Spatially resolved measurement of the distribution of solid and liquid Si nanoparticles in plasma synthesis through line-of-sight extinction spectroscopy.

In many high-temperature gas-phase nanoparticle synthesis processes, freshly nucleated particles are liquid and solidify during growth and cooling. This study presents an approach to determine the location of the liquid-to-solid phase transition and the volume fraction and number density of particles of both phases within a gas phase reactor. Spectrally-resolved line-of-sight attenuation (LOSA) measurements are applied to a silicon nanoparticle aerosol generated from monosilane in a microwave plasma reactor. A phantom-based analysis using particle number density, particle size, and temperature distribution from direct numerical simulation (DNS) of the reacting flow indicates that the contributions from the two particle phases can be decoupled under practical conditions, even with noisy data. The approach was applied to analyze spatially and spectrally resolved LOSA measurements from the hot gas flow downstream of the plasma zone where both solid and liquid silicon particles coexist. Extinction spectra were recorded along a line perpendicular to the flow direction by a spectrometer with an electron-multiplying charge-coupled device (EMCCD) camera, and two-dimensional projections were deconvolved to obtain radial extinction coefficient distributions of solid and liquid particles across the cross-section of the flow. Particle number densities of both particle phases were retrieved simultaneously based on the size-dependent extinction cross-sections of the nanoparticles. The particle-size distribution was determined via thermophoretic sampling at the same location with subsequent transmission electron microscopy (TEM) analysis. The particle temperature distribution was determined from the particle's thermal radiation based on line-of-sight emission (LOSE) measurements. The approach for phase-selective data analysis can be transferred to other materials aerosol systems as long as significant differences exist in extinction spectra for the related different particle classes.

A Compact Fiber-Coupled NIR/MIR Laser Absorption Instrument for the Simultaneous Measurement of Gas-Phase Temperature and CO, CO2, and H2O Concentration.

A fiber-coupled, compact, remotely operated laser absorption instrument is developed for CO, CO2, and H2O measurements in reactive flows at the elevated temperatures and pressures expected in gas turbine combustor test rigs with target pressures from 1-25 bar and temperatures of up to 2000 K. The optical engineering for solutions of the significant challenges from the ambient acoustic noise (~120 dB) and ambient test rig temperatures (60 °C) are discussed in detail. The sensor delivers wavelength-multiplexed light in a single optical fiber from a set of solid-state lasers ranging from diodes in the near-infrared (~1300 nm) to quantum cascade lasers in the mid-infrared (~4900 nm). Wavelength-multiplexing systems using a single optical fiber have not previously spanned such a wide range of laser wavelengths. Gas temperature is inferred from the ratio of two water vapor transitions. Here, the design of the sensor, the optical engineering required for simultaneous fiber delivery of a wide range of laser wavelengths on a single optical line-of-sight, the engineering required for sensor survival in the harsh ambient environment, and laboratory testing of sensor performance in the exhaust gas of a flat flame burner are presented.

Phase-sensitive detection of gas-borne Si nanoparticles via line-of-sight UV/VIS attenuation.

The distinct optical properties of solid and liquid silicon nanoparticles are exploited to determine the distribution of gas-borne solid and liquid particles in situ using line-of-sight attenuation measurements carried out across a microwave plasma reactor operated at 100 mbar. The ratio between liquid and solid particles detected downstream of the plasma varied with measurement location, microwave power, and flow rate. Temperatures of the liquid particles were pyrometrically-inferred using a spectroscopic model based on Drude theory. The phase-sensitive measurement supports the understanding of nanoparticle formation and interaction and thus the overall gas-phase synthesis process.

Spatial distribution of gas-phase synthesized germanium nanoparticle volume-fraction and temperature using combined in situ line-of-sight emission and extinction spectroscopy.

In this study, emission and extinction spectroscopy were combined to in situ measure temperature and volume fraction distributions of liquid germanium nanoparticle gas-phase synthesized in an argon/hydrogen/germane flow through a microwave plasma. Emission of the hot particles and extinction against a continuous background were recorded by a spectrometer in the 380-703 nm and 230-556 nm ranges, respectively, selected based on the specific optical properties of the material. Absorption coefficients were deconvoluted from line-of-sight attenuation (LOSA) measurements by a least-square algorithm and then used to determine the local volume fraction distribution. The temperature field was derived from the line-of-sight emission (LOSE) spectra with the prior knowledge of absorption coefficients. A multi-wavelength reconstruction model was developed for the determination of the spatially-resolved distribution of the measured quantities assuming a stationary axisymmetric flow. Advantages of the method include experimental simplicity, low cost, and adaptability to up-scaled reactor sizes.

Threshold photoionization shows no sign of nitryl hydride in methane oxidation with nitric oxide.

Methane was doped with nitric oxide and oxidized in a high-pressure flow reactor. The nitrogen chemistry during partial oxidation was studied using photoelectron photoion coincidence spectroscopy with vacuum ultraviolet synchrotron radiation. The adiabatic ionization energy of nitrous acid, HONO, has been determined as 10.95 ± 0.03 eV. The HONO breakdown diagram was plotted based solely on the measured parent signal and the computed Franck-Condon envelope of trans-HONO, confirming the trans-HONO dissociative photoionization threshold to NO+ + ˙OH at 11.34 eV. The spectra show strong indication for the presence of cis-HONO. We expected the m/z 47 photoion mass selected threshold photoelectron signal to rebound near 12 eV, i.e., at the ionization energy of nitryl hydride, the third HNO2 isomer. Recent computational studies suggest nitryl hydride is formed at a rate similar to trans-HONO, is more thermally stable than nitrous acid, its cation is bound, and its photoelectron spectrum is predicted to exhibit a strong origin band near 12 eV. The absence of its mass selected threshold photoelectron signal shows that nitryl hydride is either not formed in measurable amounts or is consumed faster than nitrous acid, for instance by isomerization to trans-HONO.

Characterization of tracers for two-color laser-induced fluorescence thermometry of liquid-phase temperature in ethanol, 2-ethylhexanoic-acid/ethanol mixtures, 1-butanol, and o-xylene.

The fluorescence spectra of dye solutions change their spectral signature with temperature. This effect is frequently used for temperature imaging in liquids and sprays based on two-color laser-induced fluorescence (2cLIF) measurements by simultaneously detecting the fluorescence intensity in two separate wavelength channels resulting in a temperature-sensitive ratio. In this work, we recorded temperature-dependent absorption and fluorescence spectra of solutions of five laser dyes (coumarin 152, coumarin 153, rhodamine B, pyrromethene 597, and DCM) dissolved in ethanol, a 35/65 vol.% mixture of ethanol/2-ethylhexanoic acid, ethanol/hexamethylsiloxane, o-xylene, and 1-butanol to investigate their potential as temperature tracers in evaporating and burning sprays. The dissolved tracers were excited at either 266, 355, and 532 nm (depending on the tracer) for temperatures between 296 and 393 K (depending on the solvent) and for concentrations ranging between 0.1 and 10 mg/l. Absorption and fluorescence spectra of the tracers were investigated for their temperature dependence, the magnitude of signal re-absorption, the impact of different solvents, and varying two-component solvent compositions. Based on the measured fluorescence spectra, the tracers were analyzed for their 2cLIF temperature sensitivity in the respective solvents. Coumarin 152 showed for single-component solvents the overall best spectroscopic properties for our specific measurement situation related to temperature imaging measurements in spray-flame synthesis of nanoparticles as demonstrated previously in ethanol spray flames [Exp. Fluids61, 77 (2020)10.1007/s00348-020-2909-9].

Excitation wavelength dependence of the fluorescence lifetime of anisole.

Photo-physical models that describe the pressure- and temperature-dependent fluorescence quantum yield of organic fluorescence tracers rely on an accurate prediction of the initial excited-state population, collision-dependent relaxation processes, and state-dependent relaxation processes. In case the initial excited-state population distribution reached after the laser excitation equals on average the thermal distribution, the fluorescence quantum yield becomes pressure independent. This initial distribution critically depends on the temperature-dependent ground-state population before excitation as well as the excitation wavelength. The ability to predict this behavior is a critical check for the validity of the existing photophysical models. The dependence of the effective fluorescence lifetime of anisole on the excitation wavelength (256-270 nm) was investigated at temperatures between 325 and 525 K for pressures between 1 and 4 bar. For each temperature, a unique excitation wavelength was found where the fluorescence lifetime is pressure-independent. The comparison of the experimental results with the predictions based on the established photophysical step-ladder models revealed a systematic underestimation of the required excitation photon energies for direct excitation into the thermalized level. An improved modeling approach based on quantum chemistry calculations for implementing simulated excitation spectra and state-dependent transition probabilities overcomes these limitations. Our results show for the example of anisole that the fluorescence step-ladder models that exist for aromatic fluorescence tracers must be modified to correctly predict the effect of the excitation wavelength.

IR spectroscopy of pyridine-water structures in helium nanodroplets.

We present the results of an IR spectroscopic study of pyridine-water heterodimer formation in helium nanodroplets. The experiments were carried out in the frequency range of the pyridine C-H stretch region (3055-3100 cm(-1)) and upon water deuteration in the D-O stretch region (2740-2800 cm(-1)). In order to come to an unambiguous assignment we have determined the angle between the permanent dipole and the vibrational transition moment of the aggregates. The experiments have been accompanied by theoretical simulations which yielded two minimum structures with a 16.28 kJ mol(-1) energy difference. The experimentally observed bands were assigned to two structures with different H-bonds: an N···H bond and a bifurcated O···H-C bond.

High resolution spectroscopy of HCl-water clusters: IR bands of undissociated and dissociated clusters revisited.

We report a detailed study on the IR spectroscopy of HCl-water complexes in superfluid helium nanodroplets in the frequency range from 2660 to 2675 cm(-1). We have recorded spectra of HCl-H2(16)O as well as of HCl-H2(18)O complexes and compared these results with theoretical predictions. In addition, we have carried out mass-selective intensity measurements as a function of partial pressure of HCl as well as of H2(18)O (pick-up curves). The results support a scenario where the IR-absorption in this part of the spectrum contains contributions from undissociated as well as from dissociated clusters with Cl(-)(H2O)3(H3O)(+) being the smallest dissociated complex. These findings are corroborated by additional electric field measurements yielding the orientation of the vibrational transition moment with respect to the permanent dipole moment. As a result we are able to assign a broad absorption band starting at 2675 cm(-1) to dissociated HCl-water clusters (HCl)1(H2O)n with n ≥ 4. The two narrow absorption lines at 2667.9 cm(-1) and 2670 cm(-1) are assigned to an undissociated cluster, in agreement with previous studies.