Polarization dependence of near-resonant two-photon absorption of Na2 vapor

We report theoretically and experimentally on the polarization dependence of two-photon absorption of diatomic molecules for the case where only one intermediate rotationvibration level (off-resonance Δ) contributes to the transition strength. In this case, in the classical limit (J → ∞), the polarization between is (i) independent of the electronic symmetries, and (ii) it can be different for the regime Δ ? Δv_D and Δ ≈ Δν_D, due to velocity-tuning. The experiments have been performed on the 16601.88 cm^?1 two-photon transition of Na₂; they show that this transition belongs to the Q-branch.     

An Ab initio molecular orbital study of the systematics in the photoelectron spectra of halomethanes

Ab initio molecular orbital calculations on fluoro- and chloro-methanes, CH_4—nX_n, predict the correct trends in the photoelectron spectra except in the case of the C(2s) bands of fluoromethanes. Thus, the calculated energies corresponding to the lone pair, σ (CX) and C(1s) bands increase with increasing n as found experimentally; the C(2s) energy in chloromethanes decreases with increasing n, again, in agreement with experiment.     

An experimental and theoretical evaluation of the nitrous oxide-acetylene flame as an atomization cell for flame spectroscopy

The formation of free atoms from aerosols of metal-containing solutions introduced into nitrous oxide-acetylene flames is examined by: (a) inference from well identified reactions and equilibria prevailing in cooler flames; (b) calculations employing a thermodynamic flame model; and (c) experimental observation of relative free-atom number densities in the flames as a function of stoichiometry. The calculated partial pressures of the major natural flame species and some of the spectroscopically observed minor species are presented as a function of the flow ratio of nitrous oxide to acetylene (p). Predicted relative number densities of Na, Mg, Cu, Fe, Li, Be, Al, W, Ti and Si as a function of p are compared with measured free-atom absorbances in an argon-shielded flame. These comparisons were completed for various heights above the burner tip. The data reported show that: (a) the degree of metal atomization in the nitrous oxide-acetylene flame can be adequately described by the equilibrium state; (b) in general, when solute vaporization is complete, there exists a value of ρ at which atomization is complete for metals that form monoxides with dissociation energies less than ~ 6.5 eV; and (c) certain metals may form carbon-containing compounds in the interconal zone.     

Spectroscopic flame temperature measurements and their physical significance—I. Theoretical concepts—A critical review

In this review the basic theoretical principles of temperature measurements of completely uniform flames (with no temperature or concentration gradients of the thermometric species) by the line reversal, emission-absorption, slope, and two-line Spectroscopic techniques are first presented. Since there have been no unifying accounts on the extension of the basic theoretical concepts to more realistic non-isothermal, non-homogeneous flames, a major portion of this review is devoted to a systematic examination of the validity of temperature data obtained by Spectroscopic techniques for these flames. This examination reaffirms several well known, as well as several less familiar, limitations of the temperature values obtained for real flames. Thus, the values obtained depend on: (a) the measurement method employed; (b) the energy of the quantum states involved in the line producing transition (s); (c) the particular temperature gradient in the flame; and (d) the concentration distribution of the thermometric species. Moreover, the values observed do not represent either the average or a weighted average flame temperature, but correspond to an apparent temperature that yields the ratio of the total number of particles in the relevant quantum states found within the viewing field of the spectrometer, i.e.,

where T_app is the apparent temperature, q and p refer to the two quantum states, g is the degeneracy, n_x is the particle density and E is the energy of the quantum level. This result is quite different from the Boltzmann equation

which defines the temperature T_x at the point x in the flame.     

Semiclassical calculation of cross sections and rate constants for vibrational deactivation of HD (v = 1) colliding with 4He

A semiclassical collision model has been used to calculate the rate constant for vibrational relaxation in HD (v = 1, j = 0) colliding with ⁴He. The He + HD potential surface was obtained from an analytical He + H₂ surface previously used for similar calculations on He + H₂ and He + D₂. The theoretically calculated rate constant is about 50% below that experimentally determined in the temperature range 80–300 K.     

Kinetics of rich ammonia flames

We report laser absorption measurements of NH₃ decay within the flame front region of rich, atmospheric pressure ammonia flames. These data are combined with earlier OH, NH, and NH₂ measurements to obtain new estimates for the oscillator strength of NH₂. This value, f_i = 6.4 × 10^?5 for the ^PQ_1,7 line in the (0,9,0) ← (0,0.0) vibrational band of the A²A₁ ← X²B₁ transition, suggests ΔH(NH) ? 87 kcal/mol. The ammonia profiles were also combined with previous data on NO, NH, NH₂, and OH to provide an extensive database at fuel equivalence ratios (ø) of 1.28, 1.50, and 1.81 for comparison to our kinetic model predictions. This modeling used a one-dimensional flame code which explicitly accounts for the diffusional component in our flame experiments. Modeling results using a conventional mechanism predicted concentration profiles which deviated markedly from our observations. It was possible to obtain much more satisfactory fits by postulating reactions between various NH_i (i = 1, 2) species to form N—N bonds. The N₂H_j (j = 1–3) species could then lose H atoms via dissociation to ultimately form N₂. Inclusion of these reactions in the mechanism allowed us to predict concentration—distance profiles for five different species at three different equivalence ratios that are in good agreement with experiment. The most important component of this mechanism is the recognition that the NH_i + NH_i reactions dominate the kinetics in rich flames. A most satisfying aspect of these calculations is that the key rate constants in the NH_i + NH_i sequence were estimated using simple RRK theory.     

Identification of decomposition reactions for HMDSO organosilicon using quantum chemical calculations

Hexamethyldisiloxane [HMDSO, (CH₃)₃-SiOSi-(CH₃)₃] is an important precursor for SiO₂ formation during flame-based silica material synthesis. As a result, HMDSO reactions in flame have been widely investigated experimentally, and many results have indicated that HMDSO decomposition reactions occur very early in this process. In this paper, quantum chemical calculations are performed to identify the initial decomposition of HMDSO and its subsequent reactions using the density functional theory at the level of B3LYP/6-311+G (d, p). Four reaction pathways—(a) Si O bond dissociation of HMDSO, (b) Si C bond dissociation of HMDSO, (c) dissociation and recombination of Si O and Si C bonds, and (d) elimination of a methane molecule from HMDSO—have been examined and identified. From the results, it is found that the barrier of 84.38 kcal/mol and Si O bond dissociation energy of 21.55 kcal/mol are required for the initial decomposition reaction of HMDSO in the first pathway, but the highest free energy barrier (100.69 kcal/mol) is found in the third reaction pathway. By comparing the free energy barriers and reaction rate constants, it is concluded that the most possible initial decomposition reaction of HMDSO is to eliminate the CH₃ radical by Si C bond dissociation.     

Experimental and theoretical investigation of radiative lifetimes in neutral gallium

Using laser spectroscopic techniques the natural radiative lifetimes of 4s ² n ₁ s ² S and 4s ² n ₂ d ² D states of neutral gallium have been measured forn ₁ = 6 to 11 andn ₂ = 4 to 9. These states, as well as previously measured4s ² np ² P states, have been investigated theoretically using multi-configuration Hartree-Fock calculations. Oscillator strengths to all lower-lying states have been calculated and theoretical lifetimes of the investigated states evaluated. The² D sequence is strongly influenced by the 4s4p ^{2 2} D perturber, and strong cancellation effects in the radiative decay are observed both theoretically and experimentally.     

Evaluation of recent scaling free methods to predict phenol and phenoxyl radical vibrational properties—Guidelines for more complex models of lignin or protein moieties

Phenol and phenoxyl radical are simple models of neutral or oxidized states of important moieties in biomacromolecules such as lignin or proteins. The evaluation of methods to predict their vibrational properties provides a guideline for predictions based on more complex and adequate model compounds. Fundamental frequencies of phenol and phenoxyl radical are predicted from harmonic frequencies ν_h obtained from B3LYP calculations, and anharmonic corrections Δν_anh from HF, MP2 or B3LYP calculations. The complete basis set limit is estimated for ν_h and IR intensities. Basis set qualities are evaluated for these properties and for Raman activities. Δν_anh values compare well between methods, except that MP2 is for the basis sets used unreliable for out‐of‐plane modes. Correlation effects increase Δν_anh(HF) values by an average factor ～1.1, but the correlated values do not improve predicted fundamental frequencies. The B3LYP functional is inadequate for the phenoxyl radical. Errors of approximately 5–6 cm^?1 were obtained for the phenol in‐plane low‐frequency modes. For out‐of‐plane modes basis set dependencies are critical to a degree not previously recognized. The predictions appear mainly limited by the method(s) used to calculate ν_h and Δν_anh, and the basis set dependency of Δν_anh. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Flame temperatures from Raman scattering

Pure rotational, as well as vibrational Raman scattering has been observed from stable diatomic molecules in high-temperature flames. Accurate (rotational) flame temperatures may be determined from the rotational line intensities. For a hydrogen diffusion flame in air T_rot(N₂) = 1880 ± 21K, T_rot(H₂) = 1890 ± 80 K, T_vib(N₂) = 1995 ± 130 K, and T_vib(H₂) =1900 ± 150 K.     

Enthalpic and entropic contributions to substituent effects for the ionization of meta- and para-hydroxybenzoic acids in water—dimethylsulfoxide mixtures at 25°C

Separate enthalpic and entropic contributions to substituent effects on the dissociation of m-hydroxybenzoic acid in water—dimethylsulfoxide mixtures ranging from 0.0 to 0.57 mole fraction of DMSO are presented. The effect of the medium on the reaction constant is explained in terms of solute—solvent interactions. The entropy reaction constant values ?_s of the meta-hydroxy compound are much greater than those of the meta-chloro and nitro compounds in the examined mole fraction range.For the para-hydroxy isomer only a trend of σ_s and ?_s values can be calculated. This fact is due to the electron-releasing resonance effect which overlaps and prevails over the inductive effect.     

Polymer flammability. I

The transport of oxygen by diffusion from the environment into a gas stream was investigated as a model for the analogous process in a diffusion flame. The amount transported at steady-state conditions depended on the flow rate, diameter, and spatial orientation of the gas stream. A change of the same extrinsic parameters in a diffusion flame caused changes of burner surface temperature, maximum flame temperature, and flame height. These responses were correlated and yielded an overall activation energy of the rate-controlling reaction step in the combustion process equal to 49 kcal/mole. This value was the same for several types of diffusion flames examined and appeared to be associated with the CO/CO₂ conversion process at the high-temperature flame boundary. Flame quenching was demonstrated to occur at a minimum fuel flow rate and minimum environmental oxygen concentration which were characteristic for a given fuel. Quenching conditions were related to the diffusion rate of oxygen into the product effluent stream. Quenching of a polymer flame by depletion of environmental oxygen was governed by the same processes. The effect of extrinsic parameters on polymer flames is discussed in Part II.     

The semiclassical and the quasiclassical partition function of a quartic anharmonic oscillator

After a substitution a known Laplace-type integral is used to derive quantum corrections to the classical partition function of a quartic anharmonic oscillator in the framework of the Wigner—Kirkwood perturbation expansion. By straightforward calculations results are given in a closed form allowing the analytical formulation of the thermodynamic functions H, E, S, C_υ. The numerical results agree for arbitrary anharmonicity and for high and intermediate temperatures with the numerical partition function calculated from the Hioe—Montroll eigenvalues. Furthermore, the same integral type is used for the analytical calculation of a “quasiclassical” partition function and of “quasiclassical” moments. In the trace formulation of the partition function all commutators are neglected. The harmonic oscillator density matrix is applied to the evaluation of the truncated trace expressions. The “quasiclassical” partition function is an exact upper bound and lies always below the classical partition function.     

Vibration—vibration energy exchange between carbon monoxide and carbon dioxide

Measurements have been made on the vibration—vibration (V—V) energy exchange rate between carbon monoxide and carbon dioxide in the temperature range 180 to 345 K. A steady-state vibrational fluorecence quenching technique was used in conjunction with an open flow gas system. Vibrational excitation of the carbon monoxide was accomplished by absorption of infrared radiation from prospane—oxygen flames. The measured rate constant for the process CO* (υ = 1) + CO₂ → CO + CO^*₂(001) increased linearly with temperature, and after correction for the V—V exchange rate fo the back reaction, the rate constant has a value of (2.2 ± 0.3) × 10³ torr^?1 s^?1 at 296 K. The data are compared to results at highest temperatures and to available theoretical calculations.     

Burning velocity measurement of HFC-41, HFC-152, and HFC-161 by the spherical-vessel method

The burning velocities of fluoromethane (HFC-41), 1,2-difluoroethane (HFC-152), fluoroethane (HFC-161) and ethane were measured by the spherical-vessel (SV) method at room temperature and at initial pressures of 80-107 kPa over a wide range of HFC/air equivalence ratios (?). The burning velocities were determined from the measured pressure increases by application of a spherical flame model. Schlieren photography was used to directly observe flame propagation behavior in a cylindrical vessel equipped with optical windows. The time evolution of the flame radii derived from the pressure increases agreed with the time evolution observed with the Schlieren technique. The maximum burning velocities of HFC-41, HFC-152, HFC-161 and ethane were 28.3 cm s⁻¹ at ? = 1.01, 30.1 cm s⁻¹ at ? = 1.07, 38.3 cm s⁻¹ at ? = 1.07 and 40.9 cm s⁻¹ at ? = 1.05, respectively. The maximum burning velocities for the HFCs, including previously reported C₁ and C₂ fluoroalkanes, decreased with increasing F-substitution rate (the ratio of the number of F atoms to the sum of the number of H and F atoms). The concentrations of chemical species in the flames were investigated by means of an equilibrium calculation, and the results suggested that the burning velocity was correlated with the concentrations of H and OH radicals that were not deactivated by F radicals in the flame. The results also suggested that the burning velocities were linearly related to the heats of combustion of the C₁ and C₂ fluoroalkanes.     

Quantum chemical ab initio calculations for excited states of F IV

Quantum chemical ab initio calculations have been performed for the ground state and for several excited states of the F³⁺ ion (F IV). Three levels of accuracy have been used: Frozen-core SCF calculations (FRC-SCF) to determine orbital energies ε _nl and quantum defects δ _l for excited Rydberg orbitalsnl; frozen-core SCF followed by CI calculations (FRC-CI) which account for multiplet splittings and configuration mixings, and multi-configuration coupled-electron-pair approximation (MC-CEPA) calculations which include dynamic correlation effects. The accuracy of the calculated excitation energies is in the order of 5000 cm^?1 at the FRC-CI level and in the order of 500 cm^?1 at the MC-CEPA level. This latter error amounts to about 0.1% for excitation energies in the range of 400000 to 600000 cm^?1. The MC-CEPA calculations have been performed for 17 experimentally known states and for 14 experimentally unknown states, in particular for the configurations 2s2p ² (² D)3s, 2s 2p ²(² S)3s, 2s ² 2p 4p, and 2s ² 2p 5p.     

Some causes of bending of analytical curves in atomic emission flame spectrometry

Factors affecting the shape of analytical curves in atomic emission flame spectrometry are discussed. Equations are presented by which the effect on the analytical curve of self-absorption, ionization, compound formation, variation in solution flow rate and atomization efficiency, entrance optics, and multiple spectral lines can be considered quantitatively. Theoretical and experimental curves are compared for Na introduced as aqueous NaCI solution into H₂/air, H₂/O₂, and C₂H₂/O₂ flames. The portion dealing with the effect of ionization, compound formation, and variation in solution flow rate and atomization efficiency on the atomic concentration in the flame applies as well to atomic absorption and atomic fluorescence flame spectrometry.     

Cadmium(II) binding by soil-derived fulvic acid measured by anodic stripping voltammetry

Titrations of aqueous solutions of soil fulvic acid (30, 45, and 60 mg l^-1) with cadmium ion solutions at pH 6 and 7 reveal unusual shapes in the stripping current (i_s) vs. total cadmium ion (C_cd²⁺) curves. The expected inflections occur in the titration curves at 8–16 μM. at pH 6 and 12–26 μM at pH 7. In addition, there is an initial rapid increase in i_s at very low C_cd²⁺. The initial rapid increase in current is attributed to labile cadmium—fulvic acid complexes that contribute to i_s by rapid dissociation. Subsequent addition of cadmium ion results in moderately labile complexes and i_s becomes partially kinetically controlled. The stripping current was corrected for kinetic current contributions from dissociation of complexes, and total ligand concentrations, conditional stability constants, and upper slopes were calculated from data well past the titration end-point. The use of upper slopes after kinetic current corrections as in situ calibration curves, allowed calculations of equilibrium cadmium concentrations. The data show that both kinetic current corrections and in situ calibration curves are necessary to avoid substantial errors in calculations of equilibrium cadmium concentrations from anodic stripping voltammetric experiments.     

Quantum-chemical calculations and electron diffraction study of the equilibrium molecular structure of vitamin K3

The equilibrium molecular structure of 2-methyl-1,4-naphthoquinone (vitamin K₃) having C _s symmetry is experimentally characterized for the first time by means of gas-phase electron diffraction using quantum-chemical calculations and data on the vibrational spectra of related compounds.