Radiation induced oxidation of hydroxy indoles by NO and Br radicals: effect of pH

The reactions of NO and Br radicals with 5‐hydroxyindole (HIn), 5‐hydroxytryptophol (HTpl), 5‐hydroxytryptophan (HTpn) and 5‐hydroxytryptamine (HTpe) were studied using pulse radiolysis. The rate constants for their reaction with NO radical were found to vary from 10⁵ to 10⁷ dm³ mol^?1 s^?1 in the pH range 5–9 but a higher value (k = 1.4 ± 0.01 × 10⁸ dm³ mol^?1 s^?1) was noticed in HTpe at pH 9. The gradual increase in reactivity with pH is due to the decrease in the reduction potentials of indoloxyl radicals with E = 0.55 V at pH 9. In contrast, the rate constants with Br radical were found to be diffusion controlled and remained unaffected by the pH. The transient spectra measured are attributed to the indoloxyl radical formed on oxidation with λ_max at 420 nm. The indoloxyl radicals further react with the parent hydroxy indole derivative forming the radical adduct and their decay was found to be pH dependent in derivatives containing an amino group. At pH 5, no decay of the radical adducts was seen in all derivatives up to 5 ms whereas those with the amino group decayed faster at pH 9. The total yields of the oxygen centred and carbon centred radicals formed in the reaction of NO radical with hydroxy indoles were found to be nearly equal to G(NO). Our results suggest that NO radical is inefficient in oxidizing hydroxy indoles under physiological conditions preventing the formation of toxic dimers of indole derivatives. Copyright © 2010 John Wiley & Sons, Ltd.     

Nature of the transient species formed in the pulse radiolysis of 4‐hydroxybenzyl alcohol in aqueous solutions: observation of equilibrium in the reaction of OH‐adducts with HPO ions

Reactions of ^. OH/O ^.? radicals, H‐atoms as well as specific oxidants such as N and Cl radicals with 4‐hydroxybenzyl alcohol (4‐HBA) in aqueous solutions have been investigated at various pH values using the pulse radiolysis technique. At pH 6.8, ^. OH radicals were found to react with 4‐HBA (k = 6 × 10⁹ dm³ mol^?1 s^?1) mainly by contributing to the phenyl moiety and to a minor extent by H‐abstraction from the ? CH₂OH group. ^. OH radical adduct species of 4‐HBA, i.e., ^. OH‐(4‐HBA) formed in the addition reaction were found to undergo dehydration to give phenoxyl radicals of 4‐HBA. Decay rate of the adduct species was found to vary with pH. At pH 6.8, decay was very much dependent on phosphate buffer ion concentrations. Formation rate of phenoxyl radicals was found to increase with phosphate buffer ion concentration and reached a plateau value of 1.6 × 10⁵ s^?1 at a concentration of 0.04 mol dm^?3 of each buffering ion. It was also seen that ^. OH‐(4‐HBA) adduct species react with HPO ions with a rate constant of 3.7 × 10⁷ dm³ mol^?1 s^?1 and there was no such reaction with H₂PO ions. However, the rate of reaction of ^. OH‐(4‐HBA) adduct species with HPO ions decreased on adding KH₂PO₄ to the solution containing a fixed concentration of Na₂HPO₄ which indicated an equilibrium in the H⁺ removal from ^. OH‐(4‐HBA) adduct species in the presence of phosphate ions. In the acidic region, the ^. OH‐(4‐HBA) adduct species were found to react with H⁺ ions with a rate constant of 2.5 × 10⁷ dm³ mol^?1 s^?1. At pH 1, in the reaction of ^. OH radicals with 4‐HBA (k = 8.8 × 10⁹ dm³ mol^?1 s^?1), the spectrum of the transient species formed was similar to that of phenoxyl radicals formed in the reaction of Cl radicals with 4‐HBA at pH 1 (k = 2.3 × 10⁸ dm³ mol^?1 s^?1) showing that ^. OH radicals quantitatively bring about one electron oxidation of 4‐HBA. Reaction of ^. OH/O ^.? radicals with 4‐HBA by H‐abstraction mechanism at neutral and alkaline pH values gave reducing radicals and the proportion of the same was determined by following the extent of electron transfer to methyl viologen. H‐atom abstraction is the major pathway in the reaction of O ^.? radicals with 4‐HBA compared to the reaction of ^. OH radicals with 4‐HBA. At pH 1, transient species formed in the reactions of H‐atoms with 4‐HBA (k = 2.1 × 10⁹ dm³ mol^?1 s^?1) were found to transfer electrons to methyl viologen quantitatively. Copyright © 2009 John Wiley & Sons, Ltd.     

Oxidation of substituted triazines by sulfate radical anion (SO) in aqueous medium: a laser flash photolysis and steady state radiolysis study

Laser flash photolysis has been used to determine the bimolecular rate constants and the spectral nature of the intermediates obtained by the reaction of sulfate radical anion (SO) with 1,3,5‐triazine (T), 2,4,6‐trimethoxy‐1,3,5‐triazine (TMT), 2,4‐dioxohexahydro‐1,3,5‐triazine (DHT), and 6‐chloro N‐ethyl N'‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine (atrazine, AT). The rate constants determined were in the range 4.6 × 10⁷–3 × 10⁹ dm³ mol^?1 s^?1 at pH 6. The transient absorption spectra obtained from the reaction of SO with T, TMT, DHT and AT has an absorption maximum in the region 320–350 nm and was found to undergo second‐order decay. The intermediate species is assigned to N‐yl C(OH) radical of T (TOH^?), carbon centered neutral radical of TMT, an OH‐adduct of AT and an N‐centered radical in the case of DHT. The interpretations on the experimental results obtained from TMT are supported by DFT calculation using Gaussian 03. Steady state radiolysis technique has also been used to investigate the degradation of AT induced by SO. The degradation profile indicated that about 99% of AT has been decomposed after an absorbed gamma‐radiation dose of 7.5 kGy. The degradation yield of AT (expressed as G(‐AT)) was found to be 0.26 µ mol J^?1. The degradation reactions initiated by SO may thus be employed as a potential alternative for ^?OH‐induced degradation of triazines. Copyright © 2007 John Wiley & Sons, Ltd.     

‘Self‐terminating radical cyclizations’ – new insight into the mechanism of the termination step from computational studies

Computational methods were used to gain detailed insight into the mechanism of self‐terminating radical cyclizations, which are initiated by intermolecular addition of O‐centred radicals XO^? to alkynes. The calculations were performed for the reaction of NO, SO, and AcO^? with cyclodecyne ( 1 ) and 5‐cyclodecynone ( 2 ), respectively. Whereas radical addition and the subsequent transannular radical translocation steps are energetically highly favourable processes for the various XO^?, the terminating homolytic β‐fragmentation of the O? X bond in the intermediate α‐oxy radicals 10 – 13 shows a strong dependence on the nature of X. Using simplified model systems, the fragmentation was explored in detail, which revealed that the rate of this step is primarily determined by the strength of the O? X bond and only to a minor extent by the ability of the X moiety to stabilize an unpaired electron in the transition state. However, the cleavage is exothermic, when the released radical X^? is resonance stabilized, e.g. NO, SO, and Bn^?, respectively. In those cases where the unimolecular β‐fragmentation of the O? X bond is slow, termination could also proceed through a bimolecular radical chain process involving the α‐oxy radical intermediate 10 – 13 and the precursor of XO^?, e.g. the Barton PTOC ester 18 or Kim's dithiocarbamate 20 , respectively. Alternative termination mechanisms via oxidation of 10 – 13 can be ruled out under the usual experimental conditions of self‐terminating radical cyclizations. Copyright © 2010 John Wiley & Sons, Ltd.     

The reaction of •OH with O2, the decay of O and the pKa of HO – interrelated questions in aqueous free‐radical chemistry

In the reactions of ozone with organic compounds in aqueous solution, O is an abundant intermediate. A basic aspect of its conversion into ^?OH is addressed here. The reactions O^?? + O₂ ? O (1), H⁺ + O^?? ? ^?OH (8), ^?OH + O₂ ? HO (6), and H⁺ + O ? HO (5) are interconnected by a thermodynamic cycle. For equilibria (1) and (8) reliable equilibrium constants, and hence Gibbs energies are available (ΔG⁰(1) = ?32 kJ mol^?1, ΔG⁰(8) = 67 kJ mol^?1). For reaction (6), a Gibbs energy of ΔG⁰(6) = 47 kJ mol^?1 (K₆ = 10^?8.2 M) has now been calculated by G1. From the thermodynamic cycle one hence arrives at ΔG⁰(5) = ?12 kJ mol^?1. This relates to pK_a(HO) = ?2.1. Thus, the HO radical is a very strong acid. This value agrees with a value of ?2.0 obtained from the Bielski and Schwarz relationship for pK_a values of O_xH_y compounds. Reaction (6) must be very slow, 0.1 < k₆ < 10⁴ M^?1 s^?1. Copyright © 2010 John Wiley & Sons, Ltd.     

Primary kinetic hydrogen isotope effects in deprotonations of a nitroalkane by intramolecular phenolate groups

Rate constants and kinetic isotope effects have been determined for the formation of nitronate anions from the ethers 1‐(2‐methoxyphenyl)‐2‐nitropropane, 7 (X = H, L = H and D) and 1‐(2‐methoxy‐5‐nitrophenyl)‐2‐nitropropane, 7 (X = NO₂, L = H and D), and from the corresponding phenols, 1‐(2‐hydroxyphenyl)‐2‐nitropropane, 3 (X = H, L = H and D), and 1‐(2‐hydroxy‐5‐nitrophenyl)‐2‐nitropropane, 3 (X = NO₂, L = H and D), in aqueous basic medium. For the ethers 7 , rates of deprotonation by hydroxide are comparable with those found for deprotonations of 2‐nitropropane, with k^H/k^D (25 °C) = 7.7 and 7.8, respectively. In both the cases, the isotope effects are conventionally temperature dependent. For the corresponding phenols 3 , conditions have been established under which the deprotonations of the nitroalkane are dominated by intramolecular deprotonation by the kinetically first‐formed phenolate anion, with an estimated effective molarity EM ～ 250. For 3 (X = H, L = H or D), k^H/k^D (25 °C) = 7.8, with E ? E = 6.9 kJ mol^?1 and A^H/A^D = 0.5. For 3 (X = NO₂, L = H or D), rates of intramolecular deprotonation are reduced 30‐fold, and an elevated kinetic isotope effect is found (k^H/k^D (25 °C) = 10.7). Activation parameters (E ? E = 17.8 kJ mol^?1 and A^H/A^D = 0.008) are compatible with an enhanced tunnelling contribution to reactivity in the H‐isotopomer. Copyright © 2009 John Wiley & Sons, Ltd.     

A novel probe for determination of electrical surface potential of surfactant micelles: N,N'‐di‐n‐octadecylrhodamine

The peculiarities of the structure of the fluorescent dye N,N'‐di‐n‐octadecylrhodamine advantage its using as an interfacial acid–base probe in aqueous micellar solution of colloidal surfactants. Two long hydrocarbon tails of the dye provide similar orientation of both cation and zwitterion on the micelle/water interface, with the ionizing group COOH exposed to the Stern region in all the systems studied. Further, the charge type of the acid–base couple, A⁺B^±, ensures similar values of the ‘intrinsic’ contribution, pK, to the ‘apparent’ pK value in micelles of different surfactants. This makes the indicator suitable for determination of electrical surface potentials, Ψ. The pKs have been obtained in cationic, anionic, zwitterionic, and nonionic surfactant systems, at various salt background. In total 17 systems were studied. At bulk counterion concentration of ca. 0.05 M, the pK values vary from 2.14 ± 0.07 in n–C₁₈H₃₇N(CH₃)Cl^– micelles to 5.48 ± 0.06 in n–C₁₆H₃₃OSONa⁺ micelles. The Ψ values, corresponding to the Stern region of micelles, have been evaluated as Ψ = 59.16 pK–pK for T = 298.15 K. The pK parameter was equated to the average value of 4.23 in nonionic surfactants (4.12–4.32, depending on the surfactant type). For cetyltrimethylammonium bromide and sodium n‐dodecylsulfate micelles, the Ψ values (±(7–11) mV) appeared to be +118 mV and at bulk Br^? concentration 0.019 M and ?76 mV at bulk Na⁺ concentration 0.020 M, respectively. This satisfactorily agrees with the theoretical values +111 and ?84 mV, estimated using the Oshima, Healy, and White equation for these well‐defined colloidal systems. Finally, not only absorption, but also fluorescence spectra display the same response to changes in bulk pH. Copyright © 2007 John Wiley & Sons, Ltd.     

Remarkable emissions in diprotonated 2,2′:6′,2″‐terpyridine derivatives

A series of new metal‐free blue emission compounds, i.e., diprotonated terpyH₂ClPF₆ ( 1 ), tterpyH₂ClPF₆ ( 2 ), ClterpyH₂ClPF₆ ( 3 ), and BterpyH₂(PF₆)₂ ( 4 ), were prepared and characterized by electrospray ionization mass spectrometry, UV–vis spectroscopy, and cyclic voltammetry (CV). Abbreviations used are terpy = 2,2′:6′,2″‐terpyridine, tterpy = 4′‐(4‐tolyl)‐2,2′:6′,2″‐terpyridine, Clterpy = 4′‐chloro‐2,2′:6′,2″‐terpyridine, and Bterpy = 4,4′,4″‐tert‐butyl‐2,2′:6′,2″‐terpyridine. The X‐ray crystal structures of the three new compounds 1, 2, and 4 were determined. Both protonated pyridine rings of the terpyridine derivatives are hydrogen bonded intermolecularly to the adjacent Cl^? ion in compounds 1 , 2, and 3 . The π–π* absorption bands in the UV region for 1, 2, 3, and 4 in acetonitrile were red‐shifted relative to those of the corresponding neutral compounds. All the compounds exhibited stronger emissions (around 400 nm) than their neutral counterparts. All the CVs for the diprotonated species, terpyH, tterpyH, ClterpyH, and BterpyH, showed the first reduction waves around ?0.6 V, which were more positive than those of the neutral ones. Density functional theory was applied to interpret the remarkable differences in the interaction of the Cl^? ion. The attachment of two protons to the two terminal Bterpy nitrogens in 4 elicits remarkable characteristics. Both positive charges on the nitrogens are delocalized over the conjugated pyridine systems and the tertiary carbonium ions are stabilized to lead to stronger emission (Φ = 0.35) than the corresponding neutral Bterpy (Φ = 0.045). CCDC 732045–732047 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetic Study of the Emission of the First Negative System in an Argon-Nitrogen Plasma at Atmospheric Pressure

Quantitative optical spectroscopy measurements of the emission spectra of the N(B²∑_u,)ν′→X²∑_gν″ transition (first negative system) in an Ar-N₂ microwave discharge at atmospheric pressure have allowed determination of the rate coefficient of the production of N molecules in the B² ∑_u, state with vibrational level ν′ = 0. The N(B²∑_u, ν′) molecules are produced by the reaction in a surface-wave-induced microwave discharge (2450 MHz) sustained in an open-ended dielectric tube. The rate coefficient K (T) has been obtained for ν′ν″ = 0 for different gas temperatures by varying the incident microwave power. The K₀₀(T) values are between 7.10^?10 and 4.10^?10 cm³ s^?1 for the temperature range 2500 to 3450K.     

Gas‐phase basicities of acetophenones toward lanthanum cation [La(OMe)]

The relative free energy changes (lanthanum cation basicity, LaCB[L₂]) for the reaction [La(OMe)₂]L ? La(OMe) + 2L were determined in the gas phase for m‐ and p‐substituted acetophenones based on the measurement of ligand exchange equilibria using an FT‐ICR mass spectrometer. The substituent effect on ΔLaCB[L₂] of acetophenone is described in terms of the Yukawa–Tsuno equation, ΔG = ρ(σ° + r⁺ Δ σ ), with a ρ value of ?11.2 and an r⁺ value of 0.49. From this result, a ρ value of ?7.0 and an r⁺ value of 0.49 were estimated for the monomeric complex [LLa(OMe)] with the aid of theoretical calculations. This ρ value was found to be significantly smaller than that for protonation, and even smaller than Li⁺ basicity. Such a small ρ value has been attributed to the largely ionic (ion–dipole interaction) nature of the bonding interaction between La(OMe) and the carbonyl oxygen atom and, in part, to the long distance between La(OMe) and the substituent. Contrary to the ρ value, the r⁺ value is identical in both La(OMe) and Li⁺ basicities, suggesting that the r⁺ value of 0.49 can be regarded as a limiting one in a series of Lewis cation basicities of the acetophenone system, H⁺ (0.86) > Me₃Si⁺ (0.75) > Me₃Ge⁺ (0.71) > Cu⁺ (0.60) > Li⁺ = La(OMe) (0.49). Since the binding interaction between La(OMe) or Li⁺ and a neutral ligand is mostly electrostatic, the moderate r⁺ was interpreted to result from the redistribution of the induced positive charge within the acetophenone moiety upon binding with a metal ion rather than transfer of positive charge from a metal ion to the aromatic moiety. Copyright © 2010 John Wiley & Sons, Ltd.     

A non‐enzymatic model based on an acyl transfer reaction for the formation of energy‐rich acetyl phosphate in organic solvents and in a biphasic system

The formation of acetyl phosphate (AcP), an energy‐rich phosphate compound, was studied through the reaction of 2,4‐dinitrophenyl acetate with H₂PO solubilized with Kryptofix® 222 or as a tetra‐n‐butylammonium ((n‐C₄H₉)₄N⁺) salt in organic media. The results indicated that the rate of the reaction in acetonitrile is strongly inhibited by the addition of water, suggesting that the water added to the medium preferentially solvates the H₂PO anion, inhibiting its action as a nucleophile and allowing it to act as a general base catalyst, which leads to the hydrolysis of the ester. The utilization of various organic solvents in the acetyl transfer process demonstrated that the specific interaction of the solvent with water accelerates the process, by desolvation of H₂PO, which can act as a nucleophile. Finally, a formation/transformation cycle of AcP was studied in a biphasic system (water/CH₂Cl₂) using Kryptofix® 222 and (n‐C₄H₉)₄N⁺BF as both the carrier and solubilizing agent for KH₂PO₄. Copyright © 2010 John Wiley & Sons, Ltd.     

An ab initio study of CX3‐substitution (X = H,F, Cl,Br, I) effects on the static electric polarizability and hyperpolarizability of diacetylene

We report a systematic ab initio and density functional theory (DFT) study of the electric properties of the X₃C? C≡C? C≡C? H (X = H, F, Cl, Br, and I) sequence of substituted diacetylenes. We rely on finite‐field Møller–Plesset perturbation theory and coupled‐cluster calculations with large, flexible basis sets. Our best values at the second‐order Møller–Plesset perturbation theory level for the mean dipole polarizability and second hyperpolarizability are $\overline {{\alpha} } $ /e²aE = 64.46 (? CH₃), 65.59 (? CF₃), 110.11 (? CCl₃), 138.90 (? CBr₃), 184.98 (? CI₃) and $\overline {{\gamma} } $ /e⁴aE = 21020 (? CH₃), 13469 (? CF₃), 32708 (? CCl₃), 57599 (? CBr₃), and 105251 (? CI₃). For comparison, the analogous MP2 values for diacetylene [P.Karamanis and G.Maroulis, Chem. Phys. Lett. 2003 , 376, 403.] are $\overline {{\alpha} } $ /e²aE = 49.17, and $\overline {{\gamma} } $ /e⁴aE = 16227. For the mean first hyperpolarizability we report $\overline {{\beta} } $ /e³aE = ?205.8 (? CH₃), ?55.7 (? CF₃), 120.8 (? CCl₃), 443.8 (? CBr₃), and 725.4 (? CI₃). Copyright © 2010 John Wiley & Sons, Ltd.     

Unimolecular Decay of Metastable Cluster Anions: (O2)N−* → (O2) + O2

Unimolecular dissociation of metastable excited states of (O₂)_n^?* into (O₂) cluster anions has been detected and studied quantitatively for n in the range of 13 to 22. The apparent metastable (decay rates determined increase with cluster size in from ～ 5000 s^?1 (for n = 13) up to ～ 8500 s^?1 for n = 22).     

DC Conductivity and Peierls instability in the quasi-one-dimensional organic charge density wave conductor (fluoranthene)2X

Due to the high anisotropy of the dc conductivity (σ_|/σ_⊥ ≈ 10⁴) the organic conductor (fluoranthene)₂X can be regarded as a model system for studying the Peierls instability in quasi-one-dimensional systems. The temperature dependence of the dc conductivity σ_| (T) along the highly conducting crystal axis exhibits the typical behaviour of a quasi-one-dimensional metal with a Peierls transition at about 180 K to a charge density wave (CDW) ground state. As expected for a highly one-dimensional conductor the exact transition temperature depends on three-dimensional coupling effects and therefore on the size of the counterion X^? = PF, AsF, SbF. Above the Peierls transition σ_| (T) can be described quantitatively within a model of CDW fluctuations leading to a pseudo gap in the electronic density of states. Below, the existence of a real energy gap at the Fermi level with a BCS-like temperature dependence determines the charge transport over more than eight orders of magnitude in the electrical resistance. For the intrinsic energy gaps 2 Δ (0), which characterize the ground state of the Peierls semiconductor, values of 120-180 meV have been found for different crystals.     

Electron Emission from Clean Solid Surfaces by Fast Ions

Total backward electron yields from 27 elemental, non-crystalline, clean solids were measured during bombardment by H⁺-, H-, H-, He⁺- and Ar⁺-ions in the energy range from 100 keV to 800 keV. The yields were found to exhibit an oscillatory dependence on the atomic number of the target material correlated with the periods of the periodic system. These Z₂-oscillations are relatively insensitive to the type of projectile and the impact energy at the high projectile energies of this experiment. Present theories of electron emission cannot explain the main experimental results. The reasons for this failure are discussed.     

Experimentelle Untersuchungen zur Ionisierung und Umladung von Alkanolen durch 6-25 keV Wasserstoffionen

Experimental Investigations of Ionisation and Charge-transfer of Alcohols by 6-25 keV Hydrogen Ions Ionisation and charge-transfer cross sections were measured for the interaction of H⁺, H and H ions (in the energy range 6-25 keV) with alcohols (methanol-octanol). The cross sections depend significantly on the target molecular structure like molecular length, ionisation potential, polarizability and atomic bonds in the target molecule.     

Hyperfine Structure of the Configuration lll

The matrix elements for the hyperfine structure of the configuration lll in SL-Kopplung are expressed as linear combinations of the electron coupling constants α_li(10), α_li(01), α_li(12).     

Substituent effect investigation of 3‐(2, 4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one. Part 1. Correlation analysis of 13C NMR chemical shifts

A series of substituted chlorinated chalcones namely, 3‐(2,4‐dichlorophenyl)‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one, have been synthesized, X being H, NH₂, OMe, Me, F, Cl, CO₂Et, CN, and NO₂. Dual substituent parameter (DSP) models of ¹³C NMR chemical shift (CS) have revealed that π‐polarization concept could be utilized to explain the reverse field effect at CO, the enhanced substituent field effect at CO, C‐2, and C‐5, and the decreased sensitivity of substituent field effect at C‐6. Chlorine atoms dipole direction at the benzylidene ring either enhances or reduces substituent effect depending on how they couple with the substituent dipole at the probe site. The correlation of ¹³C NMR CS of C‐2, C‐5, and C‐6 with σ and σ indicates that chlorine atoms in the benzylidine ring deplete the ring from charges. Both MSP of Hammett and DSP of Taft ¹³C NMR CS models give similar trends of substituent effects at C‐2, C‐5, and C‐6. However, the former fail to give a significant correlation for CO and C‐6 ¹³C NMR CS. MSP of σ_q and DSP of Taft and Reynolds models significantly correlated ¹³C NMR CS of C_β. MSP of σ_q fails to correlate C‐1′ ¹³C NMR CS. Investigation of ¹³C NMR CS of non‐chlorinated chalcones series: 3‐phenyl‐1‐(4′‐X‐phenyl)‐2‐propen‐1‐one has revealed similar trends of substituent effects as in the chlorinated chalcones series for C‐1′, CO, C_α, and C_β. In contrast, the substituent effect of the non‐chlorinated chalcone series at C‐2, C‐5, and C‐6 did not correlate with any substituent constant. Copyright © 2010 John Wiley & Sons, Ltd.     

Nichtstationäre Reflexion und Reemission von Wellen bei Herausbildung elektromagnetischer Strukturresonanzen im Plasma

The nonstationary resonant reflectivity of a density profile is studied which consists of two homogeneous overcritical layers, separated by a vacuum region whose width increases linearly in time. Analytical expressions are derived for the time dependence of both the reflexion coefficient and the cavity field in the case of normal incidence. If the configuration passes through a structure resonance, where the eigenfrequency of the density cavity coincides with the frequency of the external wave, the cavity field is strongly enhanced. The temporary formation and the subsequent decay of this resonant field leads to oscillations of the reflexion coefficient. In the case of a loss-less plasma it may reach the extreme values |R| = 0 |R| ≈ 3 and the moving plasma configuration temporarily acts as an energy storage (|R|² < 1) or an emitter (∣R∣² > 1).     

Vibrational Population of Hydrogen Molecules Excited by an RF Discharge in an Expanding Thermal Arc Plasma as Determined by Emission Spectroscopy

This work is devoted to the determination of the vibrational population of hydrogen molecules in the ground and excited electronic states from the analysis of visible spectra of the H₂ molecules excited by an RF discharge in an expanding thermal arc plasma. Comparison of the experimental results on relative electron-impact excitation cross sections for the transition H₂(X¹Σ, υ⁰ = 0)→ H₂(d³II_u, υ′) with other experiments, and with calculations based on the Franck-Condon principle, shows good agreement. This means, that for plasma under investigation: 1) in the ground electronic state H₂(d³II_u,υ′), only the lowest vibrational level with υ⁰ = 0 is significantly populated, and 2) direct electron exictation of H₂(d³II_u, υ′, υ′) state from the ground state H₂(X¹Σ, υ⁰ = 0) dominates.