Detection of chlorinated hydrocarbons via Laser-atomization/Laser-induced fluorescence

Chlorinated hydrocarbons (CHC), photodissociated at 193 nm, are detected with high sensitivity by observing the atomic chlorine fragment via laser-induced fluorescence (LIF). Photofragment emission spectra from CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄, C₂HCl₃, and C₂Cl₄ demonstrate that photofragment fluorescence and chemiluminescence are negligible in the region 700–800 nm where the 3p ⁴4p ⁴ S ⁰ 3p ⁴4s ⁴ P fluorescence from atomic chlorine is detected. There is also negligible interference for photodissociation in Ar, N₂, and air bath gases. Total CHC can be readily detected in air flows at mixing fractions less than 20 ppb and averaging times less than 1 minute. Techniques for considerable improvement in this detection limit are discussed.Supported by the NSF     

The effect of acids on fluorescence of coumarin-6 in organic solvents

The effect of acids (HCl, HClO₄, HNO₃, and CH₃COOH) on the fluorescence of coumarin-6 in organic solvents (acetonitrile, acetone, butanol-1, and ethanol) is studied. The regions of acid (HCl, HClO₄, HNO₃) concentrations that lead to a change in the fluorescence spectra are determined for each of the solvents. It is shown that, for all the solvents studied, acetic acid with a concentration within the region 10^?1–10^?6 M causes no significant changes in the fluorescence spectrum of coumarin-6. A mechanism of the coumarin-6 protonation is proposed.     

Spectrofluorometric investigation of salicylal- dithiocarbazinic acid esters

The fluorescent properties of salicylaldithiocarbazinic esters which can be described by RR¹C= N-NH-CSSR² (where R = 0HO-C₆H₄; R¹ = H or CH₃; R² =CH₃, C₆H₅-CH₂ or p-C1-C₆H₄-CH₂) have been investigated.The investigations were made in DMF, in different DMF-water mixtures, and in aqueous media. It could be stated that the change of the R² group has an unimportant effect on the intensity of fluorescence and on the excitation and emission spectra in all examined solvents. But an important effect is caused by the change of the group R¹. As in DMF solution, only the excitation spectrum changes on substituting R¹ = H by R¹ = CH₃, in DMF: water i.e. aqueous media, the change in the intensity of fluorescence, too, is very great.In the case of R¹ = H, the intensity of fluorescence is about five times greater than in R¹ = CH₃. The intensity of fluorescence can be increased, if an electrophil substituent is put into the place of group R¹.     

Charge Transfer Photophysics of Tetra(α-amino) Zinc Phthalocyanine

The absorption, fluorescence, and transient absorption spectra of Tetra(α-amino) zinc phthalocyanine, ZnPc(α-NH₂)₄, have been measured in polar solvents and compared with that of ZnPc(α-R)₄ (R?=?H, NO₂, OCH(CH₃)₂). While the latter three showed the typical photophysics of phthalocyanines, ZnPc(α-NH₂)₄ exhibits distinct spectral properties, a very low fluorescence quantum yield and a relatively long fluorescence lifetime. These observations are explained by the substantial charge transfer characters in the absorption and fluorescence spectra of ZnPc(α-NH₂)₄. NMR indicates that intramolecular H-bonding makes atoms in NH₂ actually coplanar with other elements of ZnPc(α-NH₂)₄. The local excited state is non emissive and the weak emission is assigned to its charge transfer state. The transient absorption bands from laser flash photolysis located at 630 nm, 645 nm is assigned to the mono-charge transfer state, while that at 545 nm is assigned to the di-charge transfer state.     

Methane activation on Ni(111) at high pressures

High pressure CH₄ decomposition experiments on Ni(111) have been conducted to discriminate between a direct reaction mechanism involving hot CH₄ molecules and a precursor-mediated mechanism. It has been found that the direct process for CH₄ decomposition is kinetically significant in depositing adsorbed carbon. Using absolute carbon coverage measurements the absolute efficiency of carbon deposition on Ni(111) at 600 K was found to be 4 × 10⁻⁸ C atom per CH₄ collision at 1.0 Torr CH₄ pressure. By the use of He, Ne, and Ar buffer gases, it has been shown that the major process leading to CH₄ dissociation involves heating CH₄ followed by return of excited CH₄ back to the surface where the CH₄ dissociates. These experiments demonstrate that CH₄ translational excitation and vibrational excitation, as studied by molecular beam methods, are the kinetically important CH₄ activation processes leading to dissociation in the high pressure regime.     

The 330-nm S1-S0 electronic spectrum of 1-pyrazoline: Probable hindered pseudorotation in S1 and confirmation of the ring-puckering potential function in S0 by single vibronic level fluorescence spectroscopy

The S₁-S₀ absorption spectrum of 1-pyrazoline is rotationally sharp but vibrationally extremely irregular, and other techniques are necessary to aid its assignment. The relaxed fluorescence spectrum shows a very long progression in the NN twisting vibration, suggesting that the ring is twisted in S₁ whereas, in S₀, this part of the ring is planar but the CH₂ group in position 4 is puckered. With a twisted ring in S₁ it seems likely that the NN twisting and CH₂(4) puckering modes in S₀ should be combined and newly described as radial and hindered pseudorotational modes in S₁. The vibronic transitions accompanying such an S₁-S₀ electronic transition are derived. Single vibronic level fluorescence spectra from many vibronic levels of S₁ show progressions in both the NN twisting and CH₂(4) puckering vibrations in S₀, but only with Δv even. This strongly supports the suggestion that these two modes are heavily mixed in S₁, and indicates that the fluorescing states are either above the barrier to pseudorotation or not far below it, so that there is appreciable tunnelling through the barrier. The progressions in the CH₂(4) puckering vibration allow us to assign uniquely the puckering quantum number, in S₀, of the band in which excitation took place. In addition, the spacings in these progressions further confirm the preferred potential function derived from the far-infrared spectrum and confirmed previously from the microwave spectrum.     

Distinct transport properties of O2 and CH4 across a carbon nanotube

It is of fundamental importance to investigate either O₂ or CH₄ molecules across nanochannels in many areas such as breathing or separation. Thus, many researches have focused on such a single type of molecules across nanochannels. However, O₂ and CH₄ can often appear together and crucially affect human life, say, in a mine. On the basis of molecular dynamics simulations, here we attempt to investigate the mixture of O₂ and CH₄, in order to identify their different transport properties in a nanochannel. We take a single-walled carbon nanotube (SWCNT) as a model nanochannel, and find that their transport properties are distinctly different. As the concentration of O₂ increases up to a high value of 0.8, it is always faster for CH₄ molecules to transport across the SWCNT, and the total number of gas molecules transporting across the SWCNT is decreased. Meanwhile, CH₄ molecules are always dominant in the SWCNT, and the total number of O₂ or CH₄ inside the SWCNT is a constant. By calculating the van der Waals interaction between the SWCNT and O₂ or CH₄, we find that the net interaction between CH₄ and the SWCNT is much stronger. Our findings may offer some hints on how to separate CH₄ from O₂, and/or store CH₄ efficiently.     

A density functional theory study of CH4 dehydrogenation on Co(1 1 1)

The dehydrogenation of CH₄ on the Co(1 1 1) surface is studied using density functional theory calculation (DFT). It is found that CH₄ is favored to dissociate to CH₃ and then transforms to CH₂ and CH by sequential dehydrogenation, and CH₄ activation into CH₃ and H is the rate-determining step on the Co(1 1 1) surface. CH₂ is quite unstable on Co(1 1 1) surface. CH dehydrogenation into C and H is difficult. CH₃ and H prefer to adsorb on 3-fold hollow hcp and fcc sites, and CH₂, CH and C prefer to adsorb on hcp sites.     

The photoelectron spectra of the tetrafluoro and tetramethyl compounds of the group IV elements

The photoelectron spectra of CF₄, SiF₄, GeF₄, C(CH₃)₄, Si(CH₃)₄, Ge(CH₃)₄, Sn(CH₃)₄, and Pb(CH₃)₄ h     

Vibrational relaxation time measurements in CH4 and CH4-rare gas mixtures

With a spectrophone vibrational relaxation times in CH₄ and in mixtures of CH₄ with rare gases were measured. Both the amplitude and the phase method were used. The two infrared active modes of CH₄ (ν₄ and ν₃) were investigated separately. The relaxation times, at one atmosphere, after exciting the lowest mode ν₄, were found to be: τ(CH₄-CH₄) = 1.65 μs; τ(CH₄-He) = 1.97 μs; τ(CH₄-Ne) = 8.6 μs; τ(CH₄-Ar) = 12 μs and τ(CH₄-Kr) ≈ 60 μs. From these values one may in that vibrational-rotational (V-R) energy transfer is the dominant relaxation mechanics. By exciting the higher mode the first step in the deactivation of ν₃ was found to be a V-V transfer to the lowest modes ν₄, ν₂.     

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.     

激光诱导荧光法研究CH2Cl2分子的振动能量转移

本文中用激光诱导荧光法首次研究了有机分子CH₂Cl₂分子的振动能量转移。实验测得了CH₂Cl₂分子的ν₃和ν₃/ν₉振动模的激活和消激活速率常数,以及稀有气体分子对CH₂Cl₂分子的ν₃/ν₉振动模消激活的影响。用SSH理论计算了CH₂Cl₂-稀有气体碰撞,CH₂Cl₂分子ν₃/ν₉的V-T能量转移相对几率,分析讨论了CH₂Cl₂分子的振动能量转移的通道。 关键词：     

ESR of Gamma Irradiation and Ultraviolet Photolysis Products in Some Tetrasubstituted Ammonium Compounds

Abstract

The electron spin resonance of gamma and ultraviolet irradiated tetrabutylammonium halides [CH₃(CH₂)₃]₄NX (X=I,Cl,Br), tetrabutylammonium hydrogen sulfate [CH₃(CH₂)₃]₄NHSO₄, tetrabutylammonium periodate [CH₃(CH₂)₃]₄NIO₄, and ultraviolet photolyzed tetramethylammonium iodide, (CH₃)₄NI, and tetramethylphosphonium iodide, (CH₃)₄PI have been investigated between 140 and 350 K. The gamma and ultraviolet irradiation damage centers in tetrabutylammonium compounds were attributed to CH₃—CH^…—CH^…—CH₂, radicals, and ultraviolet photolysis damage centers in tetramethylammonium and phosphonium iodides were attributed to ?H₃ radicals. The g values of both radicals are found to be isotropic and g = 2.0030 and 2.0037 respectively to the methylallyl and the methyl radicals. The hyperfine coupling constants of the free electron to the protons in the radicals are reported and discussed.     

Absorption spectra of tripositive thulium ion in acetate complexes

Optical absorption studies have been carried out on Tm³⁺ in NH₄ (CH₃COO), Mg(CH₃COO)₂, K(CH₃COO) and Na(CH₃COO) 3H₂O. Second derivative spectra of Tm³⁺ complexes have been recorded. The second derivative spectra exhibited splittings in the ³F₄, ³F₃ and ¹G₄ levels. These splittings have been tentatively explained as caused by superpositions of vibrational frequencies in the electronic bands. The frequency differences between the split levels are comparable with the observed infrared frequencies. Interaction and Judd-Ofelt parameters are evaluated. Radiative transition probabilities for fluorescence levels of ³P₀, ¹I₆, ¹D₂, and ¹G₄ in these complexes are estimated theoretically. The radiative lifetime for Tm³⁺ in acetate complexes is greatest for ¹G₄ and least for ³P₀.     

Structure and dynamics of CH2O,OH, and the velocity field of a confined bluff-body premixed flame,using simultaneous PLIF and PIV at 10 kHz.

This study characterizes the structure and dynamics of a confined, bluff-body-stabilized turbulent premixed flame by simultaneously employing formaldehyde (CH₂O) and hydroxyl (OH) planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV), at a rate of 10?kHz. The large field-of-view (>170 cm²) CH₂O-PLIF is enabled by use of a burst-mode laser delivering 10-kHz pulse trains of 355-nm at 350 mJ/pulse, resulting in a CH₂O signal-to-noise of 47:1 during PIV seed flow. Two cases illustrative of the CH₂O dynamics are presented. A statistically stationary turbulent combustion case highlights the development of the CH₂O layers in space and time. Notably, presumed CH₂O-vortex dynamic interactions are observed where the CH₂O accumulates broadly near the Kelvin–Helmholtz vortex core and remains thin near the vortex braid, contributing to a distribution of CH₂O preheat zone thickness from 1 to 10 times of the calculated laminar value. The second case highlights the CH₂O dynamics during a self-excited combustion instability. Two short-duration increases in CH₂O are produced during the elevated velocity portion of the acoustic cycle. The first CH₂O increase is caused by the reactant mass flux impulse as the velocity starts to increase. The second CH₂O increase is the result of the upper and lower shear layers merging downstream and entraining fresh reactants that burn in intense, distributed regions inside the wake. Estimating the time delay between the CH₂O and the heat release, it is suggested that the secondary CH₂O increase may contribute to damping of the acoustic instability, because of its out-of-phase relationship with pressure, while the first CH₂O increase appears to drive the instability.     

氩气掺入对类金刚石沉积过程的影响

在电子助进化学气相沉积(EACVD)类金刚石薄膜中,使用和比较CH₄/H₂与CH₄/H₂/Ar两种体系,用静电探针测量这两种体系的电子温度、电子密度,结果表明,在CH₄/H₂/Ar体系中,电子密度较高,使成膜的速率增加。 关键词：     

Effect of buffer gas on the generation of eye-safe 1.54 μm radiation using the stimulated Raman scattering technique in CH4

Experimental results on the generation of 1.54 μm eye-safe radiation in pure CH₄, CH₄:He and CH₄:Ar mixtures pumped by the fundamental of an Nd:YAG laser, using the stimulated Raman scattering (SRS) technique, are described. A decrease in the energy conversion efficiency and degradation in the beam quality of S₁ was observed in pure CH₄ at high pump energies. This problem was overcome in CH₄:He and CH₄:Ar mixtures. Compared with the first Stokes (1.54 μm) energy conversion efficiency in pure CH₄, at a pump energy of 126 mJ, an enhancement of 50% in energy conversion efficiency was observed in the CH₄:Ar mixture (60% argon concentration) and as much as 100% in the CH₄:He mixture (60% helium concentration). The use of these buffer gas mixtures improved the spatial beam quality of the Stokes radiation considerably and also resulted in raising the pump threshold for optical breakdown of the Raman gain medium.     

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.     

Time-resolved LIF of OH in the flame front of premixed and diffusion flames at atmospheric pressure

Time-resolved fluorescence measurements are reported for the OH radical in the flame fronts of laminar atmospheric pressure flames. Effective (collision-limited) lifetimes were determined following excitation of theA ²⁺, = 0 state of OH using a picosecond dye-laser system based on the distributed-feedback principle. Measurements were made in a premixed knife-edge burner for rich CH₄/N₂O/N₂ and CH₄/air flames and in a counterflow diffusion burner for a CH₄/air flame. In the accessible range, results indicate a net dependence of lifetime on temperature intermediate betweenT andT ^0.5 for these flames.     

A Spectroscopic Study of 2-[4′-(Dimethylamino)phenyl]-benzothiazole Binding to Insulin Amyloid Fibrils

The spectroscopic properties of 2-[4′-(dimethylamino)phenyl]-benzothiazole (BTA-2) in solution and in the presence of amyloid fibrils were investigated using absorption and fluorescence spectroscopy. Solution studies show that BTA-2 forms micelles in aqueous solutions, but that the dye can be solvated upon the addition of acetonitrile (CH₃CN). BTA-2 binds to amyloid fibrils in solution leading to a characteristic blue-shift in the emission spectrum and an increase in fluorescence intensity. However, in solutions with increasing CH₃CN concentration, there was a marked decrease in binding of the BTA-2 to fibrils. Studies demonstrating the effect of BTA-2 concentration on binding were performed. A comparison with the standard amyloid fluorescent marker, thioflavin T (ThT), showed that BTA-2 is more fluorescent, making it an excellent dye to label amyloid samples.