Nonequilibrium vibrational populations and dissociation rates of oxygen in electrical discharges

Nonequilibrium vibrational distributions and dissociation rates of molecular oxygen in both electrical and thermal conditions have been calculated by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The dissociation constant under thermal conditions (i.e. without electrons) follows an Arrhenius law with an activation energy of 120 kcal/mole, while the corresponding rates under electrical conditions (5000 ? T_e ? 15000 K, 300 ? T_g ? 1000 K, 10¹¹ ? n_e ? 10¹² cm^?3,5 ? p ? 20 torr) increase with decreasing gas (T_g) and electron (T_e) temperatures and pressure (p) and with increasing electron density (n_e). These results are explained on the basis of the different interplay of V-V and V-T energy exchanges and are rationalized by means of simplified models proposed in the literature. The accuracy of the present results is discussed paying particular attention to the dependence of V-V and V-T rate coefficients on the vibrational quantum number. A comparison of the calculated dissociation rates with the corresponding ones obtained by the direct electron impact mechanism shows that the present mechanism prevails at low electron and gas temperatures. Finally a comparison is shown between theoretical and experimental dissociation rates under electrical and thermal conditions.     

Vibrational relaxation of ozone by para and normal hydrogen

The relaxation of O₃(001) by p- and n-H₂ was measured over the temperature range 167–424 K. n-H₂ quenched ozone more efficiently, with the difference between rate constants increasing with temperature. These observations are explained by a near-resonant V—R process, in agreement with the Sharma—Brau theory.     

Critical properties of the vapor—liquid equilibria of the binary system acetone—n-pentane

Hajjar, R.F., Cherry, R.H. and Kay, W.B., 1986. Critical properties of the vapor—liquid equilibria of the binary system acetone—n-pentane. Fluid Phase Equilibria, 25: 137–146.The P—V—T—x properties for the acetone—n-pentane azeotrope-forming system have been measured by the technique developed by Kay. The experimental uncertainties have been estimated at 0.05 K, 2.4 kPa and 0.06% of the volume. The results for eight different compositions covering the range 180 to 200°C and 2.5 to 4.0 MPa are given. Smoothed critical loci have been plotted. The critical temperatures and pressures were correlated by a quadratic relationship of the form Q_cm = Σ_iΣ_jx_ix_jQ_cij where i, j = 1, 2. The maximum discrepancy in the critical temperature was 0.6 K and in the critical pressure was 0.012 MPa.     

Temperature dependence of the vibrational phase relaxation in gases: Application to H2-rare gas mixtures

The vibrational dephasing rate is calculated for H₂-rare gas mixtures between 85 and 300 K. The semiclassical calculation which only considers unbounded trajectories is based on binary interaction of particles through a Lennard—Jones potential. A comparison is made with three-dimensional and with collinear results obtained when a purely repulsive potential is considered. It is shown that the temperature dependence of the vibrational dephasing rate increases monotonically as T rises. Experimental investigation of Q₁ (1) Raman lines for n-H₂ perturbed by neon, argon and krypton between 100 and 300 K has been realized. The comparison between calculated and measured linewidths suggests that bounded states have to be taken into account at the lowest temperatures considered here.     

Vibrational energy transfer in hydrogen liquid and its isotopes

The transfer of vibrational energy (V-V) from H₂ to isotopic impurities (HD or D₂) has been studied in the liquid state, between 15 and 30 K. The subsequent relaxation (V-T) of the excited impurity by the H₂ liquid host has also been measured and contrasted with the vibrational relaxation behaviour of pure H₂ and D₂ liquids. The isothermal density dependence of both V-V and V-T transfer has been investigated in the fluid state at 30 K. High density relaxation rates are also compared to our data in the pure gases and to other available gas phase results. Measurements in the solid, near the triple point temperature, are equally reported for each process studied.     

Delayed fluorescence in a pyrene-doped naphthalene crystal: homofusion and heterofusion of triplet excitons

Prompt and delayed emissions from a naphthalene crystal doped with pyrene have been investigated in the temperature range 77–300 K. It is shown that delayed fluorescence (DF) in this crystal originates from heterofusion and homofusion of two different types of trap triplets at low temperature (T< 130 K) and from homofusion of pyrene triplets at high temperature (T >200 K). Pyrene concentration effects on DF suggest that free triplet excitons are trapped more efficiently at monomeric pyrene sites than at nascent excimeric pyrene sites.     

Kinetic models for the CH4(ν3) deactivation in CH4-CH4 collisions. Interpretation of experimental results

Various kinetic models for the CH₄(ν₃) deactivation in CH₄-CH₄ collisions at low temperatures (T ? 300 K) are proposed and applied to interpret recently published experimental results. We discuss the value of the rate constant of the single-quantum process CH₄(2ν₄) → CH₄(ν₄) (V-T,R process).     

Energy transfer from a platinum—nickel excited state to platinum clusters in quasi-one-dimensional Ba(Pt,Ni)(CN)4·nH2O crystals

Laser-induced luminescence in quasi-one-dimensional Ba(Pt, Ni)(CN)₄·nH₂O crystals has been measured from 5 to 300 K. A well-defined luminescence peak is observed, corresponding to an excited electronic state of platinum and nickel. Energy transfer from this Pt—Ni excited state to Pt(CN)₄^2? clusters is found to be very much dependent on the temperature.     

A joint vibro-electronic mechanism in the dissociation of molecular hydrogen in nonequilibrium plasmas

A new mechanism Of H₂ dissociation in electrical discharges (10¹¹ ? n_e ? 10¹² cm^?3, 2.10^?16 ? E/N ? 3.10^?16 V cm², 300 ? T_g ? 1000 K, 3 ? p ? 30 torr) is presented and discussed. In this mechanism, called joint vibro-electronic mechanism (JVE), the electrons of the discharge create a strong vibrational disequilibrium with respect to the gas temperature (T_g) and promote electronic transitions from all vibrational levels of ¹Σ_g H₂ state to the repulsive ³Σ_u one. Moreover the V-V (vibration-vibration) and V-T (vibration-translation) energy exchanges are considered for building up the vibrational distribution of ¹Σ_g state. A complete set of e - D cross sections (e + H₂(¹Σ_g,ν) → e + H₂ (³Σ_u) → + 2H, ν = 0,14) is calculated by using an extension of the semiclassical Gryzinski theory in combination with the Franck-Condon principle. Dissociation rates calculated according to JVE are larger either than those obtained by the pure vibrational mechanism (PVM) discussed in our previous work or than those from the direct electronic impact mechanism (DEM) from the ground vibrational level. The behaviour of JVE rates as a function of gas temperature (T_g), of E/N, of electron density (n_e) and of pressure is then reported. The results show strong differences as compared, with the corresponding values obtained, with PVM. Finally the influence of the atoms as well as their recombination on the dissociation rates is discussed. The results have been obtained by solving a system of vibrational master equations.     

Vibrational energy exchange of the DF (ν=2) state with DF (ν=0)

The energy transfer rate for the reaction DF (ν=1) + DF (ν=1)^k_νν→ DF (ν=0) + DF (ν=2) + ΔE=91.6 cm^?1 has been studied in a combined shock-tube laser-induced fluorescence experiment at temperatures from 295 to 720°K. The rate coefficient k_νν for the exothermic reaction was found to vary as T^?1 when expressed in units of cm³/mole sec. At T=295°K, the probability of the reaction is approximately 0.2 per collision.     

Thermal decomposition of C3-substituted peroxyacyl nitrates

The gas phase thermal decomposition rates of C₃-substituted peroxyacyl nitrates, RC(O)OONO₂ have been measured at ambient temperature (287–298 K) and 1 atm. of air. Two saturated compounds (PnBN, R = n-C₃H₇- and PiBN, R = i-C₃H₇-) and two unsaturated compounds (MPAN, R = CH₂=C(CH₃)- and CPAN, R = CH₃CH=CH-) have been studied. In the narrow temperature range studied, thermal decomposition rates for PiBN, PnBN and MPAN exhibited linear Arrhenius behavior with, in units of 10^-4 s^-1, and at 298 K, k = 2.2 for PiBN, 2.3 for MPAN, and 2.7 for PnBN. The thermal decomposition rate of CPAN was 1.6 x 10^-4 s^-1 at 291.6 K and 1.73 x 10^-4 s^-1 at 293.2 K. These thermal decomposition rates are of the same magnitude as that for PAN, R = CH₃. Implications for the atmospheric persistence of C₃- substituted peroxyacyl nitrates are briefly discussed.     

Expansion approach to photodissociation dynamics. Effect of anharmonicity

The effect of anharmonicity in the residual molecule A-B in the photodissociation process A-B-C + hv → A-B + C is examined. It is shown that even if the transition probability for the V-T energy transfer process A-B (u = n) + C → A-B (v ? n') + C is not appreciably affected by the anharmonicity, the partial linewidth

can be dramatically modified.     

Vibrational relaxation induced population inversions in laser pumped polyatomic molecules

Conditions for population inversion in laser pumped polyatomic molecules are described. For systems which exhibit metastable vibrational population distributions [slow vibration—translation/rotation (V—T/R) relaxation], large, long lived inversions are possible even when the vibrational modes are strongly coupled by rapid collisional vibration—vibration (V—V) energy transfer. Overtone states of a hot mode are found to invert with respect to fundamental levels of a cold mode even at V—V steady state. Inversion persists for a V—T/R relaxation time. A gain of 4 m^?1 for the 2v₃ → v₂ transition in CH₃F (λ ≈ 15.9 μ) was found assuming a spontaneous emission lifetime of 10 s for this transition. General equations are derived which can be used to determine the magnitude of population inversion in any laser pumped, vibrationally metastable, polyatomic molecule. A discussion of factors controlling the population maxima of different vibrational states in optically pumped, V—V equilibrated metastable polyatomics is also given.     

Single crystal EPR study of Mn(II)-doped cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II): Mn(II) in imidazole ligated low symmetry site

EPR of Mn(II)-doped single crystals of cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II) has been studied in the temperature range 300-90 K. Mn(II) replaces Cd(II) substitutionally giving rise to two magnetically inequivalent sites. The large magnitude and high asymmetry component of the D-tensor is in accordance with the low symmetry of the substitutional site. Due to the non-first order nature of the spectrum all the work had to be carried out at Q-band frequency. Forbidden transitions corresponding to Δm_I = ± 1 and ±2 are observed for a number of orientations and their positions have been calculated by perturbation theory taking into account the second order admixtures due to cross terms in D and hyperfine coupling constant A. The spin—Hamiltonian parameters at 300 K are A = −83.5, D = −790.0, E = −80.4 (all in units of 10⁻⁴ cm⁻¹) and g = 2.0014.     

Determination of arsenic and antimony in electrolytic copper by anodic stripping voltammetry at a gold film electrode

A simple procedure is described for the determination of arsenic and antimony in electrolytic copper. The copper is digested with nitric acid and copper is separated from arsenic and antimony by passing an ammoniacal solution of the sample through a column of Chelex-100 resin. After digestion with sulphuric acid and reduction to arsenic(III) and antimony(III) with sodium sulphite in 7 M sulphuric acid at 80°C, both arsenic and antimony are deposited at-0.30V and their total is determined by anodic stripping; antimony is then selectively deposited at -0.05 V for anodic stripping. The lower limits of determination are 56 ng As and 28 ng Sb per gram of copper; relative standard deviations (n = 5) are in the ranges 6.1–15.0% for 5.5—0.5 ppm arsenic in copper and 4.1–6.8% for 2.6—0.6 ppm antimony.     

Effect of tetra-n-alkylammonium bromides on the volumetric properties of glycine,l-alanine and glycylglycine at T=298.15 K

The effect of tetra-n-alkylammonium bromides, R₄NBr (R=CH₃, C₂H₅, C₄H₉) on the densities, ρ, of glycine, l-alanine and glycylglycine are reported at T=298.15 K. The apparent molar volumes of amino acids in aqueous tetra-n-alkylammonium salts, φ_VAJW, and of tetra-n-alkylammonium bromides in aqueous amino acids and peptide, φ_VJAW, are calculated from the measured densities. Both φ_VAJW and φ_VJAW have been analysed accurately using a simple equation. Positive transfer volumes are observed for glycine, l-alanine and glycylglycine in the presence of R₄NBr. Tetra-n-butylammonium bromide shows almost double increase in the transfer volumes of amino acids or peptide than tetramethyl- or tetraethylammonium bromides. Negative transfer volumes for the tetra-n-alkylammonium bromide salts are noted in aqueous amino acids or peptide due to large tetra-n-alkylammonium cation undergoing hydrophobic hydration.     

The relaxation processes of several long-chain aliphatic ketones and amines in glassy media

The dielectric absorption of 2-alkanones (CH₃(CH₂)_nCOCH₃) (where n = 4, 5, 6, 7, 8, 10, 12, 14 and 16) and 1-aminoalkanes (CH₃(CH₂)_nNH₂) (where n = 2, 3, 5, 7, 8, 9, 10 and 11) has been studied in a polystyrene matrix between 80 and 300 K in the frequency range 10–10⁵ Hz. 2-alkanone with n = 8 has also been examined in polyphenylether, glassy o-terphenyl and a polypropylene matrix. For both series two sets of absorption peaks were obtained, one around 80–150 K and the other in approximately the 170–300 K region. The lower-temperature absorption can be accounted for by a model involving segmental motion about the CC bonds with consequent displacement of the polar end group. The enthalpy of activation for this intramolecular motion is independent of whether the polar end group is -CH₂Br, -COCH₃, or -CH₂NH₂. The higher-temperature absorption may be ascribed to a molecular relaxation process.     

A three-parameter cubic equation of state for asymmetric mixture density calculations

A new three-parameter cubic equation of state of the van der Waals type with one parameter temperature dependent, P = RT/(V − b) − a(T)/[V(V + c) + b(3V + c)], has been developed for representation of liquid volumes (or densities) for asymmetric mixtures such as CO₂C₁₉ and C₁C₁₀. The calculated results are better than those obtained from the two-parameter Peng—Robinson equation, the three parameter Schmidt—Wenzel equation, the volume-translated Soave—Redlich—Kwong equation proposed by Peneloux et al., and the volume-translated Peng—Robinson equation developed in this work. The parameters of the new equation have been generalized in terms of the acentric factor ω and reduced temperature T_r.     

First rate constants for reactions of OH radicals with amides

Rate constants have been measured for the reaction of OH radicals with four amides, R₁N(CH₃)—C(O)R₂ (R₁ = H or Methyl, R₂ = Methyl or Ethyl), at 300 and 384 K using flash photolysis/resonance fluorescence. Reactants are introduced under slow flow conditions and are controlled by two independent methods, gas saturation and continuous injection. It turns out that the reactivities of the amides are considerably lower than those of the corresponding amines. The pattern of rate constants obtained at 300 K: 14, 21, 5.2, and 7.6 · 10⁻¹² cm³/s for N,N-Dimethylacetamide (dmaa), N,N-Dimethylpropionamide (dmpa), N-Methylacetamide (maa), and N-Methylpropionamide (mpa), respectively, indicates a single, dominating reaction center and strong electronic effects of the substituents at both sides of the amide function. Correspondingly, the observed negative temperature dependence (E/R = − 400 to − 600 K) excludes a direct abstraction mechanism. © 1997 John Wiley & Sons, Inc.     

An electron spin resonance study on γ-irradiated poly( -lactic acid) and poly( -lactic acid)

The effect of gamma irradiation on poly( -lactic acid) ( -PLA) and poly( -lactic acid) ( -PLA), has been examined using ESR spectroscopy and through analysis of the changes in molecular weight. The G values for radical formation of both polylactic acids have been calculated at 77 and 300 K; G(R) = 2.0 at 77 K and G(R) = 1.5 at 300 K for -PLA and G(R) = 2.4 at 77 K and G(R) = 1.2 at 300 K for -PLA. The ESR spectrum at 300 K for the polymers was assigned to one radical, resulting from H atom abstraction from the quaternary carbon atom. The G values for crosslinking and scission have also been determined for the polymers at 300 K; G(S) = 2.3 and G(X) = 0.0 for -PLA, G(S) = 2.4 and G(X) = 0.28 for -PLA.