Laser-induced gratings (LIGs) were generated in neat O2 at ambient temperature by photoexcitation of molecular species with nanosecond laser pulses at 1064 nm. LIGs were created at atmospheric pressure at different laser frequencies around that of the RR(11) transition of the O2(a 1ag, v = 1) <- O2(X 3Eg- , v = 0) absorption band of O2 monomers, and over a wide pressure range (1-130 bar) at a fixed frequency. LIGs were probed by diffracting continuous wave laser radiation. The proposed modeling indicates that at small delays (0-2 mu s) LIGs are formed, in addition to electrostriction, by "instantaneous" collisional exchange of energy between the molecular species and the environment resulting from radiation absorption by O2-O2 dimols and the subsequent breakdown of the laser-excited species, with collisional emergence of electronically and vibrationally excited O2 monomers. At large delays (20-1000 mu s), LIGs are created by the "slow" collisional relaxation of the produced excited monomers.

The present work is aimed at studying how spatially periodic modulations of the refractive index of the medium, i.e., laser-induced gratings (LIGs), generated in a gas mixture containing methane (CH4) by nanosecond pulses of resonant mid-infrared laser radiation, can be used to measure various gas parameters. It is investigated to what extent the temporal profiles of the LIG signals, recorded as the power of the diffracted by LIGs continuous wave probe radiation, are specific to the composition, pressure, and temperature of a selected buffer gas. This specificity is illustrated by the LIG signal profiles recorded in the experiments in different gas mixtures under various conditions. Experimental data show that large LIG signals can be obtained even in mixtures with CH4 concentrations as low as ∼100 parts per million due to the strong absorption of the excitation light and subsequent rapid, highly exothermic, and partner-dependent collisional energy exchange of the laser-excited molecules with the environment. These two factors ensure high LIG generation efficiency by a small number of CH4 molecules and high sensitivity of signal strength and profile to variations of gas parameters.

Pump-laser absorption of ammonia at 1064nm may generate a thermal contribution to a laser-induced grating (LIG) in ammonia-nitrogen gas mixtures. We demonstrate a simplified approach for simultaneous mixture composition and temperature evaluation in this system using the LIG technique.

Using two-color planar laser-induced fluorescence (PLIF) of OH hydroxyl radicals excited at two wavelengths and coherent anti-Stokes Raman scattering (CARS) spectroscopy of H2 molecules in a laboratory burner, we have measured spatial temperature distributions in the central section of a stationary atmospheric laminar flame of a methane–air mixture with partial preliminary mixing of the components. The results of a comparative analysis of the data on two-dimensional PLIF and point CARS thermometry of a spatially inhomogeneous flame are presented, and possibilities of complementary application of these methods for diagnostics are shown.

The stationary diffusion combustion of a boron nanoparticle suspension in isopropanol in oxygen coflow and the pulsed laser photolytic initiation of this combustion were studied. Experiments were carried out using a number of spectroscopic methods. Coherent anti-Stokes Raman scattering spectroscopy was used to determine the transverse distributions and concentrations of oxygen molecules diffusing into the fuel jet and the flame temperature variation at different distances from the edge of the burner nozzle due to the addition of boron nanoparticles into the fuel. The dimensions of the region of laser ignition of the combustible mixture were determined by laser-induced fluorescence spectroscopy of electronically excited O-2(*) molecules. Chemiluminescence spectroscopy of intermediate products of gas-phase reactions (OH* and BO2* radicals) from the ignition region made it possible to characterize the spatio-temporal dynamics of this process. The variations in the temperature field and ignition dynamics due to the addition of boron nanoparticles are explained based on an analysis of the obtained data. In particular, it is assumed that the characteristic rise in temperature in the region of the flame front is primarily due to an increase in the burning rate of the fuel with nanoparticles.

Multiparameter determination in the gas phase using the versatile laser-induced grating (LIG) technique is a challenging task due to interdependence of observables on multiple thermodynamic parameters. In C O 2-N 2 mixtures, simultaneous determination of species concentration and gas temperature can be achieved by using an additional C O 2 concentration-dependent contribution to the LIG signal, which appears if 1064 nm pump pulses are employed. This contribution can be attributed to a direct, quasi-resonant two-color four-wave mixing (TCFWM) of the pump and probe radiations in C O 2. A detailed study of the laser power and beam polarization, as well as mixture composition, pressure, and temperature dependencies of the TCFWM intensity relative to that of the LIG signal, allowed for the formulation of analytical relations enabling simultaneous mixture composition and temperature determination.

Концентрация молекул NO измерялась в диффузионном пламени атмосферного давления смеси изопропанола с азотом в кислороде как функция температуры пламени и концентрации О2 при ЛИФ (лазерно-индуцированной флуоресценции) диагностике.

Продемонстрировано определение временных и пространственных коэффициентов корреляции локальных колебаний температуры газа в турбулентном пламени с помощью спектроскопии когерентного антистоксова рассеяния света (КАРС).

The laser induced grating (LIG) technique is a powerful, versatile, non-intrusive measurement technique that employs the generation of a density grating by application of the strong electric fields of a laser. Studying the resulting energy release in the time domain gives access to a manifold of thermodynamic, fluiddynamic and material properties. Typically, the determination of different properties is mutually exclusive i.e., all other properties need to be known for the accurate determination of an unknown. We demonstrate concurrent determination of temperature and concentration in CO2-N2 mixtures from the same measurement exploiting the occurrence of a four-wave mixing signal contribution to the LIG signal.

A four-wave mixing signal contribution has been found in 1064 nm pumped laser-induced grating (LIG) experiments in CO2 that allows the otherwise challenging simultaneous determination of multiple thermodynamic parameter.

In this paper, temperature distribution in the diffusion flame during combustion of isopropanol or isopropanol with addition of boron nanoparticles in the amount of 0.5 wt % in oxygen was studied experimentally by using coherent anti-Stokes Raman scattering (CARS) spectroscopy. The addition of boron was found to result in an increase in temperature in the flame front by 150 K.

In surface-enhanced Raman-scattering experiments that use plasmonic nanostructures as substrates, the scattering spectrum contains a broad background usually associated with photoluminescence. This background exists above and below the frequency of the incident wave. The low-frequency part of this background is similar to the scattering spectrum of a plasmon nanoparticle, while the high-frequency part follows the Gibbs distribution. We develop a theory that explains experimentally observed features in both the high- and low-frequency parts of the photoluminescence spectrum from a unified point of view. We show that photoluminescence is attributed to the cascade Brillouin scattering of the incident wave by metal phonons under the plasmon resonance conditions. The theory is in good agreement with our measurements over the entire frequency range of the background.

The feasibility to determine the timescale of pulsations of “instantaneous” local temperatures in a turbulent flame at a microsecond time scale by using coherent anti-Stokes Raman scattering (CARS) spectroscopy is demonstrated for the first time to our knowledge. A laboratory laser measurement complex was utilized, based on two CARS-spectrometers employing synchronized pulse-repetitive lasers with 10 ns pulse duration. The system enabled to record, with high temporal resolution (in one single laser shot) and at a variable delay between two sequential shots following each other in pairs at a repetition rate of 10 Hz, series of CARS spectra of N2 molecules from a probe volume as small as 0.03×0.03×2 mm 3 . From the spectra, “instantaneous” temperatures at a given delay were derived. The obtained values enabled calculation of the correlation coefficient of temperature pulsations vs the delay. The results are presented for the series of 500 single-shot coupled measurements, at the delays in the range 1 μs – 0 ms, of local gas temperatures in a few points of an open turbulent partially premixed methane-air flame of a model burner with visually distinguishable stability of combustion. The average temperatures were between 1500 K and 1800 K. The measurements allowed temperature correlation times in the selected points of the flame to be derived.

Surface-enhanced Raman scattering (SERS) by biochemically relevant organic reporter molecules, spread out over a promising localized surface plasmon resonance (LSPR) structure of randomly arranged silver nanoscale particles (AgPs) of various dimensions and shape, was induced using tightly focused continuous wave (CW) 785-nm laser radiation, and the spectra were registered simultaneously in the anti-Stokes and Stokes spectral ranges. The spectra were recorded as a function of the excitation laser power density in the range of their profile reproducibility. The power density dependences of the line strength ratio for three respective pairs of vibrational lines of a thiolate of 2-nitrobenzoic acid (TNB) in SERS spectra were derived. Using these data, we specify and quantify the contributions responsible for the discrepancy between this ratio and that defined by the thermal equilibrium populations of the upper and lower vibrational levels corresponding to the Raman-active transitions. These contributions are the following: (i) the spectral profile of an LSPR contour, (ii) local heating of the reporter molecule/AgP conjugates by the 785-nm radiation, and (iii) optical (Raman) pumping of the upper vibrational levels of the transitions involved. The extraction of the latter contribution enabled us to estimate the cross-sections of the TNB/AgP conjugates Raman vibrational pumping by the radiation for each of the three vibrational modes.

The influence of triphenyl phosphate additives on the characteristics of flame propagation along a polymethyl methacrylate (PMMA) sample has been shown by laser-induced fluorescence and coherent anti-Stokes light scattering spectroscopy.

The decrease of the induction length during the ignition of the syngas/air mixture in the low-pressure flow reactor was obtained after the exposure of nitrogen and air to the glow discharge.

Optical diagnostic techniques have been employed at a combustion chamber test rig for investigation of the liquid fuel atomization and mixing with air, and of combustion of the fuel-air mixture formed in the flow. Certain characteristics of the kerosene spray and of the kerosene-air mixture combustion have been determined. An axisymmetric single-burner section with a pneumatic atomizer in the flame tube head was investigated at excess air ratios α = 0.9-1.5. The transverse spatial distributions of average kerosene droplet dimensions and the axial component of the average volume flux of the fuel have been measured in the axial cross-section of the mixture flow by using the Particle Shadow Velocimetry (PSV) technique. The results prove the efficiency of the investigated atomizer in the formation of finely dispersed kerosene- air flow with a regular structure which enables the stability of the combustion process. By employing Coherent Anti-Stokes Raman Scattering (CARS) during combustion of the produced kerosene-air mixture the statistical characteristics of "instantaneous" local gas temperature fluctuations in the flame and transverse distributions of average temperatures in the flame axial cross-section have been measured. The results demonstrate reasonable temporal stability of the obtained kerosene-air mixture combustion over the measurement period, and the possibility to describe the transverse flow structure and its modifications with the flow composition. The set of the complementary data provided by the PSV and CARS techniques at different excess air ratios characterizes the operation of the combustion chamber and expands the variety of possible flame tube head tests.

The experimental results are presented of implementation of a built-up laser-based diagnostic system at a test rig for investigations of the flame tube heads of aircraft gas turbine engines, both existing and being under development. At this test rig the processes of liquid fuel atomization and mixing with air by flame tube heads of model combustion chambers equipped with newly-developed atomizers, and of combustion of the fuel–air mixture formed in the flow have been studied. These studies enable development and optimization of advanced low-emission engines satisfying the established environmental requirements for the levels of polluting species in the exhaust gases. By using the particle shadow velocimetry technique with a pair of 10 μs delayed 5 ns laser pulses the transverse spatial distributions of average kerosene droplet dimensions, the axial components of their average velocity, and of the average volume flux of the fuel have been measured in the mixture flow axial cross-section. By employing coherent anti-Stokes Raman scattering with 10 ns pump laser pulses at 10 Hz repetition rate during combustion of the produced kerosene–air mixture the statistical characteristics of local ‘instantaneous’ gas temperature fluctuations in the flame, slow variations of these temperatures in time during the combustion process, and transverse distributions of average temperatures in the flame axial cross-section have been measured.