Diagnosis of Multiple Gases Separated from Transformer Oil Using Cavity-Enhanced Raman Spectroscopy

The Raman signal of gas molecules is very weak due to its small scattering cross section. Here, a near-confocal cavity-enhanced Raman detection system is demonstrated. In the cavity, a high power light of 9W is achieved by using a cw 200mW 532nm laser, which greatly enhances the detection sensitivity of gas species. A photomultiplier tube connected to a spectrometer is used as the detection system. The Raman spectra of the mixed gases separated from transformer oil has been observed. The relationship of absolute Raman intensity and gas pressure is also obtained. To our knowledge, this is the first Raman system to detect the gases separated from transformer oil.     

Raman Spectrum Analysis on the Solid-Liquid Boundary Layer of BGO Crystal Growth

We study the Raman spectra of Bi4GeO12 crystal at different temperatures, as well as its melt. The structure characters of the single crystal, melt and growth solid-liquid boundary layer of BGO are investigated by their high-temperature Raman spectra for the first time. The rule of structure change of BGO crystal with increasing temperature is analysed. The results show that there exists [GeO4] polyhedral structure and Bi ion independently in BGO melt. The bridge bonds Bi-O-Bi and Bi-O-Ge appear in the crystal and at the boundary layer, but disappear in the melt. The structure of the growth solid-liquid boundary layer is similar to that of BGO crystal. In the melt, the long-range order structure of the crystal disappears. The thickness of the growth solid-liquid boundary layer of BGO crystal is about 50 μm.     

Raman diagnostics of heterogeneous chemical processes

The suitability of cw Raman spectroscopy for characterization of mass transport phenomena in connection with heterogeneous chemical processes is demonstrated for the example of the catalytic hydrogenation of acetylene. Temperature and concentration profiles in a model reactor were derived simultaneously from H₂ pure rotational lines and from the most intenseQ branches of C₂H₂ and of C₂H₄. The temperature dependence of the band contours has been considered; by careful calibration the systematic error of the derived concentration values is limited to less than 2%. The measurements allow the separation of the effects of thermal diffusion and of the reaction.     

Silver fractal networks for surface-enhanced Raman scattering substrates

Based on diffusion-limited aggregation process, a convenient nanotechnique is demonstrated to obtain large silver fractal networks for a surface-enhanced Raman scattering (SERS)-active substrate. The silver fractal networks are of high SERS enhancement factor, large dynamical range. The observed SERS efficiency can be explained in terms of strongly localized plasmon modes relative to the single particle plasmon resonance.     

In Situ Raman Spectroscopy Study on Dissociation of Methane at High Temperatures and at High Pressures

We investigate the stability and dissociation of methane, which is the most abundant organic molecule in the universe, using diamond anvil cell （DAC） with in situ Raman spectroscopy up to 903K and 21 GPa. At the temperatures of 793 and 723 K and the corresponding pressures of 16.15 and 20.30 GPa, methane dissociates to form carbon ＇soot＇ and heavier hydrocarbons involving C=C and C≡C bonds. However, if the pressure is not very high, methane remains stability up to the highest temperature of 903 K of the work. The four symmetric C-H bonds of methane split at high temperatures and at high pressures, and there is at least one phase transition of crystalline symmetry from face centred cubic （fcc） to hexagonal close packed （hcp） before dissociation.     

Discrete Dipole Approximation Aided Design Method for Nanostructure Arrays

A discrete dipole approximation （DDA） aided design method is proposed to determine the parameters of nanostructure arrays. The relationship between the thickness, period and extinction efficiency of nanostructure arrays for the given shape can be calculated using the DDA. Based on the calculated curves, the main parameters of the nanostructure arrays such as thickness and period can be determined. Using this aided method, a rhombic sliver nanostructure array is designed with the determinant parameters of thickness （40 nm） and period （440 nm）. We further fabricate the rhombic sliver nanostructure arrays and testify the character of the extinction spectra. The obtained extinction spectra is within the visible range and the full width at half maximum is 99nm, as is expected.     

Interferometric coherent Raman spectroscopy with incoherent light: Some applications

The theory of Interferometric Coherent Raman Scattering (ICRS) using twin broadband fields has recently been extended to include both resonant and nonresonant contributions to the signal. It is reviewed here. Valid for both ICARS and ICSRS, it is applied to new ICARS experiments on neat toluene and on alcoholic solutions of a methine dye. In each case the damped Rabi Detuning Oscillations (RDO), that are characteristic of ICRS, lead directly to vibrational frequencies and their dephasing times. The theory is also applied as a new way to determine the ratio of the nonresonant to resonant hyperpolarizabilities, accurately for toluene, and approximately for the methine dye solution whose (vibrationally) nonresonant contribution is negative.     

Structural changes in excited states and photoisomers determined by time-resolved Raman scattering

Molecules in the excited singlet and triplet state and photoproducts occuring after photoisomerization or proton transfer are investigated by time-resolved resonance Raman and resonance CARS spectroscopy with nanosecond time resolution. From the time-resolved spectra information on structures and bond strengths of these short-lived species and on their kinetics are obtained.     

A study on Raman scattering cross section of carbon tetrachloride at low concentrations

Using the technique of the liquid-core optical fiber (LCOF), we measured the Raman spectra intensities of CCl₄ in CS₂ at concentrations ranging from 90 to 0.8% in volume. According to the relative peak area ratios and peak height ratios, two kinds of RSCS values of the 459 cm^–1 CCl₄ band for each concentration, _{R A} and _{R H}, were obtained by using Onsagers model and the internal standard method. The two kinds of RSCS values and their changing tendencies are in agreement with each other. It is also found that both _{R A} and _{R H} increase with decreasing solution concentration. When the concentration is higher than 4%, both of them are flat and linear with the volume concentration of CCl₄ in CS₂. When the concentration is lower than 4%, they increase rapidly with decreasing concentration. At 0.8% both RSCS values increase to ten times that of the 459 cm^–1 band of CCl₄ in pure liquid. A tentative theory exploration for the experimental results is given, which introduces dispersion forces into Onsagers model. Additionally, some likely influencing factors on experimental results, such as numerical aperture (NA) of LCOF and baseline subtraction, were researched.     

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.     

Raman diagnostics of CVD systems: Determination of local temperatures

The suitability of local temperature measurements by cw Raman spectroscopy for the CH₄/H₂ CVD system has been established. The temperature profiles in a model reactor were derived from H₂ pure rotational lines and from hot bands of thev ₁ vibrational band of CH₄. Experimental results are presented for substrate temperatures of 773 K and of 1473 K. High accuracy of measurement and excellent agreement with theoretical solutions for the temperature field within the reactor were found.     

System for precise measurement in inverse Raman spectroscopy

A system for precise measurement was developed for inverse Raman spectroscopy, using a cw argon laser and a pulsed dye laser pumped by a YAG laser. The frequency accuracy was assured by monitoring the frequency of both of the lasers with an iodine fluorescence cell or a Fabry-Perot etalon dynamically calibrated by a stabilized HeNe laser. The spectrometer system employed a digitally controlled mechanism to handle the complicated measurement procedure and hence to reduce the overall measurement time. The performance of the system was tested by measuring the CH₄ v₁ lines and the H₂ Q(1) line.     

Solvent solute interactions probed by picosecond transient Raman spectroscopy:S 1 1,4-diphenyl-1,3-butadiene in the linear alkanes

We present theS ₁ Raman spectra of 1,4-DiPhenyl-1,3-Butadiene (DPB) in a series of linear alkanes (pentane, hexane, heptane, octane, decane, and dodecane). Bands assignable to both the 1¹ B _u and 2¹ A _g states are present, suggesting that the state we are observing in solution is a mixed state with both 1¹ B _u and 2¹ A _g character. The relative intensities of several bands associated with C_oC_o stretching motions in the 2¹ A _g and 1¹ B _u states change systematically through the solvent series. The relative intensity changes reflect a changing distribution ofs-trans conformers inS ₁ DPB as the solvent is varied. We suggest that the distribution ofs-trans conformers inS ₁ DPB controls the nature of the mixing between the 2¹ A _g and 1¹ B _u states and that the distribution of conformers is controlled by the solvent viscosity. Changes in the peak position and bandwidth of the phenyl C=C stretch with delay reflect vibrational relaxation processes inS ¹ DPB. We observe anomolous behavior in pentane that we attribute to the effect of the solvent structure on the ability of DPB to exchange energy with pentane.     

Raman spectroscopy of infrared multiple-photon excited molecules

The method of spectroscopy of spontaneous Raman scattering (RS) with time resolution has been applied for the first time to diagnose the process of multiphoton ir molecular excitation (MPE). Some aspects of RS diagnostics of MPE processes are being analyzed. It has been shown experimentally on SF₆ and CF₃I molecules that it is possible to study such important characteristics of excitation process as the fraction of molecules involved in the process of excitation, vibrational energy distribution of molecules, stochastization of inner molecular energy.     

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.     

Determination of resonance Raman scattering cross sections of laser dyes using a traveling-wave amplifier

Received: 1 April 1998/Revised version: 20 April 1998     

Spectrum Simulation of Li-Like Aluminium Plasma

X-ray emission spectra for L-shell of Li-like aluminium ions are simulated by using the flexible atomic code based on the collisional radiative model. Atomic processes including radiative recombination, dielectronic recombination, collisional ionization and resonance excitation from the neighbouring ion （Al^9＋ and Al^11＋ ） charge states of the target ion （Al^10＋） are considered in the model. In addition, the contributions of different atomic processes to the x-ray spectrum are analysed. The results show that dielectronic recombination, radiative recombination, collisional ionization and resonance excitation, other than direct collisional excitation, are very important processes.     

Soft X-ray gain in hydrogenic ions: Line pumping,ionization pumping and transient excitation

In a plasma containing hydrogenic ions which is irradiated by appropriate X-ray radiation, gain on the n=3n=2 (Balmer-) transition can be generated. Depending on the spectrum of the pump radiation, there are two different ways of obtaining the inversion: direct excitation of the upper laser level by line radiation, or ionization of the hydrogenic ions by broad band radiation with subsequent population of the upper level by recombination.In the first part of this paper, we present numerical calculations which compare the two pumping methods. The gain is evaluated under idealized conditions, i.e., with a pump spectrum containing only radiation useful for generating an inversion and under more realistic conditions, with a pump spectrum containing radiation at other frequencies as well.In the second part of the paper the possibility of obtaining Balmer- gain in hydrogenic ions under highly transient conditions is investigated, assuming a pump pulse with a duration comparable to the decay time of the lower laser level. Considerable gain is predicted even for a black-body pump spectrum.     

Space- and time-resolved investigation of gain lines in Na-like copper

We report space- and time-resolved measurements of the gain coefficient for four gain lines in sodium-like copper. The lines investigated include the twon = 1 transitions 5g–4f and 5f–4d at 11.1 nm and 10.3nm and the twon = 2 transitions 6g–4f and 6f–4d at 7.2 nm, and 6.9 nm. The investigations were carried out for four irradiation intensities from 4 × 10¹² W/cm² to 3 × 10¹³ W/cm² using the Asterix IV high-power iodine laser at Garching (wavelength 1.315 µm, pulse duration 450 ps).The main results may be summarized as follows: On varying the laser intensity it was found that the highest values of the gain could be seen at an irradiation of 8 × 10¹² W/cm². For then = 1 lines the spatial maximum of the gain occurred at a distance of 300 µm from the target, and for then = 2 lines at 200 µm. The temporal gain maximum occurred at a time of 1.8 ns after the pulse maximum. The gain values range up to 2.6 cm^–1.Dedicated to the memory of the late Prof. Shi-shen Chen, who contributed to the early phase of this work     

Rapid and quantitative detection of ethanol proportion in ethanol-gasoline mixtures by Raman spectroscopy

The paper reports a Raman spectral in-situ detection technique to measure the ethanol proportion in an ethanol-gasoline mixture rapidly and quantitatively. The transformed information of Raman spectra for different mixtures of gasoline and ethanol have been observed and recorded. The relative intensity ratio of two typical Raman peaks for ethanol and gasoline satisfies a linear relation with a variation in volume ratio which could be used as a quantitative measure. This method, compared with the direct intensity measurement of a Raman peak, may not only eliminate the random measurement error due to dark current, but also confirm the accurate the proportion of ethanol and gasoline.