High‐resolution stimulated Raman study of the first vibrational hot band of 14N2. Separate observation of the spectra of the ortho and para species.

We present high‐resolution stimulated Raman spectra of the first hot band of nitrogen, ¹⁴N₂, obtained under conditions of low sample pressure and temperature. The use of a Raman optical pumping system allowed us to transfer a significant amount of population to the v = 1 vibrational state, which made possible the observation of the aforementioned hot band with good S/N ratio without resorting to the use of high temperatures or pressures. This has resulted in a more precise and accurate determination of the peak positions than previously existing ones, which in turn has yielded improved values for the molecular constants obtained from the analysis. The rotational selectivity of the pumping process (only one rotational state is initially populated in v = 1), coupled with the negligible probability of nuclear spin interconversion in the time frame of the experiment, has allowed us to observe the hot band spectra of the ortho and para species of ¹⁴N₂ separately. Copyright © 2013 John Wiley & Sons, Ltd.     

CARS studies of vibrationally excited nitrogen at low pressures

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational temperature of microwave-excited nitrogen in a N₂–CO–He mixture. CARS spectra, originating from the N₂-vibrational levelsv=0 up tov=3, have been recorded by both narrowband scanning of the resonance region as well as by broadband OSA detection. For the microwave-excited N₂ molecules a vibrational temperatureT _v (N ₂ = (2130±110K) and a lower limit of detection forN ₂(v = 3) = 1.2 x 10¹⁵ cm^–3 was established. The CARS results were independently confirmed by simultaneously recorded and spectrally resolved CO infrared fluorescence studies.     

Raman band shapes and the dynamics of liquid N2O4

The Raman spectra of the totally symmetric A _g modes, v ₁, v ₂ and v ₃, of the N₂O₄ molecule have been measured in the liquid state at 262, 279 and 297 K. The vibrational and the rotational correlation functions are calculated. The long-time exponential decay of the rotational correlation functions of all the A _g modes reflects an asymptotic diffusional behaviour of molecular reorientation. The rotational relaxation rate is found to increase with increasing temperature. A marked point of inflection from the short time inertial correlation to the long time exponential decay appears at about 0·35 ps for the v ₂ mode. This is an indication of orientational rebound arising from the librational motion in a temporary solvent cage. The isotropic bandwidth increases in the order v ₁ < v ₂ < v ₃, which is also the order of decreasing vibrational frequency. The temperature dependence of the peak frequency and of the bandwidth are also found to increase in the same order. These observations are analysed qualitatively in terms of two models of vibrational dephasing which take into account the effect of vibrational anharmonicity.     

Utilization of rotational and vibrational temperatures for plasma diagnostics of high-pressure discharges

In the spectra of high-pressure discharges excited in molecular gases, very intensive molecular spectral bands may usually be observed. We may determine the rotational and vibrational temperatures without difficulty, however, the rotational and vibrational temperatures (T _r, T_v) do not offen equal to the temperature of neutral gas (T ₀) or to that of electrons (T _e). If the collision cross sections of electronic, atomic, and molecular excitation (deexcitation) are known, we may then calculate the dependence of the rotational and vibrational temperatures onT _e,T ₀,N _e and the pressure of the gas. The calculations have been performed for pure N₂ and for an Ar-N₂ mixture at atmospheric pressure. The computed graphs make it possible to determine some of the values 4T _e,T ₀,N _e if the temperaturesT _r andT _v are known.The author wishes to extend his thanks to Prof. V. Truneek for valuable comments and to Mr. A. Struka for the preparation of the diagrams.     

The application of CARS for temperature measurements in high pressure combustion systems

The determination of accurate temperatures from CARS N₂ Q-branch spectra in premixed flames is discussed for pressures up to 40 bar. The influence of collisional line narrowing in the CARS spectra is modelled by a MEG fitting law. It takes into account collisions of N₂ with CO₂ and H₂O. The analysis of the CARS data showed that the non-resonant background has an increasing influence on temperature with increasing pressure. Little influence on the quality of the fit between theory and experiment was found. Since there is a danger of residual systematic temperature deviations, which cannot be identified from the quality of the fit, spontaneous rotational Raman scattering is employed as an independent measuring technique.     

Coherent anti-Stokes Raman spectroscopy (CARS) of thev 3 band of methane in supersonic molecular beams

Supersonic molecular beams of methane are investigated in the expansion region using coherent anti-Stokes Raman scattering (CARS). Raman spectra of thev ₃ vibration with resolved rotational structure at low temperatures are reported. Comparison with calculated CARS spectra shows that the rotational distribution in the beam may be well described by a Boltzmann distribution. Temperatures are the same for all three nuclear spin modifications within the experimental error.     

Theoretical study of Raman scattering from pure liquids

A theory is presented to study the Raman spectra of liquids formed by diatomic molecules in the fundamental spectral region. This theory is based on the fact that the Hamiltonian of a liquid sample is invariant under the operations of the full N-symmetric group. It is found, in particular, that the isotropic Raman scattering is a partially coherent and, if the vibrational rotational correlation is neglected, the anisotropic Raman scattering is a totally incoherent process.     

H2 CARS thermometry in a fuel-rich,premixed, laminar CH4/air flame in the pressure range between 5 and 40 bar

To establish H₂ CARS thermometry at high pressure, accumulated H₂ Q-branch CARS spectra were recorded in the exhaust of a fuel-rich CH₄/air flame at pressures between 5 and 40 bar. Temperatures were deduced by fitting theoretical spectra to experimental data points. The Energy-Corrected Sudden (ECS) scaling law was employed to set up an empirical model for the calculation of H₂ linewidths in high-pressure hydrocarbon flames with H₂ as a minority species. Experimental H₂ CARS spectra could be simulated very accurately with this model. The evaluated temperatures agreed well with reference temperatures obtained by spontaneous rotational Raman scattering of N₂.     

Etude des mouvements moléculaires en milieu liquide à partir du profil des bandes de vibration dans l'infrarouge et des fonctions de corrélation

We have studied the fundamental bands of chloroform and bromoform, both pure and in solution in various solvents; also the first harmonics of v ₁, and a few other harmonics and combination bands. The correlation functions of v ₁ and v ₂ and band moments of v ₁ have been calculated. The comparison of our results with those obtained in microwave and far IR and Raman spectra offers an opportunity to discern, in widths and correlation functions, what can be attributed to vibration and what originates in rotational diffusion. Our results are interpreted with the assumption that rotational diffusion is produced by small angles jumps and that the vibrational effect is very important. The v ₁ band shows an additional widening not accounted for in existing theories.     

Efficiently generated rotational and vibrational stimulated raman emission by means of a two-color laser beam

When a two-color laser beam is introduced into pressurized hydrogen, about 40 laser emission lines are generated from the ultraviolet to the visible regions. This phenomenon is ascribed to the stimulated Raman effect due to a combination ofJ=1 J=3 rotational andv=0 v=1 vibrational transitions. By introducing the two-color laser beam, the threshold for generation of the rotational line is substantially reduced. The present phenomenon is attributed to four-wave mixing, which allows efficient generation of higher-order rotational and vibrational Raman lines.     

Study of a non-equilibrium pulsed nanosecond discharge at atmospheric pressure using coherent anti-Stokes Raman scattering

Time-resolved CARS measurements of rotational and vibrational temperatures of nitrogen in nanosecond pulsed discharge at atmospheric pressure are reported. Experiment is first performed with a discharge in pure air where spatial and temporal evolution of temperature distribution is recorded by delaying the probe lasers relative to the discharge pulse in the range 10 ns to 1 ms. The experiments demonstrate that a strong vibrational non-equilibrium can be sustained in N₂ at 1 bar. The effect of different colliding partners on the vibrational relaxation of N₂ is studied for discharges in CH₄/air mixtures with different equivalence ratio. The observed temperature distributions suggest that thermal equilibrium is not fully achieved in this mixture. Effect of the discharge on the ignition of a premixed CH₄/air flame is also investigated for various equivalence ratio.     

Non‐degenerate second‐order scattering and difference combination bands in infrared‐pump/anti‐Stokes resonance Raman‐probe experiments

Non‐degenerate second‐order scattering due to interaction of infrared and ultraviolet pulses is observed in picosecond infrared‐pump/anti‐Stokes Raman‐probe experiments under electronic resonance conditions. We detected resonance hyper‐Rayleigh scattering at the sum frequency of the pulses as well as the corresponding frequency‐down‐shifted resonance hyper‐Raman lines. Nearly coinciding resonance hyper‐Raman and one‐photon resonance Raman spectra indicate conditions of A‐term resonance Raman scattering. Second‐order scattering is distinguished from transient anti‐Stokes Raman scattering of v = 1 to v = 0 transitions and v = 1 to v′ = 1 combination transitions by taking into account their different spectral and temporal behaviour. Separating these processes is essential for a proper analysis of transient vibrational populations. Copyright © 2007 John Wiley & Sons, Ltd.     

Low frequency Raman scattering of anatase titanium dioxide nanocrystals

TiO₂ nanoparticle of size 7.8 nm are synthesized by wet chemical route and characterized by low-frequency Raman scattering (LFRS), transmission electron microscopy (TEM) and X-ray diffraction. The low frequency peaks in the Raman spectra have been explained using the Lamb's theory that predicts the vibrational frequencies of a homogeneous elastic body of spherical shape. Our results show that the observed low-frequency Raman scattering originates from the spherical (l=0) and quadrupolar vibrations (l=2) of the spheriodal mode due to the confinement of acoustic vibrations in TiO₂ nanoparticles. In addition to the low-frequency peak due to the vibrational quadrupolar and spheriodal modes, a band is also observed, which is assigned to the Raman forbidden torsional l=2 mode originating from the near spherical shape of the TiO₂ nanoparticles. The size distribution is also obtained from LFRS, which is in good agreement with TEM.     

Rotational and Vibrational Temperature Measurements in a High‐Pressure Cylindrical Dielectric Barrier Discharge (C‐DBD)

The rotational (T_R) and vibrational (T_v) temperatures of N₂ molecules were measured in a high‐pressure cylindrical dielectric barrier discharge (C‐DBD) source in Ne with trace amounts (0.02 %) of N₂ and dry air excited by radio‐frequency (rf) power. Both T_R and T_v of the N₂ molecules in the C ³Π_u state were determined from an emission spectroscopic analysis the 2^nd positive system (C ³Π_u → B³Π_g). Gas temperatures were inferred from the measured rotational temperatures. As a function of pressure, the rotational temperature is essentially constant at about 360 K in the range from 200 Torr to 600 Torr (at 30W rf power) and increases slightly with increasing rf power at constant pressure. As one would expect, vibrational temperature measurements revealed significantly higher temperatures. The vibrational temperature decreases with pressure from 3030 K at 200 Torr to 2270 K at 600 Torr (at 30 W rf power). As a function of rf power, the vibrational temperature increases from 2520 K at 20 W to 2940 K at 60 W (at 400 Torr). Both T_R and T_v also show a dependence on the excitation frequency at the two frequencies that we studied, 400 kHz and 13.56 MHz. Adding trace amounts of air instead of N₂ to the Ne in the discharge resulted in higher T_R and T_v values and in a different pressure dependence of the rotational and vibrational temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Role of intramolecular interactions in Raman spectra of N2 and O2 molecules

The effect of vibration-rotation interactions and anharmonicities on the polarizabilities matrix elements, Raman scattering cross-sections, and depolarization degrees of N₂ and O₂ molecules for vibrational transitions v→v, v+1, v+2, v+3 have been investigated.     

单脉冲喇曼散射技术测量高压燃烧场组分浓度

介绍了自发喇曼散射技术的基本原理、实验方法及对高压燃烧场的测量结果。利用Nd:YAG激光的三倍频输出激发振动喇曼散射,在单脉冲条件下测量了高压模拟燃烧室内不同化学配比条件下以氢气-空气预混燃烧场为主要组分(N2,O2,H2O,H2等)的喇曼光谱,获得了主要组分浓度随燃烧时间、燃烧场压力的变化规律。实验中利用偏振技术有效地提高了信噪比。通过优化激光光束形状及光路设计避免了等离子体光谱对喇曼信号的干扰。     

Rotational CARS for simultaneous measurements of temperature and concentrations of N2, O2, CO, and CO2 demonstrated in a CO/air diffusion flame

Rotational coherent anti-Stokes Raman spectroscopy (CARS) has over the years demonstrated its strong potential to measure temperature and relative concentrations of major species in combustion. A recent work is the development and experimental validation of a CO₂ model for thermometry, in addition to our previous rotational CARS models for other molecules. In the present work, additional calibration measurements for relative CO₂/N₂ concentrations have been made in the temperature range 294-1246 K in standardized CO₂/N₂ mixtures. Following these calibration measurements, rotational CARS measurements were performed in a laminar CO/air diffusion flame stabilized on a Wolfhard-Parker burner. High-quality spectra were recorded from the fuel-rich region to the surrounding hot air in a lateral cross section of the flame. The spectra were evaluated to obtain simultaneous profiles of temperature and concentrations of all major species; N₂, O₂, CO, and CO₂. The potential for rotational CARS as a multi-species detection technique is discussed in relation to corresponding strategies for vibrational CARS.     

The Stimulated Raman Spectrum of Cyanogen

The Raman spectrum of cyanogen¹²C₂¹⁴N₂has been investigated at nearly Doppler resolution by means of the Stimulated Raman technique. The regions around theQbranches of the ν₁(2330 cm⁻¹) and ν₂(845 cm⁻¹) vibrations have been recorded. Besides the fundamentals, hot bands arising fromv₅= 1,v₅= 2, andv₄= 1 have been observed. The spectra have been analyzed, and rotational constants for the excited states have been obtained. Computer simulations of the Raman contours have been carried out as a test of the assignments.     

Resonance Raman investigation of the short-time dynamics of the ultraviolet photodissociation of bromoform

We report ultraviolet resonance Raman spectra of bromoform (CHBr₃) in cyclohexane solution. The resonance Raman spectra show significant intensity in the overtones of the nominal Br-C-Br symmetric bend (v ₆), the nominal H-C-Br asymmetric bend (v3), the nominal Br-C-Br symmetric stretch (v ₂) and the nominal Br-C-Br asymmetric stretch (v ₅) vibrational modes suggesting that the short-time photodissociation dynamics have noticeable multidimensional character. The lack of strong combination bands between several of the Franck-Condon active modes suggests that more than one electronic transition contribute to the resonance Raman spectra. We briefly discuss the ultraviolet short-time photodissociation dynamics of bromoform and the potential implications for the secondary photodissociation reactions of the initially formed CHBr₂ radical.     

The rotational Raman spectra and cross sections of H2O, D2O, and HDO

We report the experimental rotational Raman spectra of H₂O, and of a mixture of D₂O and HDO in the vapor phase at room temperature, and their interpretation in terms of rotational–vibrational energies, wavefunctions, and transition moments of the molecular polarizability. These transition moments are based on high-level ab initio calculations of the wavelength dependent polarizability surface, and on wavefunctions where the rotational–vibrational coupling is considered in detail. As a byproduct of this analysis several tables have been compiled including scattering strengths and assignments for individual rotational transitions of the three species. From these tables the rotational Raman spectra can be simulated over the range of temperatures up to 2000 K for H₂O, and up to 300 K for D₂O and HDO.