Evaluation method for Raman depolarization measurements including geometrical effects and polarization aberrations

In this article, we address the notoriously difficult problem to quantitatively link measured Raman depolarization values to theoretical polarizability tensor quantities, since quantum calculations do not incorporate experimental parameters. For this, we introduce a numerical model to calculate, for realistic experimental configurations, effective Raman line strength functions, Φ, which find their way into depolarization ratios, ρ. The model is based on interlinked integrations over the angles in the light collection path and a finite Raman source volume along the excitation laser beam. The model deals also with the conditional aperture parameters, associated with more than one optical component in the light collection path. Finally, we also can take into account polarization aberrations introduced by the sample cell windows. The procedure was fully tested for Raman depolarization spectra of selected hydrogen isotopologues. Distinct aspects affecting Raman depolarization data were validated, namely: (1) excitation polarization impurities; (2) extended Raman excitation volumes; (3) Raman light collection over finite solid angles; and (4) polarization aberrations introduced by optics in the light collection path. The correction of the experimental measurement data for the aforementioned effects resulted in depolarization ratios for the Q₁(J " ) Raman lines of H₂ and T₂, which mostly differed by less than 5% from those obtained by quantum‐calculations. Copyright © 2013 John Wiley & Sons, Ltd.     

Precise measurement of the depolarization ratio from photoacoustic Raman spectroscopy

A new method for the accurate determination of the Raman depolarization ratio is reported with an improved setup for photoacoustic Raman spectroscopy (PARS). The precise measurement is achieved by measuring the dependence of the acoustic signal intensity on the cross‐angle between the polarizations of two incident laser beams. We demonstrate this sensitive and simple method with several gaseous molecules, such as CH₄ and H₂. The measured results of depolarization ratios agree well with the theoretical values with an upper error limit of ± 0.005, which is comparable to that with polarization‐resolved CARS spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.     

Monitoring of all hydrogen isotopologues at tritium laboratory Karlsruhe using Raman spectroscopy

We have recorded Raman spectra for all hydrogen isotopologues, using a CW Nd:YVO₄ laser (5 W output power at 532 nm) and a high-throughput (f/1.8) spectrograph coupled to a Peltier-cooled (200 K) CCD-array detector (512 × 2048 pixels). A (static) gas cell was used in all measurements. We investigated (i) “pure” fillings of the homonuclear isotopologues H₂, D₂, and T₂; (ii) equilibrated binary fillings of H₂ + D₂, H₂ + T₂, and D₂ + T₂, thus providing the heteronuclear isotopologues HD, HT, and DT in a controlled manner; and (iii) general mixtures containing all isotopologues at varying concentration levels. Cell fillings within the total pressure range 13–985 mbar were studied, in order to determine the dynamic range of the Raman system and the detection limits for all isotopologues. Spectra were recorded for an accumulation period of 1000 s. The preliminary data evaluation was based on simple peak-height analysis of the ro-vibrational Q₁-branches, yielding 3σ measurement sensitivities of 5 × 10⁻³, 7 × 10⁻³, and 25 × 10⁻³ mbar for the tritium-containing isotopologues T₂, DT, and HT, respectively. These three isotopologues are the relevant ones for the KATRIN experiment and in the ITER fusion fuel cycle. While the measurement reported here were carried out with static-gas fillings, the cells are also ready for use with flowing-gas samples.     

High-resolution stimulated Raman spectroscopy of O2

High-resolution (0.002 cm^?1) stimulated Raman spectroscopy has been applied to the study of both normal and satellite Q branches of the fundamental vibrational band of molecular oxygen. Using a pulsed molecular free-expansion jet to adiabatically cool the oxygen sample, satellite Q branches at 1554 and 1558 cm^?1 that arise due to the splitting of the ³Σ_g ground state by spin-spin and spin-rotation interactions were completely resolved for the first time. Measured intensity ratios for the ${}^{\mathrm{<mtext>\Delta </mtext><mtext>N</mtext>}}\text{Δ}\text{J(J, N) =}{}^{\mathrm{Q}}\text{0(2, 1)}$ and ^QR(1, 1) lines, and for the ^QS(0, 1) and ^QP(2, 1) lines compare favorably with that for a coupling case intermediate between Hund's cases (a) and (b). Depolarization ratios, measured for a series of ^QQ-branch (unresolved) triplets, give a value 0.164 ± 0.004 for the depolarization ratio of the fundamental vibrational band.     

Polarized Fluorescence of Jet-Cooled indole and Carbazole and Their Complexes with Water

The spectra of the fluorescence excitation within the rotational contours of the bands of the pure electronic long-wavelength S ₀-S ₁ transitions of jet-cooled indole and carbazole molecules and their complexes with water are measured. For the carbazole-water complex, a contour with three maxima is registered, which is possibly related to the occurrence of two isomers, differing in a slight displacement of hydrogen between the nitrogen atom of the imine group of carbazole and the oxygen atom of the water molecule. The degrees of polarization of integral fluorescence upon excitation within the rotational contours of the S ₀-S ₁ electronic transition bands of the above molecules and their complexes with water are determined for the first time. The coincidence of the calculated (7.7%) and measured (7.3%) values of the degree of polarization upon excitation in the rotational Q branch of the ^b L ₁-A electronic transition of indole confirms the accepted intramolecular orientation of the transition dipole moment at an angle of 38.3° with respect to the principal axis of inertia A. Upon excitation of indole, its complex with water, and carbazole into the P and R branches, the measured and calculated degrees of polarization are also close to each other and amount to 2–3%. This confirms the occurrence of contributions to the fluorescence polarization due to the rotations of the indole molecules around the principal axes of inertia A and C.     

Raman spectrum and cross sections of n-C3F7I. Implications for a large-scale atomic iodine laser

The Raman spectrum of gaseous n-C₃F₇I has been recorded.Raman cross sections, depolarization ratios, and line widths were measured for the four strongest Raman-active modes. Using the measured values for the 280 cm^-1 mode, the stimulated-Raman gain for a large-scale iodine laser was calculated. The results indicate that losses due to stimulated-Raman scattering will be negligible for a fusion-driver iodine laser.     

First evidence for vibrational excitations of large atomic clusters in amorphous semiconductors

Raman and depolarization spectra of GeSe₂ alloyed with Se or As₂Se reveal a sharp vibrational doublet, only one component of which can be attributed to a vibrational excitation of the GeSe₂ local atomic cluster. The composition dependence of the intensity and polarization of the second feature identify it as an excitation involving a substantially larger cluster with high symmetry. The large cluster is a ring of Ge atoms inter-connected through single Se atoms     

The resonant secondary emission of the F center in alkali halides

The secondary radiation after resonant excitation of F center and its linear polarization P correlated to the polarization of resonant light have been measured for five alkali halides at low temperatures. In KC1, the spectrum of P over the whole Stokes region is divided into three successive regions, the depolarization range at the one-phonon Raman scattering, the near plateau range, and the depolarization range down to vanishing. The former two have common relevance to resonant energy and symmetries of coupled phonons. These relevances are interpreted adopting a configuration coordinate model for 2s- and 2p-like excited states.     

Vibrational Raman scattering temperature measurements

Experimental data and analyses are presented for the determination of gas temperature by measurements of vibrational Raman scattering intensity ratios of Stokes Q-branch fundamental bands. The method is demonstrated for two thermal equilibrium experiments: (1) CO₂ (a gas well-suited for use in multi-component mixtures near ambient temperatures) in a test cell, and (2) N₂ (a gas well-suited for use at elevated temperatures) in a flame. This method of temperature measurement is of particular value for non-thermal equilibrium conditions, for which vibrational excitation temperatures can be assigned to each pair of vibrational level corresponding to observable Raman bands.     

使用斯塔克诱导的绝热拉曼通道技术实现D2分子的高效相干布居转移

使用斯塔克诱导的绝热拉曼通道技术,成功地将分子束中的D₂分子从(v=0, J=0)转移至(v=1, J=0). 激发效率达到了75%.该技术将为交叉分子束和分子束-表面散射实验研究氢分子的振动激发对化学反应的影响提供一个独特的工具.     

Polarized Fluorescence of Jet‐Cooled Indole

The polarization of indole fluorescence on excitation in the P, Q, and R branches of a purely electron transition has been investigated. Relations are given to calculate the fluorescence polarization with arbitrary orientation of the dipole moments of transitions with absorption and emission and with excitation in the Q branch of the vibronic line. The coincidence of the measured (7.3%) and calculated (7.7%) values of the polarization degree confirms the reported intramolecular orientation of the dipole moment of transition. The measured fluorescence polarization degree on nonselective excitation is about 3%.     

Raman scattering from p-benzoquinone

The Raman spectrum of powdered p-benzoquinone and p-benzoquinone-d₄ has been investigated at liquid nitrogen temperature using Kr⁺-laser excitation. Several new lines were observed. To facilitate the assignment, depolarization ratios have been measured in the melt and in acetone solution. Overlapping peaks were resolved with the aid of a computer program.     

Differential Raman cross section of dimethyl sulfide

Biogenic sulfur compounds such as dimethyl sulfide (DMS) are important contributors to the global carbon cycle. The differential Raman cross section of DMS relative to the nitrogen fundamental, σ_DMS, has been measured at several excitation wavelengths in order to assess the applicability of Raman spectroscopy for the direct quantitative measurement of this compound. At 488 nm, σ_DMS for the ν₆ carbon–sulfur stretching mode was found to be 4.9 ± 1.6, while for the ν₂ carbon–hydrogen stretching mode it was 2.8 ± 0.9. Using, KrF laser excitation, values for σ_DMS could be measured simultaneously at two excitation wavelengths, 248.32 and 248.69 nm. The average values of σ_DMS for 248‐nm excitation based on measurements at these two excitation wavelengths were 3.5 ± 1.4 for the carbon–sulfur stretching mode and 4.6 ± 0.6 for the carbon–hydrogen stretching mode. The results indicate that no significant resonance enhancement of σ_DMS for either mode occurs, although they show some slight enhancement of the cross section for the ν₂ band (C H stretching mode). It was concluded that the measured values of σ_DMS are high enough to allow the quantitative detection of DMS at the millimolar level. Copyright © 2010 John Wiley & Sons, Ltd.     

Reconstruction of the Q band spectrum of the 1285-cm−1 transition in the CO2 molecule from measurements of a pulsed response

By using the method of nonstationary spectroscopy of coherent anti-Stokes Raman scattering, pulsed responses of the Q band of the vibrational-rotational transition in the CO₂ molecule at a frequency of 1285 cm^?1 are measured under conditions of broadening close to the Doppler type. The ratios of the amplitudes of Q-band components at known frequencies are obtained by fitting to the measured pulsed responses.     

Orientation of sp2 carbon nanoclusters in ion‐implanted polymethylmethacrylate as revealed by polarized Raman spectroscopy

Raman spectroscopy is widely used for the characterization of bonding type in carbon‐based materials, including carbonized surface layer in ion‐implanted polymers. Studies of the polarization properties of Raman scattering from amorphous carbonaceous materials, however, are very scarce. In this paper, we investigate the polarized Raman spectra of polymethylmethacrylate (PMMA) implanted with 50‐keV Si⁺ ions at fluences in the range 3.2 × 10¹⁴–1.0 × 10¹⁷ ions/cm² and for different visible excitation wavelengths. The spectra of the implanted samples are dominated by the D‐ and G‐bands of sp² carbon, which evidence strong carbonization of the ion‐modified layer. The multiwavelength excitation allowed us to resonantly probe the depolarization ratios for sp² clusters of different sizes. We established that the depolarization ratio ρ_G of the G‐band correlates with the sp² cluster size approaching the random orientation limit of 0.75 for the smallest clusters and a limiting value of 0.41 for the largest clusters. The experimental findings give evidence for a preferable orientation of the larger size clusters with their hexagonal planes perpendicular to the surface of the sample. A plausible explanation for such an arrangement is that the sp² clusters form tile‐like arrangements along the ion tracks. This finding may give clues for understanding of the strong transconductance of the ion‐modified layer, and open prospects for the application of polarized Raman spectroscopy as a characterization tool for surface morphology in ion‐implanted materials. Copyright © 2010 John Wiley & Sons, Ltd.     

Lyman transitions of high rovibrationally excited H2, HD and D2 molecules

Energies and probabilities of Lyman transitions of high rovibrationally excited H₂, HD and D₂ molecules have been measured and compared with calculations. The experimental results are obtained from laser-induced fluorescence spectra that have been recorded in the spectral range from 60 500 to 83 500 cm⁻¹, covering 2/3 of the hydrogen Lyman band system. The necessary vacuum-UV radiation is produced by stimulated anti-Stokes Raman scattering, providing a widely tunable radiation source with narrow spectral bandwidth to resolve single Lyman transitions. The highest internal energies of detected hydrogen isotopologues are close to the dissociation limit. This extends the available data base of Lyman transitions from and to higher rotational states (J > 10) of HD and D₂.     

Spectroscopic determination of the relative particle densities of H<Subscript>2</Subscript>, HD,and D<Subscript>2</Subscript> Molecules in non-equilibrium hydrogen–deuterium plasma: II. Experimental verification

The relative intensities of the Balmer series lines of hydrogen and deuterium atoms and the Q-branch lines in the Fulcher-α band system of the H₂, HD, and D₂ molecules were measured. These lines were emitted by non-equilibrium plasma surrounded by the cylindrical channel of an additional molybdenum electrode located between the cathode and anode of glow (in the deuterium with a minor hydrogen impurity) and arc (in a mixture of D₂, H₂, and Ne in comparable quantities) discharges at pressures of 6–8 Torr and current densities of 0.4 and 8.5–17 A/cm², respectively. The measured intensity ratios and gas temperature were used for the estimation of the relative particle densities of the H₂, HD, and D₂ molecules in the framework of the simple models of the excitation of atoms (models 1a and 1b for the high and low values of dissociation degrees, respectively) and the rovibronic levels of isotopic molecules (model 2). The results obtained by means of models 1a and 2 were in a significant contradiction, whereas the data obtained with the use of models 1b and 2 coincided within error bars. The good agreement between two spectroscopic techniques based on two independent theoretical models and two independent sets of experimental data showed that the techniques proposed are promising for the determination of the isotopic composition of molecules in lowpressure hydrogen-deuterium plasma at least in the case of a low dissociation degree of molecules.     

Two Dimensional Rotational Temperature Measurement by Multiline Laser Induced Fluorescence of Nitric Oxide in Combustion Flame

Two-dimensional rotational temperature measurement was performed in a stable combustion flame of premixed butane and oxygen using multiline laser induced fluorescence (LIF) of nitric oxide molecules. Multiple rotational absorption lines of A²∑⁺Π;X²II(0,0) Q₁ and Q₂ lines were excited by laser light around 226 nm, and the LIF signal was observed by an image-intensified digital camera. Temperature was determined through least squares fitting correlation between LIF intensity and excitation rotational quantum number for the Boltzmann distribution function. The measured LIF intensity was approximated by the Boltzmann distribution with good accuracy, and the temperature obtained was between 500 K and 1800 K for the test flame. The measuring error of the temperature was evaluated and found to be 80 K, which corresponded to 8% of the measured fluorescence intensity. The two-line LIF scheme was evaluated by different pairs of excitation lines (Q₁(31.5)/Q₁(16.5) and Q₁(18.5)/Q₁(16.5)) for comparison with the multiline LIF approach. Temperature which was obtained by two-line LIF scheme corresponded well with multiline LIF results for Q₁(31.5)/Q₁(16.5) excitation. However, for Q₁(18.5)/Q₁(16.5) excitation, the obtained temperature did not agree with the multiline LIF result because the population of rotational states J=18.5 and J=16.5 is similar at high temperatures. We found that two-line LIF temperature measurement was reliable when excitation lines were suitably selected.     

Vibrational Spectra and Assignments for 3,4-Dibromothiophene

Abstract

The infrared spectrum of 3,4-dibromothiophene has been studied from 4000 to 200 cm^?1. The Laser Raman spectrum has also been recorded and depolarization values have been measured. An assignment of the 21 fundamental vibrations is proposed based on group frequency correlations, Raman polarization data and comparison with the spectra of parent and some halogeno-substituted molecules.     

用CARS技术确定拉曼退偏比的一种数据处理新方法

Yuika等人利用偏振CARS技术可以准确地确定分子的拉曼退偏比.其方法是，首先对不同检偏角_d所对应CARS谱峰的频率分布进行数学模拟，然后由所得系数随检偏角φ_d的变化求得使CARS信号中共振项消失的偏振角φ^０_d，最后由消失条件ρ＝-1/(tanθtanφ⁰_d)求出退偏比ρ，θ为产生CARS光的Pump光与Stokes光偏振方向的夹角.本文提出的数据处理方法，即交点法.同Yuika等人处理数据的方法相比，交点法毋需关于谱峰频率分布的知识，做法也更为简便.