Absolute electronic excitation cross sections for low-energy electron (5-12 eV) scattering from condensed thymine

The absolute cross sections for electronic excitations of thymine by electron impact between 5 and 12 eV are determined by means of electron-energy loss (EEL) spectroscopy for the molecule deposited at submonolayer coverage on an inert Ar substrate. The lowest EEL features at 3.7 and 4.0 eV are attributed to the excitation of the triplet 1 3A'(pi --> pi*) and 1 3A'(n --> pi*) valence states of the molecule. The higher EEL features located at 4.9, 6.3, 7.3, and 9 eV with a weak shoulder around 6 eV are ascribed mostly to triplet valence (pi --> pi*) excitation manifold of the molecule. The energy dependence of the cross section for both the lowest triplet valence excitations shows essentially a peak at about 5 eV reaching a value of 2.9 x 10(-17) cm2. The cross sections for the higher EEL features are generally characterized by a common broad maximum around 8 eV. The latter reaches a value of 1.36 x 10(-16) cm2 for the combined 6 and 6.3 eV excitation region. The maxima in the present cross sections are found to correspond to the resonances that have been reported at about the same energies in the O- yield from electron impact on thymine in the gas phase.     

Absolute vibrational and electronic cross sections for low-energy electron (2-12 eV) scattering from condensed pyrimidine

Low-energy vibrational and electronic electron-energy-loss (EEL) spectra of pyrimidine condensed on a thin film of solid argon held at 18 K are reported for the incident-energy range of 2-12 eV. Sensitivity to symmetry and spin forbidden transitions as well as correlations to the triplet states of benzene make it possible to ascribe the main features, below 7 eV in the electronic part of the EEL spectrum, to triplet transitions. The lowest EEL feature with an energy onset at 3.5 eV is attributed to a transition to the (3)B(1)(n-->pi(*)) valence electronic state and the next triplet n-->pi(*) transition to a (3)A(2) state located around 4.5 eV. The remaining EEL features at 4.3, 5.2, 5.8, and 6.5 eV are all assigned to pi-->pi(*) transitions to states of symmetry (3)B(2), (3)A(1), (3)B(2), and (3)B(2)+(3)A(1), respectively. The most intense maximum at 7.6 eV is found to correspond to both (1)B(2) and (1)A(1) transitions, as in the vacuum ultraviolet spectra. Absolute inelastic cross sections per scatterer are derived from a single collision treatment described herein. Their values are found to lie within the 10(-17) cm(2) range for both the electronic and the vibrational excitations. Features in the energy dependence of the cross sections are discussed, whenever possible, by comparison with data and mechanisms found in the gas phase. A maximum over the 4-5 eV range is attributed to a B (2)B(1) shape resonance and another one observed in the 6-7 eV range is ascribed to either or both sigma(*) shape resonances of (2)A(1) and (2)B(2) symmetries.     

The photoionization cross section for H2 from threshold to 30 eV

The photoionization cross section of H₂ is calculated from threshold to 30 eV. The initial state is the self-consistent-field ground state of H₂ and the final states are the ground electronic state of H⁺₂ and single-center continuum states calculated in the field of H⁺₂. Exchange and screening effects are included.     

Total reactive cross section for K + Br2 in the energy range of 0–4 eV

3-D classical trajectory calculations were performed using diabatic as well as adiabatic potential energy surfaces. Non-adiabatic transitions were allowed and localized at the avoided crossing of the two adiabatic surfaces. The transition probability was calculated according to the Landau—Zener formalism. The total cross sections for the reaction K + Br₂ → KBr + Br were calculated and compared with experimental data. The total cross sections, calculated with the aid of adiabatic potential energy surfaces, were, contrary to those with diabatic surfaces, in very good agreement with the measured total reactive cross sections over the whole energy range of 0–4 eV.     

Calculated cross section for photoionization from the 2π and 5σ orbitals of hydrogen chloride

Single-center expansion calculations or the photoionization cross section for the two highest-lying valence orbitals of HCl are presented. Both dipole-length and dipole-velocity forms have been used and agreement between the two cross sections is of the same quality as for comparable calculations on Ar. The role of various spherical-wave outgoing channels is discussed.     

Emission cross sections of balmer-α,β,γ radiation for electrons (0–2000 eV) on H2 and D2

Absolute emission cross sections (σ_em) of Balmer-α,β, γ radiation have been measured in the case of electrons (0–2000 eV) incident on H₂ and D₂. The H Balmer σ_em's are larger than the corresponding D Balmer σ_em's for every electron impact energy. It has been found that this isotope effect varies strongly as a function of the incident electron energy below 100 eV. The isotope effect will be interpreted using the H₂ and D₂ potential energy curves.     

Absolute photoionization cross section of the ethyl radical in the range 8-11.5 eV: synchrotron and vacuum ultraviolet laser measurements

The absolute photoionization cross section of C(2)H(5) has been measured at 10.54 eV using vacuum ultraviolet (VUV) laser photoionization. The C(2)H(5) radical was produced in situ using the rapid C(2)H(6) + F → C(2)H(5) + HF reaction. Its absolute photoionization cross section has been determined in two different ways: first using the C(2)H(5) + NO(2) → C(2)H(5)O + NO reaction in a fast flow reactor, and the known absolute photoionization cross section of NO. In a second experiment, it has been measured relative to the known absolute photoionization cross section of CH(3) as a reference by using the CH(4) + F → CH(3) + HF and C(2)H(6) + F → C(2)H(5) + HF reactions successively. Both methods gave similar results, the second one being more precise and yielding the value: σ(C(2)H(5))(ion) = (5.6 ± 1.4) Mb at 10.54 eV. This value is used to calibrate on an absolute scale the photoionization curve of C(2)H(5) produced in a pyrolytic source from the C(2)H(5)NO(2) precursor, and ionized by the VUV beam of the DESIRS beamline at SOLEIL synchrotron facility. In this latter experiment, a recently developed ion imaging technique is used to discriminate the direct photoionization process from dissociative ionization contributions to the C(2)H(5)(+) signal. The imaging technique applied on the photoelectron signal also allows a slow photoelectron spectrum with a 40 meV resolution to be extracted, indicating that photoionization around the adiabatic ionization threshold involves a complex vibrational overlap between the neutral and cationic ground states, as was previously observed in the literature. Comparison with earlier photoionization studies, in particular with the photoionization yield recorded by Ruscic et al. is also discussed.     

Transport coefficients and cross sections for electrons in water vapour: comparison of cross section sets using an improved Boltzmann equation solution

This paper revisits the issues surrounding computation of electron transport properties in water vapour as a function of E/n(0) (the ratio of the applied electric field to the water vapour number density) up to 1200 Td. We solve the Boltzmann equation using an improved version of the code of Ness and Robson [Phys. Rev. A 38, 1446 (1988)], facilitating the calculation of transport coefficients to a considerably higher degree of accuracy. This allows a correspondingly more discriminating test of the various electron-water vapour cross section sets proposed by a number of authors, which has become an important issue as such sets are now being applied to study electron driven processes in atmospheric phenomena [P. Thorn, L. Campbell, and M. Brunger, PMC Physics B 2, 1 (2009)] and in modeling charged particle tracks in matter [A. Munoz, F. Blanco, G. Garcia, P. A. Thorn, M. J. Brunger, J. P. Sullivan, and S. J. Buckman, Int. J. Mass Spectrom. 277, 175 (2008)].     

Induction of strand breaks by low-energy electrons (8-68 eV) in a self-assembled monolayer of oligonucleotides: effective cross sections and attenuation lengths

Self-assembled monolayers of 5'-32P-labeled 3'-thiolated oligonucleotides chemisorbed on gold were bombarded by low-energy electrons (LEE) of 8-68 eV. Shorter 5'-32P-oligonucleotides produced by LEE-induced strand breaks were separated with denaturing polyacrylamide gel electrophoresis and quantified by phosphor imaging. The yields of short oligonucleotides (y) decrease exponentially with their length (n), following the equation y=ae-bn, where a and b are constants, which are related to the average effective cross section per nucleotide for DNA strand break (sigmaeff) and the attenuation length (AL=1b) of LEE, respectively. The AL decreases with LEE energies from 2.5+/-0.6 nm at 8 eV to 0.8+/-0.1 nm at 68 eV, whereas sigmaeff increases from (3+/-1)x10(-18) to (5.1+/-1.6)x10(-17) cm2 within the same energy range. The energy dependence of sigmaeff shows a resonance peak of (2.8+/-0.9)x10(-17) cm2 at 18 eV superimposed on a monotonically rising curve. Transient electron attachment to a sigma* anion state of the deoxyribose group, followed by dipolar dissociation into H- and the corresponding positive-ion radical, leading to C-O bond cleavage, is proposed to account for this maximum.     

Observed translational energy dependence of the K + C2H5I → KI + C2H5 reaction cross section from 0.17 to 0.55 eV (c.m.)

Relative values of the total reaction cross section σ_R for the crossed molecular beam reaction K + C₂H₅I → KI + C₂H₅ have been measured over the translational energy (E_T) range 0 17–0.55 eV. It is found that σ_R decreases monotonically with E_T over this range, any maximum in σ_R(E_T) is presumed to lie below 0.17 eV.     

1-Bromo-2-trifluoroacetylcyclobutenes as novel building blocks for the construction of trifluoromethyl substituted heterocycles. Part 1: Synthesis of 5-(trifluoromethyl)-2(5H)-furanones condensed with substituted cyclobutenes

The regioselective reduction of substituted 1-bromo-2-trifluoroacetylcyclobutenes by lithium aluminium hydride affords corresponding brominated alcohols, which, under the treatment of two equivalents of butyllithium, give new lithiated cyclobutenes. Their carboxylation followed by lactonization induced by trifluoroacetic anhydride appeared to be an effective approach towards 5-trifluoromethylated furanones condensed with substituted cyclobutene rings.     

Electron transfer and CO addition to polynitrido cobalt carbonyl clusters: parallel pathways for conversion of the [Co10N2(CO)19]4- anion to the novel [Co11N2(CO)21]3- anion

The redox aptitude of the dinitrido anion [Co10N2(CO)19]4- has been tested from both chemical and electrochemical points of view, together with its reactivity toward CO that induces disproportionation. In any case, through a remarkable overlapping of intermediate steps, the new anion [Co11N2(CO)21]3- (4) is eventually obtained. A detailed study of the pathways to 4 allowed the identification of three labile intermediates by their characteristic IR spectra as well as their electrochemical and paramagnetic properties. The unprecedented structure of trianion 4 has been studied in details in two different crystalline salts.     

A novel catalyst for alkene epoxidation: a polymer-supported CoLCl2 {L = 2-(alkylthio)-3-phenyl-5-(pyridine-2-ylmethylene)-3,5-dihydro-4H-imidazole-4-one} complex

A polymer-supported complex based on 3-phenyl-5-(2-pyridylmethylidene)-2-thiohydantoin has been synthesized and employed as a catalyst for the epoxidation of alkenes using iodosylbenzene and hydrogen peroxide as oxidants.     

(Eta-C5H4R)Fe(CO)2X], X = Cl, Br, I, NO3, CO2Me and [(eta-C5H4R)Fe(CO)3]+, R = (CH2)nCO2Me (n = 0-2), and CO2CH2CH2OH: a new group of CO-releasing molecules

A new group of CO-releasing molecules, CO-RMs, based on cyclopentadienyl iron carbonyls have been identified. X-Ray structures have been determined for [(eta-C(5)H(4)CO(2)Me)Fe(CO)(2)X], X = Cl, Br, I, NO(3), CO(2)Me, [(eta-C(5)H(4)CO(2)Me)Fe(CO)(2)](2), [(eta-C(5)H(4)CO(2)CH(2)CH(2)OH)Fe(CO)(2)](2) and [(eta-C(5)H(4)CO(2)Me)Fe(CO)(3)][FeCl(4)]. Half-lives for CO release, (1)H, (13)C, and (17)OC NMR and IR spectra have been determined along with some biological data for these compounds, [(eta-C(5)H(4)CO(2)CH(2)CH(2)OH)Fe(CO)(3)](+) and [[eta-C(5)H(4)(CH(2))(n)CO(2)Me]Fe(CO)(3)](+), n = 1, 2. More specifically, cytotoxicity assays and inhibition of nitrite formation in stimulated RAW264.7 macrophages are reported for most of the compounds analyzed. [(eta-C(5)H(5))Fe(CO)(2)X], X = Cl, Br, I, were also examined for comparison. Correlations between the half-lives for CO release and spectroscopic parameters are found within each group of compounds, but not between the groups.     

Reactions of germanium atoms and small clusters with CO: experimental and theoretical characterization of Ge(n)CO (n = 1-5) and Ge2(CO)2 in solid argon

Reactions of germanium atoms and small clusters with carbon monoxide molecules in solid argon have been studied using matrix isolation infrared absorption spectroscopy. Besides the previously reported GeCO monocarbonyl, the Ge2(CO)2 and Ge(n)CO (n = 2-5) carbonyl molecules are formed spontaneously on annealing and are characterized on the basis of isotopic substitution and theoretical calculations. It is found that Ge2CO, Ge3CO, and Ge5CO are bridge-bonded carbonyl compounds, whereas Ge2(CO)2 and Ge4CO are terminal-bonded carbonyl molecules.     

Hydrido-Silyl-Komplexe. V. Strukturelle Änderungen in Hydrido-Silyl-Komplexen infolge Si?H-Wechselwirkung; Strukturvergleich zwischen (π-CH3C5H4)(CO)2Mn(H)SiR3- und analogen (π-C5H5)(CO)2FeSiR3-Komplexen

Hydrido Silyl Complexes. V. Structural Changes in Hydrido Silyl Complexes Due to Si? H Interaction; Comparison between the Structures of (π-CH₃C₅H₄)(CO)₂Mn(H)SiR₃ and Analogous (π-C₅H₅)(CO)₂FeSiR₃ Complexes The structures of the complexes Cp(CO)₂FeSiFPh₂ ( 2a ) and Cp(CO)₂FeSiCl₃ ( 2b ), containing ?normal”? metal-silicon bonds, are compared with the known structures of the complexes MeCp(CO)₂Mn(H)SiFPh₂ ( 1a ) and MeCp(CO)₂Mn(H)SiCl₃ ( 1b ), containing Mn? H? Si three-center bonds. 2a crystallizes in space group P2₁/c, a = 805.8(3), b = 1417.5(6), c = 1498.2(4) pm, β = 90.99(3)°, 2b in space group P2₁/n, a = 817(1), b = 1244(1), c = 1142(1)pm, β = 99.9(1)°. In 2a gauche conformation in respect to the Fe? Si bond is found; in 2b the silylligand is rotated 12° around the Fe? Si axis out of a staggered conformation. Possible reasons for the unsymmetrical conformations are discussed. Lengthening of the Fe? Si distance from 221.6(1)pm in 2b to 227.8(1) pm in 2a corresponds to the expected increase in the bond radius of silicon due to exchange of the substituents at silicon. Additional lengthening of the Mn? Si distance by about 3.5 pm in 1a compared with 1b is attributed to the increased delocalization of the Mn? H? Si bond in 1a .