Experimental and theoretical studies of the valence-shell dipole excitation spectrum of molecular fluorine

Electron energy loss measurements and concommitant RPAE calculations are reported of the valence-shell dipole excitation spectrum of molecular fluorine. The measured spectrum is dominated by a series of strong features in the 12–16 eV interval which are in accord with X¹Σ_g⁺→¹Σ_u⁺ bands assigned in a previously reported high-resolution optical study. These features are attributed on basis of the present RPAE calculations to configuration mixing between 1π_g→npπ_u Rydberg and 3σ_g→3σ_u intravalence excitations. A depleted X→V_σ charge-transfer excitation is correspondingly observed at ≈17 eV, in good accord with the calculated values. The appearance of the σ→σ* transition in F₂ below the 3σ_g^?1 threshold is in marked contrast to the situation in other light diatomic molecules, in which cases σ→σ* transitions appear as intravalence shape resonances in photoionization continua. Assignments are also provided of weak, irregularly spaced X¹Σ_g⁺→¹Π_u excitations the origins of which are attributed to configuration mixing between 1π_g→npσ_u and 1π_u→nsσ_g Rydberg series.     

Kinetics and mechanism of atmospheric reactions of partially fluorinated alcohols

Gas-phase reactions typical of the Earth’s atmosphere have been studied for a number of partially fluorinated alcohols (PFAs). The rate constants of the reactions of CF₃CH₂OH, CH₂FCH₂OH, and CHF₂CH₂OH with fluorine atoms have been determined by the relative measurement method. The rate constant for CF₃CH₂OH has been measured in the temperature range 258–358 K (k = (3.4 ± 2.0) × 10¹³exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(1.5 ± 1.3) kJ/mol). The rate constants for CH₂FCH₂OH and CHF₂CH₂OH have been determined at room temperature to be (8.3 ± 2.9) × 10¹³ (T = 295 K) and (6.4 ± 0.6) × 10¹³ (T = 296 K) cm³ mol^?1 s^?1, respectively. The rate constants of the reactions between dioxygen and primary radicals resulting from PFA + F reactions have been determined by the relative measurement method. The reaction between O₂ and the radicals of the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH) have been investigated in the temperature range 258–358 K to obtain k = (3.8 ± 2.0) × 10⁸exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(10.2 ± 1.5) kJ/mol. For the reaction between O₂ and the radicals of the general formula C₂H₄FO (? HFCH₂O, CH₂F?HOH, and CH₂FCH₂?) at T = 258–358 K, k = (1.3 ± 0.6) × 10¹¹exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(5.3 ± 1.4) kJ/mol. The rate constant of the reaction between O₂ and the radicals with the general formula C₂H₃F₂O (?F₂CH₂O, CHF₂?HOH, and CHF₂CH₂?) at T = 300 K is k = 1.32 × 10¹¹ cm³ mol^?1 s^?1. For the reaction between NO and the primary radicals with the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH), which result from the reaction CF₃CH₂OH + F, the rate constant at 298 K is k = 9.7 × 10⁹ cm³ mol^?1 s^?1. The experiments were carried out in a flow reactor, and the reaction mixture was analyzed mass-spectrometrically. A mechanism based on the results of our studies and on the literature data has been suggested for the atmospheric degradation of PFAs.     

CH3 + Cl = CH3Cl: RRKM/master equation modeling

Data for the title reaction have been fitted using an RRKM/master equation approach. Energy transfer was modeled using an exponential decay with downward step sizes, ΔE_d, as a fitting parameter. The low temperature (200 < T (K) < 300) combination of CH₃ with Cl atoms in He can be accommodated with ΔE_d (cm^?1) = 400. Higher temperature (1600 < T (K) < 2100) decomposition in Ar required ΔE_d(T) (cm^?1) = 694(T/300)^0.46. Previous analysis of the analogous system CH₄ = CH₃ + H required ΔE_d(T) (cm^?1) = 100(T/300) for He and ΔE_d(T) (cm^?1) = 150(T/300) for Ar. Understanding of the magnitudes and temperature dependence of ΔE_d remains the greatest detriment to quantitative calculation, extrapolation, and prediction of unimolecular rate constants. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 245–254, 2009     

Dissociation of ethane by electron impact

The absolute total dissociation cross section for ethane is reported for electron energies between 10 and 600 eV. A maximum value of 7.6 × 10^?16 cm² occurs at 80 eV while the apparent threshold is ≈ 10 eV. Dissociative ionization is more probable than dissociation into neutral fragments at all energies except in the threshold region. The data indicates that fragmentation involving methane elimination (c^? + C₂ H₆ → e^? + CH₄ + CH₂) occurs in less than 2% of the dissociative events for 50 < E < 600 eV. Arguments are presented which suggest that some of the lower excited states of ethane are stable against dissociation.     

Vibrational relaxation of N2(A3Σu+,υ = 1, 2,3) by CH4 and CF4

N₂(A, υ = 0-3) produced by the Ar(³P_0,2) + N₂ reaction and detected by laser-induced fluorescence undergoes rapid, stepwise vibrational relaxation but slow electronic quenching with added CH₄ or CF₄. Rate constants, k_Q^υ, of 1.5, 3.1, and 5.0 × 10^?12 cm³ s^?1 are measured for Q = CH₄, υ = 1-3, and 0.47, 1.8, and 5.5 × 10^?12 cm³ s^?1 for Q = CF₄, υ = 1-3, with ≈±20% accuracy (1σ). Information is also obtained for the unrelaxed, relative υ populations.     

Reactions of photogenerated fluorine atoms with dopant molecules in solid argon

The products of UV photolysis of ternary Ar?CH₄(CD₄)?F₂ mixtures (1:c:c ₀,c, c ₀=0.001–0.01) at 13–16 K were identified by ESR and FTIR spectroscopy. These products are^?CH₃ (^?CD₃) radicals of typesI andII and molecular CH₃F?HF complexes. The latter were characterized by the IR bands of the stretching C?F (1003 cm^?1) and H?F (3774 cm^?1) vibrations. The ESR spectra of radicalsI are asymmetric. The anisotropy of theg-factor (Δg～10^?3) of radicalI indicates that the structure of the radicals is nonplanar. The ESR spectrum of the typeII radical is identical to that of matrix-isolated^?CH₃ (^?CD₃) radicals with the planar structure (Δg<5·10^?5). Under the experimental conditions, the amount of complexes formed in the photolysis is equal to 0.022·c. When the photolysis is ceased, radicalI disappears after ≈10³ s and radicalII is stabilized. The limiting concentrations of the stabilized^?CH₃ and^?CD₃ radicals are equal to 2·10^?2·c and 2·10^?3·c, respectively. A mechanism of the formation of the products is suggested. It is based on the assumption that both matrix-isolated CH₄ and F₂ and their heterodimers CH₄?F₂ are present in the samples and it takes into account the long-range migration of translationally excited flourine atoms. The CH₃F?HF complexes and radicalsI are generated by the photolysis of the CH₄?F₂ heterodimers. The decay of radicalsI is caused by geminate recombination of proximate F...CH₃ pairs. RadicalsII are formed in the reaction of translationally excited fluorine atoms with isolated CH₄ (CD₄) molecules.     

Electrochemical and XPS measurements on thin oxide films on zirconium

The potentiodynamic growth of thin oxide films on zirconium electrodes was investigated by coulometric and simultaneous impedance measurements, as a function of the electrode potential (0 V ⩽ E ⩽ 9 V), the pH (0 ⩽ pH ⩽ 14) and the surface preparation (electropolishing, etching and mechanical polishing). The initial film thickness d₀ is at least 4–6 nm; with increasing potential, the oxide grows irreversibly by 2.6 nm/V (pH 0.3) up to 3.2 nm/V (pH 14). In Cl⁻- and ClO⁻₄-containing solutions the oxide growth is limited by localized corrosion. The oxide behaves like a typical insulator with a donor concentration N_D < 10¹⁹ cm⁻³ and a dielectric constant D = 31. Below −0.5 V (vs. SHE) only, th film behaves like an n-type semiconductor with N_D ≈ 3 × 10¹⁹ cm⁻³. From photoelectrochemical measurements a direct and an indirect transition with band gap energies of E_g = 5 eV and E_g = 2.8 eV could be derived. Anodic electron-transfer reactions (ETRs) are blocked at the homogeneous oxide surface, but cathodic ETRs are possible at larger overvoltages. Near the flatband potential E_fb ≈ −1.3 ± 0.2 V (vs. SHE) hydrogen evolution takes place with a simultaneous increase of the capacity which may be attributed to hydrogen incorporation. With XPS measurements the stoichiometry of the oxide film was determined as ZrO₂ at all the pH values examined, but a thin outer layer contained some hydroxide. Components of the forming electrolyte could not be detected (sulphate, borate and perchlorate < 1%), but etching in HF caused accumulation of F⁻ at the inner boundary.     

Thermische Zerfallsreaktionen von Nitroverbindungen I: Dissoziation von Nitromethan

The thermal decomposition of CH₃NO₂ highly diluted in Ar has been studied in shock waves at 900 < T < 1500 K and 1.5 · 10^?5 < [Ar] < 3.5 · 10^?4 mol/cm³. Concentration profiles of CH₃NO₂ and NO₂ were recorded. The unimolecular reaction was found to be in its fall-off range. Limiting low pressure rate constants of k₀ = [Ar] · 10^17.1 exp(?42(kcal/mol)/RT) cm³/ mol sec in the range 900 < T < 1400 K and limiting high pressure rate constants of k_∞ = 10^16.25 exp (?(58.5 ± 0.5 kcal/mol)/RT) sec^?1 have been derived. A rate constant of 1.3 · 10¹³ cm³/mol sec was found for the first subsequent reaction CH₃+NO₂ → CH₃O+NO.     

Elastic differential cross sections and intermolecular potentials for Ar + CH4 and Ar + NH3

Differential elastic cross sections are reported for CH₄ + Ar (E = μg²/2 = 8.43 kJ/mole) and NH₃ + Ar (E = 8.31 kJ/mole) in the region of the rainbow angles. Quantum interference undulations are apparently observed as well for CH₄ + Ar and, possibly, NH₃ + Ar. The measurements are fit to spherically symmetric intermolecular potentials yielding well depths and equilibrium intermolecular separations of 1.32 kJ/mole and 3.82 Å for CH₄ + Ar and 1.32 kJ/mole and 3.92 Å for NH₃ + Ar.     

Reexamination of the phosphorescence of Ba2Pt2(H2P2O5)4 via thermally modulated emission spectroscopy

Reexamination of the phosphorescence of Ba₂Pt₂(H₂P₂O₅)₄ reveals that the ≈10 K spectrum is a superposition of two electronic transitions [³A₂u(E_u,A_1u → A_1g] separated by ≈40 cm^?1. Each band displays a prominent 110 cm^?1 vibrational progression. Franck-Condon analysis yields a ≈0.25 Å distortion of the PtPt bond in the excited states, interpreted as a contraction.     

A precise determination of the first ionization potential of benzene

The first ionization potentials of benzene and benzene-d₆ have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-photon ionization spectrum of the jet-cooled molecule. The convergence of resolved transitions in two Rydberg series for principal quantum numbers as high as 14 (-h₆) and 15 (-d₆) establish adiabatic thtesholds of 74573.0 ± 2.0 cm^?1, and 74592.5 = ± 1.2 cm^?1, respectively. These results are crucial for the understanding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been obtained for several Rydberg series and their variation with principal quantum number is reported. The results strongly suggest that the R? series of Wilkinson is derived from aπ(e_1g)→ nf±1 Rydberg excitation.     

Dielectronic recombination of lithium-like Ar15+

Dielectronic recombination (DR) of Ar¹⁵⁺(1s ²2s) ions was studied in a single-pass merged-beams experiment at the UNILAC (universal linear accelerator) of GSI. Absolute recombination rates and cross sections were measured for electron-ion center-of-mass energies from 0 to 580 eV. A number of Rydberg states formed by DR with 2s → 2p (Δn=0) and 2s → 3? (Δn=1) core excitations and even individual terms in the 1s ²3?3?′ configuration could be resolved. Theoretical calculations of DR cross sections are in good overall agreement with the data. In the calculations for Δn=0 transitions, effects of electric fields have to be included to reproduce the magnitude of the measured DR rates at the limit of the 2 _{p 1/2}? and 2 _{p 3/2}? Rydberg series. Discrepancies between theory and experiment are observed at the series limits of the (1s ²3?n?′) Rydberg series.     

Electronic band structure of solid CO2 as determined from the hv-dependence of photoelectron emission

Photoelectron energy distribution curves from solid CO₂ have been determined for excitation energies from hv = 14 up to 40 eV using synchrotron radiation. A 1:1 correspondence to the gas-phase photoelectron spectrum is observed for the occupied molecular orbitals. The vertical binding energies E_B^v (E_VAC = 0) and widths (fwhm) of the valence bands of solid CO₂ are determined to be 13.0 and 0.95 eV (1π_g); 16.7 and 1.1 eV (1π_u); 17.6 and 0.85 eV (3σ_u) and 18.8 and 0.8 eV (4σ_g) for the individual bands respectively. The partial photoemission cross sections differ importantly from those of the gas phase in exhibiting pronounced maxima at 5.2 eV (1π_g), 4.4–5.3 eV (1π_u + 3σ_u) and 4.2 eV (4σ_g) above the vacuum level, which is attributed to effects of high density of final (conduction-band) states. Further weaker maxima are observed at higher photon energies. Contrary to the case for the gas phase, the resonances are unperturbed in the solid by degenerate autoionizing molecular Rydberg states. The molecular origin of the resonances in the continuum is discussed and related to X-ray absorption spectra, electron-scattering data and to theoretical cross-section calculations. It is shown that the same set of resonances is observed in the different experiments. The resonances occur however at different energies due to different Coulomb interactions. The photoemission results presented provide also a key to the hitherto unexplained optical spectrum of solid CO₂ in the VUV range, making possible an assignment of the structures observed to Frenkel-type excitons (hv ≤ 15 eV) and interband transitions (hv ? 15 eV).     

Kinetics of the gas‐phase reactions of chlorine atoms with CH2F2, CH3CCl3, and CF3CFH2 over the temperature range 253–553 K

Relative rate techniques were used to study the title reactions in 930–1200 mbar of N₂ diluent. The reaction rate coefficients measured in the present work are summarized by the expressions k(Cl + CH₂F₂) = 1.19 × 10^?17 T² exp(?1023/T) cm³ molecule^?1 s^?1 (253–553 K), k(Cl + CH₃CCl₃) = 2.41 × 10^?12 exp(?1630/T) cm³ molecule^?1 s^?1 (253–313 K), and k(Cl + CF₃CFH₂) = 1.27 × 10^?12 exp(?2019/T) cm³ molecule^?1 s^?1 (253–313 K). Results are discussed with respect to the literature data. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 401–406, 2009     

Interstate coupling and dynamics of excited singlet states of isolated diphenylbutadiene

In this paper, we report on absolute fluorescence quantum yields from photoselected vibrational states of jet-cooled 1,4-diphenylbutadiene for excess vibrational energies, E_v = 0−7500 cm⁻¹, above the apparent electronic origin of the S₁(2A_g) state. The pure radiative lifetimes, τ_r, of the strongly scrambled S₂(1B_u)—S₁(2A_g) molecular eigenstates (E_v = 1050−1800 cm⁻¹) show a marked dilution effect, (τ_r/τ_r(S₂) ≈ 40), being practically identical with the τ_r values from the S₁(2A_g) manifold (E_v = 0–900 cm⁻¹), which is affected by near-resonant vibronic coupling to S₂(1B_u) and exhibiting the dynamic manifestations of the intermediate level structure. Isomerization rates in the isolated molecule, which do not exhibit vibrational mode selectivity, were recorded over the energy range 0–6600 cm⁻¹ above the threshold.     

Conductibilité electrique aux basses températures des composés binaires Cr2X3 et Cr3X4 (X = S,Se ou Te)

Electrical resistivity of some binary compounds Cr₂X₃ and Cr₃X₄ (X = S, Se or Te) is studied on polycrystalline samples with the four point probe method, at temperatures between 4.2 and about 330 K. A metallic behavior is observed on Cr₂Te₃, Cr₃Te₄, Cr₃S₄ and the 3c′ form of Cr₂Se₃. Some other compounds are semiconductors: Cr₃Se₄ (E_300K ≈ 0.07 eV; E_4.2K = 2.07 × 10^?4 eV), the 2c′ form of Cr_2+εSe₃ (E_300K ≈ 0.074 eV; E_4.2K = 2.76 × 10^?4 eV) and the 3c′ form of Cr₂S₃ (E_275K ≈ 0.55 eV). The observed results seem to be closely related to the nature of the octahedral neighborhood of the cations.     

Interatomic potential for the X1Σ state of Be2, revisited

An extended geminal model has been applied to determine the interatomic potential for the X¹Σ state of Be₂. By adopting a (23s, 10p, 8d, 6f, 3g, 2h) uncontracted Gaussian‐type basis, the following spectroscopic parameters are obtained: R_e = 4.633 a.u. (4.63 a.u.), D_e = 945 ± 15 cm (790 ± 30 cm), G(1)–G(0) = 221.7 cm^?1 (223.8 ± 2 cm^?1), G(2)–G(1) = 175.0 cm^?1 (169 ± 3 cm^?1), G(3)–G(2) = 123.1 cm^?1 (122 ± 3 cm^?1), and G(4)–G(3) = 80.8 cm^?1 (79 ± 3 cm^?1), experimental values in parentheses. The calculated binding energy is substantially higher than the accepted experimental value. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Light absorption by manganese pairs in KMgF3:Mn2+ through a vibronically induced exchange mechanism

Double excitations to ⁴E_g and ⁴A_1g states in manganese pairs of KMgF₃:Mn²⁺ have been studied spectroscopically. Three prominent sharp bands are observed in the low temperature absorption spectrum. These bands are displaced by about 400–500 cm^?1 to higher energies from the expected electronic origins. It is proposed that the observed pair transitions gain their intensity through a vibronically induced exchange mechanism. With this mechanism the symmetric double excitations ⁶A_1g⁶A_1g → ⁴E_gu⁴E_gu, ⁴E_gv⁴E_gv and ⁴A_1g⁴A_1g become allowed in addition to the transition ⁶A_1g⁶A_1g → ⁴E_gu⁴A_1g. Analogy to the spectrum of the linear dinuclear chromium complex [(NH₃)₅CrOCr(NH₃)₅]⁴⁺, where the same mechanism has been postulated, is demonstrated.     

A MS-CASPT2 study of the low-lying electronic excited states of CH2BrCl

The low-lying singlet excited states of CH₂BrCl have been calculated using multiconfigurational CASSCF, second-order perturbation theory CASPT2 and its multistate extension MS-CASPT2. The CASSCF method shows spurious valence–Rydberg mixing and a wrong order of states. Inclusion of dynamical correlation by single root CASPT2 lowers dramatically the energy of the valences states but does not lead to a complete separation between valence and Rydberg states. This situation is improved by the MS-CASPT2 calculations, which gives two valence states for both A^′ and A″ symmetries below the lowest Rydberg state, corresponding to n(Br)→σ^*(C–Br) and n(Cl)→σ^*(C–Cl) transitions at 6.1 eV (203 nm) and 7.2 eV (173 nm), and being repulsive along C–Br and C–Cl coordinates.     

Energy dependence of rotational and vibrational distributions of N2(C) for the Ar*(3P0,2) + N2(X) excitation transfer reaction

The Ar* + N₂(X) → N₂(C, v′, N′) + Ar excitation transfer reaction has been investigated experimentally in two different atomic beam experiments. The inelastic cross sections Q_{v′ = 0}(E) and Q_{v′ = 1}(E) to the v′ vibrational level have been measured in the energy range 0.06 ⩽ E(eV) ⩽ 6, using a crossed beam machine. Both cross sections show a behaviour typical for a curve crossing mechanism, with maximum values Q₀ = 8.0 Ų and Q₁ = 1.2 Ų at E = 0.16 eV and E = 0.13 eV, respectively. The oscillatory behaviour of the ratio Q₁(E)/Q₀(E), as first observed by Cutshall and Muschlitz, is also present in our data. Within the model of Gislason et al. the results indicate a decreasing bond stretching with increasing energy. As an alternative we discuss the possibility that the oscillation is due to a different energy dependence of the cross sections for the Ar*(³P₀) and Ar*(³P₂) fine structure states in the mixed beam of metastable Ar*. The vibrational and rotational distributions have also been measured at E = 0.065 eV in a small scale atomic beam-scattering cell experiment, which can be considered as an intermediate between a bulk experiment and a crossed beam experiment. The relative vibrational populations are n_v′ = 100, 16.0, 3.03 and 0.31 for v′ = 0 through 3, with rotational “temperatures” of T_rot,v′ = 1960, 1010, 370 and 130 K. Pronounced deviations (“hump”) of the Boltzmann rotational distributions occur at N′ ≈ 27 for v′ = 0, 1 and 2, with a fractional population of 1, 3 and 11%. For v′ = 0 the “hump” is largely obscured by overlap with the v′ = 1 bandhead. These bimodal distributions are in qualitative agreement with the results of Nguyen and Sadeghi for v′ = 0. The results are discussed within the framework of a curve crossing mechanism with the Ar⁺-N⁻₂ diabatic potential as an intermediate. By assuming equal charges on both N atoms the Coulomb potential of the collinear orientation lies lower (0.45 eV at R = 2.5 Å) than the perpendicular orientation, with the consequence of different transfer probabilities for both orientations. Within a classical model or rotational excitation the final N′ values can be calculated for both orientations, resulting in much higher N′ values for the perpendicular orientation. This mechanism supplies a qualitative explanation for the observed bimodal rotational distributions.