. CARS can be used to study higher order molecular constants and to sensitively determine temperature in, e.g., H 2 up to 2000 K. Finally, CARS is applied for the investigation of pressure dependent lineshape models, which are important for the temperature evaluation from spectroscopic data. To cite this article: B. Lavorel et al., C. R. Physique 5 (2004).

Temperature measurement in gas mixtures by femtosecond Raman‐induced polarization spectroscopy

AbstractThe potential of femtosecond Raman‐induced polarization spectroscopy (RIPS) for the simultaneous determination of temperature and concentrations was investigated. These measurements were related to the rotational time response of the molecular gas mixture, which was measured as a function of the pump–probe time delay. The change of the polarizability anisotropy with respect to the vibrational levels was taken into account. The results of temperature measurements in pure CO2 showed good agreement with the values obtained by a thermocouple. The RIPS technique was also applied to a CO2–N2 gas mixture to determine simultaneously temperatures and concentrations at temperatures up to 600 K. The accuracy and the feasibility of the method applied to highest temperatures are discussed. Copyright © 2003 John Wiley & Sons, Ltd.

Application of time-resolved spectroscopy to concentration measurements in gas mixtures

Concentration measurements using femtosecond Raman Induced Polarization Spectroscopy (RIPS) are performed in binary gas mixtures CO 2–N 2 and CO 2–N 2O at room temperature. The principle of these measurements is based on the nonlinear rotational time response of each molecular component of the mixture. The general form of this molecular response is a series of periodic transients with a period related to the rotational constant B e. The relative strength of the individual responses allows an accurate determination of the concentration. Two techniques are presented using either two pulses (one pump and one probe) or three pulses (two pumps and one probe).

Concentration measurements in molecular gas mixtures with a two-pump pulse femtosecond polarization spectroscopy technique

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 CO2–N2O 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.

Femtosecond polarization spectroscopy in molecular gas mixtures: Macroscopic interference and concentration measurements

Raman-induced polarization spectroscopy (RIPS) experiments combined with homodyne detection have been conducted with a femtosecond laser at room temperature and low pressure (p<2 atm) in CO2–N2 mixtures as well as in air (O2–N2 mixtures). Each molecule of the mixture produces its own time-dependent signal, measured as a series of recurring transients. Macroscopic interference is observed when transients of both molecules overlap in the time domain. This interference leads to a large modification of the signal, which is well reproduced by calculations. The total signal recorded in CO2–N2 or O2–N2 mixtures of known concentration is analyzed in order to measure the polarizability anisotropy ratio of the two components at 800 nm. The ratio measured in an O2–N2 (air) mixture is compared with values of previous works. The knowledge of the ratio for CO2/N2 allows us to determine the concentration of CO2–N2 unknown gas mixtures. The method is presented as a relevant technique for concentration measurements in the picosecond time domain. The accuracy of the measurements and the influence of macroscopic interference in the results are discussed. The present work can be easily extended to other gas mixtures and a broad concentration range.

Rotational coherence in gaseous molecules observed by the femtosecond Raman-induced polarization spectroscopy

Femtosecond coherence-burst signals due to the rotational coherences in the ground vibronic state of gaseous molecules, air, nitrogen, and oxygen, were observed by using the Raman-induced polarization spectroscopy. The frequencies of the induced coherences between the rotational levels are obtained from the Fourier spectrum of the coherence-burst signal. Calculations using a theory of coherent beats due to the Raman-induced rotational coherence reproduced well the observed signal and spectrum.

Analysis of femtosecond Raman-induced polarization spectroscopy (RIPS) in N2 and CO2 by fitting and scaling laws

Raman-induced polarization spectroscopy (RIPS) experiments were conducted at room temperature and pressures below 2 atm in pure N2 and CO2. Both homodyne and optically heterodyne detection were used. Calculations of the signal as a function of the pump–probe delay were performed taking into account the pulse duration of the laser, the rotational dependence of the dephasing rates and the small instantaneous electronic Kerr effect. The observed decay and shape of the rotational quantum beats are in good agreement with the calculations. Several sets of linewidth coefficients corresponding to the diagonal part of the collisional–rotational relaxation matrix were used. This matrix was choosen among available models derived from wavenumber domain studies (fitting and scaling laws). The energy-corrected sudden model derived from extensive wavenumber domain studies reproduces the experimental data well. A comparison between different models is discussed. Copyright © 2000 John Wiley & Sons, Ltd.

Tensor analysis of femtosecond Raman induced polarization spectroscopy: Application to the study of rotational coherence

In this paper we present a tensor analysis of Raman induced polarization spectroscopy (RIPS), a variation of time-resolved rotational coherence spectroscopy (RCS), developed in our laboratory to study coherent rotational motions in fluids over a range of densities. Based on the irreducible tensor algebra, the analysis separates the fundamental molecular response in RIPS from the dependence of the signal on geometric factors (i.e., laser polarizations). Explicit formulas for the femtosecond RIPS response of symmetric and asymmetric tops are derived. The results are in good agreement with experimental RIPS spectra of low pressure CO2 and O3. The tensor analysis is also generalized to other types of four wave mixing experiments, allowing detailed comparison of the various frequency- and time-domain pump–probe experiments.

Femtosecond Raman-induced polarization spectroscopy studies of rotational coherence in O 2, N 2 and CO 2

Using the femtosecond Raman-induced polarization spectroscopy (RIPS) technique, we have observed ground vibronic state rotational coherence in gaseous N 2, CO 2 and O 2. Both theoretical and experimental studies are reported. The theory is in excellent agreement with experiment. At earlier pump—probe delays, the time evolution of the polarization recurrence is well described by a rigid rotor model. Higher order spectroscopic constants are extracted from longer delay time spectra. The rotational relaxation constants for ground vibronic state N 2 and O 2 are also measured using the real-time technique.