Photo-physical properties of anisole: temperature, pressure, and bath gas composition dependence of fluorescence spectra and lifetimes

Anisole is a promising candidate for use as fluorescent tracer for gas-phase imaging diagnostics. Its high-fluorescence quantum yield (FQY) and its large Stokes shift lead to improved signal intensity (up to 100 times stronger) compared with the often used toluene. Fluorescence spectra and effective fluorescence lifetimes of gaseous anisole were investigated after picosecond laser excitation at 266 nm as a function of temperature (296–977 K) and bath gas composition (varying amounts of N₂ and O₂) at total pressures in the range of 1–10 bar to provide spectroscopic data and FQY for applications, e.g., in in-cylinder measurements in internal combustion engines. Fluorescence spectra of anisole extend from roughly 270–360 nm with a peak close to 290 nm at 296 K. The spectra show a red-shift with increasing temperature (0.03 nm/K) and O₂ partial pressure (5 nm from N₂ to air). In the investigated temperature range and in pure N₂ at 1 bar total pressure the effective fluorescence lifetime drops with increasing temperature from 13.3 ± 0.5 to 0.05 ± 0.01 ns. Increasing the total pressure of N₂ leads to a small decrease of the lifetime at temperatures above 400 K (e.g., at 525 K from 4.2 ± 0.2 ns at 1 bar to 2.7 ± 0.2 ns at 10 bar). At constant temperature and in the presence of O₂ the lifetimes decrease significantly (e.g., at 296 K from 13.3 ± 0.5 ns in N₂ to 0.40 ± 0.02 ns in air), with this trend diminishing with increasing temperature (e.g., at 675 K from 1.02 ± 0.08 ns in N₂ to 0.25 ± 0.05 ns in air). A phenomenological model that predicts fluorescence lifetimes, i.e., relative quantum yields as a function of temperature, pressure, and O₂ concentration is presented. The photophysics of anisole is discussed in comparison with other aromatics.     

Laser-induced fluorescence determination of flame temperatures in comparison with CARS measurements

Temperature profiles in several premixed low pressure H₂/O₂/N₂ flames and in an atmospheric pressure CH₄/air flame were determined by laser-induced fluorescence (LIF) and by CARS experiments. In the LIF study, temperatures were derived from OH excitation spectra, CARS temperatures were deduced from N₂ Q-branch spectra. The present study is the first quantitative comparison of these two methods for temperature determination in flames burning at pressures up to 1 bar. The resulting temperatures showed good agreement.     

Fluorescence quantum yield of carbon dioxide for quantitative UV laser-induced fluorescence in high-pressure flames

The fluorescence quantum yield for ultraviolet laser-induced fluorescence of CO₂ is determined for selected excitation wavelengths in the range 215–250 nm. Wavelength-resolved laser-induced fluorescence (LIF) spectra of CO₂, NO, and O₂ are measured in the burned gases of a laminar CH₄/air flame (φ=0.9 and 1.1) at 20 bar with additional NO seeded into the flow. The fluorescence spectra are fit to determine the relative contribution of the three species to infer an estimate of fluorescence quantum yield for CO₂ that ranges from 2–8×10^?6 depending on temperature and excitation wavelength with an estimated uncertainty of ±0.5×10^?6. The CO₂ fluorescence signal increases linearly with gas pressure for flames with constant CO₂ mole fraction for the 10 to 60 bar range, indicating that collisional quenching is not an important contributor to the CO₂ fluorescence quantum yield. Spectral simulation calculations are used to choose two wavelengths for excitation of CO₂, 239.34 and 242.14 nm, which minimize interference from LIF of NO and O₂. Quantitative LIF images of CO₂ are demonstrated using these two excitation wavelengths and the measured fluorescence quantum yield.     

Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics

Two-photon laser-induced fluorescence (LIF) of ammonia (NH₃) with excitation of the C′-X transition at 304.8 nm and fluorescence detection in the 565 nm C′-A band has been investigated, targeting combustion diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH₃-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm². Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm², i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400–700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH₃-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH₃-detection in combustion diagnostics.     

Ruby fluorescence lifetime measurements for temperature determinations at high (p,T)

The lifetime of the ruby R₁ fluorescence line was measured as a function of pressure (up to about 20 GPa) and temperature (550 K) in an externally heated diamond anvil cell (DAC). At constant temperatures, the lifetime is increasing linearly with increasing pressure. The slope of the pressure dependence is constant up to a temperature of 450 K and it is decreasing at higher temperatures. At constant pressure, the lifetime is exponentially decreasing with increasing temperature. The (p, T)-dependence can be parametrized by the combination of a linear and an exponential function. This allows an accurate p, T-determination by the combination of fluorescence spectroscopy using Sm²⁺-doped strontium tetraborate and lifetime measurements of ruby, as the energy of the Sm²⁺ fluorescence is nearly temperature-independent.     

Coherence gating in optical heterodyne detection measurements of scattering and absorption in highly scattering media

Laser-Induced Fluorescence (LIF) from the S₁ state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O₂, and N₂ respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N₂ but decreased markedly for O₂. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.     

Fluorescence spectroscopy of naphthalene at high temperatures and pressures: implications for fuel-concentration measurements

Fluorescence of the S ₁→S ₀ transition of naphthalene vapour after laser excitation at 266 nm was studied in a heated cell. Experiments were carried out for temperature in the range 350–900 K, at pressure between 0.1 and 3.0 MPa and for oxygen molar fraction from 0 to 21%. The absorption cross section of naphthalene showed a non-monotonic dependence upon temperature, which may be attributed to the spectral structures present in the absorption spectrum of naphthalene. Under nitrogen atmosphere, naphthalene fluorescence bi-exponentially decreased by an order of magnitude as temperature increased, whereas it increased by about 10% with pressure. Strong influence of quenching by O₂ on naphthalene fluorescence was observed and Stern–Volmer plots were found to be linear for temperatures between 450 and 750 K. The dependence of naphthalene fluorescence on oxygen concentration suggests one to use this molecule for fuel-concentration measurements in turbulent flows.     

Effect of pressure and temperature on the wavelength shift of the fluorescence line of SrB 4O 7:Sm 2+ scale

The pressure shift of ⁷D ₀?⁵F ₀ fluorescence line of SrB ₄O ₇:Sm ²⁺ has been recalibrated at high temperature and high pressure, respectively. Combined with the high pressure and high-temperature experimental data given by previous study, a quantitative analysis of the temperature effect on pressure shift of the ⁷D ₀?⁵F ₀ fluorescence line has been performed. The results show that there is an overall negligible coupling effect of temperature and pressure on the wavelength shift of the ⁷D ₀?⁵F ₀ fluorescence line below 770 K and 35 GPa. But above 770 K, the temperature effect on pressure shift could not be ignored at least in a relatively high pressure range. A proposed calibration relation is recommended to predict more accurate pressures in high pressure and high-temperature experiments with SrB ₄O ₇:Sm ²⁺ as the pressure scale.     

Fluorescence spectroscopy of anisole at elevated temperatures and pressures

Laser-induced fluorescence of anisole as tracer of isooctane at an excitation wavelength of 266 nm was investigated for conditions relevant to rapid compression machine studies and for more general application of internal combustion engines regarding temperature, pressure, and ambient gas composition. An optically accessible high pressure and high temperature chamber was operated by using different ambient gases (Ar, N₂, CO₂, air, and gas mixtures). Fluorescence experiments were investigated at a large range of pressure and temperature (0.2–4 MPa and 473–823 K). Anisole fluorescence quantum yield decreases strongly with temperature for every considered ambient gas, due to efficient radiative mechanisms of intersystem crossing. Concerning the pressure effect, the fluorescence signal decreases with increasing pressure, because increasing the collisional rate leads to more important non-radiative collisional relaxation. The quenching effect is strongly efficient in oxygen, with a fluorescence evolution described by Stern–Volmer relation. The dependence of anisole fluorescence versus thermodynamic parameters suggests the use of this tracer for temperature imaging in specific conditions detailed in this paper. The calibration procedure for temperature measurements is established for the single-excitation wavelength and two-color detection technique.     

Effects of O2–CO2 polarization beating on femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of O2

Femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy has recently emerged as a promising laser-based temperature-measurement technique in flames. In fs-CARS, the broad spectral bandwidths of the pump and Stokes lasers permit the coupling of each ro-vibrational Raman transition via a large number of pump-Stokes photon pairs, creating a strong Raman coherence. However, the broad-bandwidth fs pulses also excite other molecular transitions that are in resonance. The polarization beating between these closely spaced Raman transitions can affect the coherence dephasing rate of the target molecule, making it difficult to extract accurate medium temperature. In a previous study our group investigated N₂/CO polarization beating in N₂ fs-CARS; in the present work we study O₂/CO₂ polarization beating in O₂ fs-CARS. O₂ fs-CARS can be particularly important for thermometry in non-air-breathing combustion in the absence of N₂. The effects of O₂/CO₂ polarization beating are investigated in the temperature range 300–900 K at atmospheric pressure and also at 300 K for pressures up to 10 bar. Unlike in the N₂/CO system, it was observed in the O₂/CO₂ system that the presence of CO₂ can significantly alter the time evolution of the Raman coherence and, hence, affect the measured temperature.     

Laser-induced fluorescence temperature determination in fuel–air mixtures without additional fluorescence tracers

A concept for temperature determination of fuel–air mixtures using Laser-Induced Fluorescence (LIF) is presented. For this purpose the fluorescence spectra of gasoline were measured after excitation by frequency quadrupled Nd:YAG laser light at 266 nm in a temperature range between 373 K and 448 K. Experiments were performed with colorless near-standard gasoline conforming to the Euro-super specifications. It is shown that the intensities of two fluorescence bands (290–302 nm and 332–344 nm) can be used to determine the temperature.     

Detection of trace nitric oxide concentrations using 1-D laser-induced fluorescence imaging

Spectrally resolved laser-induced fluorescence (LIF) with one-dimensional spatial imaging was investigated as a technique for detection of trace concentrations of nitric oxide (NO) in high-pressure flames. Experiments were performed in the burnt gases of premixed methane/argon/oxygen flames with seeded NO (15 to 50 ppm), pressures of 10 to 60 bar, and an equivalence ratio of 0.9. LIF signals were dispersed with a spectrometer and recorded on a 2-D intensified CCD array yielding both spectral resolution and 1-D spatial resolution. This method allows isolation of NO-LIF from interference signals due to alternative species (mainly hot O₂ and CO₂) while providing spatial resolution along the line of the excitation laser. A fast data analysis strategy was developed to enable pulse-by-pulse NO concentration measurements from these images. Statistical analyses as a function of laser energy of these single-shot data were used to determine the detection limits for NO concentration as well as the measurement precision. Extrapolating these results to pulse energies of ～?16 mJ/pulse yielded a predicted detection limit of ～?10 ppm for pressures up to 60 bar. Quantitative 1-D LIF measurements were performed in CH₄/air flames to validate capability for detection of nascent NO in flames at 10–60 bar.     

SrB4O7:Sm2+: an optical sensor reflecting non-hydrostatic pressure at high-temperature and/or high pressure in a diamond anvil cell

A systematic investigation on the fluorescent spectra of SrB₄O₇:Sm²⁺ was performed in detail at high-temperature up to 623?K and/or high pressure up to 23.2?GPa with different pressure-transmitting media (PTMs), respectively. Combined with experiment data of previous research, the change of the ⁷D₀–⁵F₀ line (0–0 line) full width at half maximum (FWHM) of SrB₄O₇:Sm²⁺ under different pressure environments was specifically discussed. The results indicate that the FWHM of 0–0 line is sensitive to the non-hydrostatic pressure environment in 2-propanol, and methanol and ethanol mixture (ME) PTMs at ambient temperature. The first-order and the second-order derivation of the temperature dependence of 0–0 line FWHM at ambient pressure are 1.48(±0.21)?×?10^?4?nm/K and 9.63(±0.63)?×?10^?7?nm²/K² below 623?K. The 0–0 line FWHM is also sensitive to the non-hydrostatic pressure environment in ME at high-temperature and high pressure simultaneous, the non-hydrostatic transition pressures are 9.6?GPa at 323?K, 11.0?GPa at 373?K, 14.4?GPa at 423?K, respectively. SrB₄O₇:Sm²⁺ is recommended as an optical sensor to reflect the change of pressure environment in liquid media at high-temperature and/or high pressure.     

Structural and electrical properties of lithium manganese oxide thin films grown by pulsed laser deposition

LiMn₂O₄ thin films were deposited by reactive pulsed laser deposition technique and studied the microstructural and electrical properties of the films. The LiMn₂O₄ thin films deposited in an oxygen partial pressure of 100 mTorr and at a substrate temperature of 573 K from a lithium rich target were found to be nearly stoichiometric. The films exhibited predominantly (111) orientation representing the cubic spinel structure with Fd3m symmetry. The intensity of (111) peak increased and a slight shift in the peak position was observed with the increase of substrate temperature. The lattice parameter increased from 8.117 to 8.2417 Å with the increase of substrate temperature from 573 to 873 K. The electrical conductivity of the films is observed to be a strong function of temperature. The evaluated activation energy for the films deposited at 873 K is 0.64 eV.     

XRD,XPS, SEM/EDX and broadband impedance spectroscopy study of pyrophosphate (LiFeP2O7 and Li0.9Fe0.9Ti0.1P2O7) ceramics

LiFeP₂O₇ and Li_0.9Fe_0.9Ti_0.1P₂O₇ were synthesised by solid-state reaction and ceramics were sintered. The structure of compounds was studied in the temperature range 300–700 K by X-ray diffraction. Ceramics’ surfaces were investigated by scanning electron microscope. Binding energies of Fe 2p, P 2p and O 1s core levels at ceramics’ surfaces have been determined by X-ray photoelectron spectroscopy and different valence states of Fe and P were detected. Elemental compositions of the compounds were studied by energy dispersive X-ray spectrometer. Impedance spectroscopy was performed in the frequency range 10 Hz–3 GHz and in the temperature interval 400–700 K. The changes of the activation energy of ionic conductivity at 528 and 550 K for LiFeP₂O₇ and Li_0.9Fe_0.9Ti_0.1P₂O₇, respectively, were found. The phenomena can be related to disordering in the unit cells of the compounds.     

Quenching of excited strontium atomic state measured in H2- and CO-flames

The efficiency of resonance fluorescence, Y, of the strontium resonance line (¹P₁→¹S₀ transition) at 4607.33 Å was measured in CO/N₂O, CO/O₂/Ar, and H₂/O₂/CO₂/N₂ flames at atmospheric pressure. From these data, the specific quenching cross sections, σ_qu, for CO₂ and CO were found to be (60 ± 10) Ų and  (300 ± 60) Ų, respectively. The experimental cross sections were confronted with the intermediate ionic-state curve-crossing model and chemical quenching model, respectively.     

High resolution polarization spectroscopy and laser induced fluorescence of CO<Subscript>2</Subscript> around 2μm

High resolution Infrared Polarisation Spectroscopy (IRPS) and Infrared Laser Induced Fluorescence (IRLIF) techniques were used to probe CO₂/N₂ binary gas mixture at atmospheric pressure and ambient temperature. The probed CO₂ molecules were prepared by laser excitation to an overtone and combination ro-vibrational state (12⁰1, J=15) of CO₂, centred at 4988.6612 cm^-1. IRPS and IRLIF line profiles were recorded for several CO₂/N₂ binary mixtures. The observed IRLIF line shapes have the expected Lorentzian form while the observed IRPS line shapes are narrower by a factor of two than those recorded with the IRLIF and appear to have a Lorentzian-cubed profile. The recorded line profiles provide measurements of the pressure-broadening coefficient directly at atmospheric pressure. The Full-Width-Half-Maxima (FWHM) pressure broadening coefficients are measured, based on IRLIF, to be 0.2174±0.0092 cm^-1atm^-1 and 0.1327 ±0.0077 cm^-1atm^-1 for self- and N₂ collision broadening, respectively. The broadening coefficients obtained based on IRPS were measured to be ~8% larger than those obtained with IRLIF.     

Line broadening,shifting and mixing parameters for the ν1 + ν3 band of OCS perturbed by N2 and O2

In this paper, we report measured Rosenkranz N₂- and O₂-broadening, induced pressure-shift and mixing coefficients for OCS in the ν₁ + ν₃ band, using a multi-pressure fitting technique applied to the measured shapes of the lines, including the interference effects caused by the line overlaps. These measurements were made by analysing six laboratory absorption spectra recorded at 0.004 cm^?1 resolution using the Fourier transform spectrometer Bruker IFS125HR located at the Laboratoire Interuniversitaire des Systèmes Atmosphériques, in Créteil. The spectra have been recorded in the 1850–3000 cm^?1 wave number range at 295 K, using a multipass absorption cell with an optical path of 3.249 m. The total sample pressures ranged from 5.97 to 83.28 Torr with OCS volume mixing ratios between 0.001 and 0.013 in nitrogen or oxygen. We have been able to determine the N₂- and O₂-pressure-broadening coefficients of 81 ν₁ + ν₃ transitions with rotational quantum number J up to 50. The measured N₂- and O₂-broadening coefficients range from 0.0815 ± 0.0698 to 0.1169 ± 0.1027 cm^?1 atm^?1 at 295 K, respectively. Most of the measured pressure shifts are positive. The reported N₂- and O₂-induced pressure-shift coefficients vary from about ?0.0103 ± 0.0092 to 0.0097 ± 0.0092 cm^?1 atm^?1, respectively. We have examined the dependence of the measured broadening parameters on the quantum number m (m = ?J for the P branch and m = J + 1 for the R branch) and also developed an empirical expression to describe the broadening coefficients in terms of |m|. On average, this empirical expression reproduces the measured broadening coefficients to within 2%. Using a semi-classical Robert and Bonamy formalism, the theoretical broadening coefficients have been calculated at room temperature and compared with the experimental results. The theoretical results of the broadening coefficients are in very good overall agreement with the experimental data (2%).     

Simultaneous one-dimensional fluorescence lifetime measurements of OH and CO in premixed flames

A method for simultaneous measurements of fluorescence lifetimes of two species along a line is described. The experimental setup is based on picosecond laser pulses from two tunable optical parametric generator/optical parametric amplifier systems together with a streak camera. With an appropriate optical time delay between the two laser pulses, whose wavelengths are tuned to excite two different species, laser-induced fluorescence can be both detected temporally and spatially resolved by the streak camera. Hence, our method enables one-dimensional imaging of fluorescence lifetimes of two species in the same streak camera recording. The concept is demonstrated for fluorescence lifetime measurements of CO and OH in a laminar methane/air flame on a Bunsen-type burner. Measurements were taken in flames with four different equivalence ratios, namely ? = 0.9, 1.0, 1.15, and 1.25. The measured one-dimensional lifetime profiles generally agree well with lifetimes calculated from quenching cross sections found in the literature and quencher concentrations predicted by the GRI 3.0 mechanism. For OH, there is a systematic deviation of approximately 30 % between calculated and measured lifetimes. It is found that this is mainly due to the adiabatic assumption regarding the flame and uncertainty in H₂O quenching cross section. This emphasizes the strength of measuring the quenching rates rather than relying on models. The measurement concept might be useful for single-shot measurements of fluorescence lifetimes of several species pairs of vital importance in combustion processes, hence allowing fluorescence signals to be corrected for quenching and ultimately yield quantitative concentration profiles.     

Physical investigations on pulsed laser deposited nanocrystalline ZnO thin films

The nanocrystalline ZnO thin films were deposited by pulsed laser deposition on quartz and i-Si (100) substrates at different substrate temperatures (473 K–873 K) and at different mixed partial pressures (0.05, 0.01, and 0.5 mbar) of Ar+O₂. The structural studies from XRD spectra reveals that the films deposited at 0.05 mbar and at lower substrate temperatures were c-axis oriented with predominant (002) crystallographic orientation. At 873 K along with (002) orientation, additional crystallographic orientations were also observed in case of films deposited at 0.01 and 0.5 mbar pressures. The composition of Zinc and Oxygen in ZnO films from EDAX reveals that the films deposited at lower partial pressures were have high at.% of O₂ whereas higher partial pressures and substrate temperatures had high at.% Zn. The surface microstructure of the films show that the films deposited at lower partial pressures (0.05 mbar ) and at lower substrate temperatures (473 K) were found to have nanoparticles of size 15 nm where as films deposited at 873 K have nanorods. The length of these nanorods increases with increasing Ar+O₂ partial pressure to 0.5 mbar. The optical energy gap of the film deposited at lower partial pressure and substrate temperature was 3.3 eV and decrease with the increase of substrate temperatures. The films deposited at 0.5 mbar and at 873 K emitted an intense luminescence at a wavelength of 390 nm. The measured thickness of deposited films by spectroscopic ellipsometry is around 456 nm.