Electron affinity and redox potential of tetrafluoro-p-benzoquinone: A theoretical study

The electron affinity of tetrafluoro-p-benzoquinone (2.69 eV) and the mono- (2.10 eV), 2,3-di- (2.29 eV), 2,5-di- (2.28 eV), 2,6-di- (2.31 eV) and tri- (2.48 eV) fluoro derivatives of p-benzoquinone have been calculated via standard ab initio molecular orbital theory at the G3(MP2)-RAD level of theory. Comparison of calculated electron affinities with the available experimental values shows excellent agreement between theory and experiment. The reduction potential of tetrafluoro-p-benzoquinone in acetonitrile vs. SCE (−0.03 V) has been calculated at the same level of theory and employing a continuum model of solvation (CPCM), and is also in excellent agreement with the experimental value (−0.04 V vs. SCE).     

Recombination of simple molecular ions studied in storage ring: dissociative recombination of H2O+

Dissociative recombination of vibrationally relaxed H2O+ ions with electrons has been studied in the heavy-ion storage ring CRYRING. Absolute cross-sections have been measured for collision energies between 0 eV and 30 eV. The energy dependence of the cross-section below 0.1 eV is found to be much steeper than the E-1 behaviour associated with the dominance of the direct recombination mechanism. Resonant structures found at 4 eV and 11 eV have been attributed to the electron capture to Rydberg states converging to electronically excited ionic states. Complete branching fractions for all dissociation channels have been measured at a collision energy of 0 eV. The dissociation process is dominated by three-body H + H + O breakup that occurs with a branching ratio of 0.71.     

Studies of the Auger spectrum from the (100) surface of GaAs using positron annihilation induced Auger electron spectroscopy

Measurements of the first high-resolution positron annihilation induced Auger spectrum from GaAs(100) are presented. The spectrum displays six As and three Ga Auger peaks below 110 eV, including a strong As M_4,5VV peak at 28 eV and a less intense Ga M_2,3M_4,5M_4,5 peak at 53 eV. The Auger peak intensities are used to obtain experimental annihilation probabilities for relevant core-level electrons. Experimental results are compared with first-principles calculations of positron surface states and annihilation characteristics of surface trapped positrons.     

Photoluminescence of boron carbide

The excitation of B_4.3C with an Ar-laser () yields a photoluminescence spectrum between about 1.56 and 1.58 eV with its main maximum at 1.563 eV and a weaker maximum at 1.572 eV. It is attributed to the indirect-allowed recombination of free excitons.     

Development of soft X-ray multilayer laminar-type plane gratings and varied-line-spacing spherical grating for flat-field spectrograph in the 1–8 keV region

W/C and Co/SiO₂ multilayer laminar-type holographic plane gratings (groove density 1/σ = 1200 lines/mm) in the 1–8 keV region are developed. For the Co/SiO₂ grating the diffraction efficiencies of 0.41 and 0.47 at 4 and 6 keV, respectively, and for the W/C grating 0.38 at 8 keV are observed. Taking advantage of the outstanding high diffraction efficiencies into practical soft X-ray spectrographs a Mo/SiO₂ multilayer varied-line-spacing (VLS) laminar-type spherical grating (1/σ = 2400 lines/mm) is also developed for use with a flat field spectrograph in the region of 1.7 keV. For the Mo/SiO₂ multilayer grating the diffraction efficiencies of 0.05–0.20 at 0.9–1.8 keV are observed. The FWHMs of the measured line profiles of Hf-Mα₁(1644.6 eV), Si-Kα₁(1740.0 eV), and W-Mα₁ (1775.4 eV) are 13.7 eV, 8.0 eV, and 8.7 eV, respectively.     

Effects of alkyl substitution on the energetics of enolate anions and radicals.

The photoelectron spectra of the structural isomers of the three- and four-carbon enolate anions, n-C3H5O(-), i-C3H5O(-), n-C4H7O(-), s-C4H7O(-), and i-C4H7O(-) have been measured at 355 nm. Both the X(2A' ') ground and A(2A') first excited states of the corresponding radicals were accessed from the X(1A') ground state of the enolate anions. The separation energies of the ground and first excited states (T0) were determined: T0[(E)-n-C3H5O] = 1.19 +/- 0.02 eV, T0[(Z)-n-C3H5O] = 0.99 +/- 0.02 eV, T0[i-C3H5O] = 1.01 +/- 0.02 eV, T0[n-C4H7O] = 1.19 +/- 0.02 eV, T0[(2,3)-s-C4H7O] = 1.25 +/- 0.02 eV, T0[(1,2)-s-C4H7O] = 0.98 +/- 0.02 eV, and T0[i-C4H7O] = 1.36 +/- 0.02 eV. The effects of alkyl substitution on the vibronic structure and energetics previously observed in the vinoxy radical are discussed. The X(1A')-X(2A' ') relative stability is strongly influenced by substitution whereas the X(1A')-A(2A') relative stability remains nearly constant for all of the observed structural isomers. Alkyl substitution at the carbonyl carbon affects vibronic structure more profoundly than the energetics, while the converse is observed upon alkyl substitution at the alpha carbon.     

Radical production in the radiolysis of liquid benzene

Iodine scavenging techniques have been used to examine the role of the phenyl radical in the radiolysis of benzene with γ-rays. Biphenyl, the main product in the radiolysis of neat benzene, and iodobenzene yields were determined as a function of iodine concentration. The yield of biphenyl in neat benzene is found to be 0.075 molecules/100 eV and independent of dose up to 1 Mrad. The addition of 0.1 mM iodine increases the biphenyl yield to about 0.4 molecules/100 eV. This increase is thought to be due to a reaction of iodine with the phenyl radical–benzene adduct, which apparently has a very long lifetime. Further addition of iodine leads to a decrease in biphenyl to about 0.09 molecules/100 eV at 30 mM iodine. This decrease is accompanied by an increase in iodobenzene from 0.04 to 0.32 molecules/100 eV. It appears that iodine can effectively compete with benzene for scavenging phenyl radicals.     

Dissociative recombination of fully deuterated protonated acetonitrile, CD3CND+: product branching fractions, absolute cross section and thermal rate coefficient

The dissociative recombination of fully deuterated protonated acetonitrile, CD(3)CND(+), has been investigated at the CRYRING heavy ion storage ring, located at the Manne Siegbahn Laboratory, Stockholm, Sweden. Branching fractions were measured at approximately 0 eV relative collision energy between the ions and the electrons and in 65% of the DR events there was no rupture of bonds between heavy atoms. In the remaining 35%, one of the bonds between the heavy atoms was broken. The DR cross-section was measured between approximately 0 eV and 1 eV relative collision energy. In the energy region between 1 meV and 0.1 eV the cross section data were best fitted by the expression sigma = 7.37 x 10(-16) (E/eV)(-1.23) cm(2), whereas sigma = 4.12 x 10(-16) (E/eV)(-1.46) cm(2) was the best fit for the energy region between 0.1 and 1.0 eV. From the cross section a thermal rate coefficient of alpha(T) = 8.13 x 10(-7) (T/300)(-0.69) cm(3) s(-1) was deduced.     

Experimental and quantum-chemical studies on photoionization and dissociative photoionization of CH2Br2

The dissociative photoionization of CH2Br2 in a region approximately 10-24 eV was investigated with photoionization mass spectroscopy using a synchrotron radiation source. An adiabatic ionization energy of 10.25 eV determined for CH2Br2 agrees satisfactorily with predictions of 10.26 and 10.25 eV with G2 and G3 methods, respectively. Observed major fragment ions CH2Br+, CHBr+, and CBr+ show appearance energies at 11.22, 12.59, and 15.42 eV, respectively; minor fragment ions CHBr2+, Br+, and CH2+ appear at 12.64, 15.31, and 16.80 eV, respectively. Energies for formation of observed fragment ions and their neutral counterparts upon ionization of CH2Br2 are computed with G2 and G3 methods. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. An upper limit of DeltaH0f,298(CHBr+) < or = 300.7 +/- 1.5 kcal mol(-1) is derived experimentally; the adiabatic ionization energy of CHBr is thus derived to be < or = 9.17 +/- 0.23 eV. Literature values for DeltaH0f,298(CBr+) = 362.5 kcal mol(-1) and ionization energy of 10.43 eV for CBr are revised to be less than 332 kcal mol(-1) and 9.11 eV, respectively. Also based on a new experimental ionization energy, DeltaH0f,298(CH2Br2+) is revised to be 236.4 +/- 1.5 kcal mol(-1).     

Free-electron attachment to coronene and corannulene in the gas phase

Electron attachment to the polyaromatic hydrocarbons coronene and corannulene is studied in the electron energy range of about 0-14 eV using a high-resolution crossed electron-neutral beam setup. The major anions observed are the parent anions peaking at about 0 eV with cross sections of 3.8 x 10(-20) and 1 x 10(-19) m(2), respectively. The only fragment anions formed in coronene and corannulene are the dehydrogenated coronene and corannulene anions. Other anions observed in the negative mass spectra at about 0 eV can be ascribed to impurities of the sample. High-level quantum-mechanical studies are carried out for the determination of electron affinities, hydrogen binding energies, and structures of both molecules. The behavior of coronene and corannulene upon electron attachment is compared with that of other polyaromatic hydrocarbons studied previously.     

Direct determination of the ionization energies of PtC, PtO, and PtO2 with VUV radiation

Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +/- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +/- 0.1 eV and IE(PtO2) = 11.35 +/- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +/- 0.07 eV, D0(Pt-O) = 4.30 +/- 0.12 eV, and D0(OPt-O) = 4.41 +/- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules DeltaH(0)(f,0)(PtC(g)) = 701 +/- 7 kJ/mol, DeltaH(0)(f,0)(PtO(g)) = 396 +/- 12 kJ/mol, and DeltaH(0)(f,0)(PtO2(g)) = 218 +/- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides.     

Perturbations of molecular extravalence excitations by rare-gas fluids

In this paper we report the results of an experimental study of the vacuum ultraviolet absorption spectra of molecular impurity states of methyl iodide in Ar (density range ? = 0–1.4 g cm^?3) and in Kr (? = 0–2.3 g cm^?3), of carbon disulphide in Ar (? = 0–1.4 g cm^?3) and of formaldehyde in Ar (? = 0–1.25 g cm^?3). The experimental results provide new information regarding medium perturbations of intravalenc transitions, of the lowest extravalence transitions and of transitions to mixed valence—Rydberg configurations, which serve as a diagnostic tool to distinguish between different types of electronic excitations. All the lowest extravalence molecular excitations exhibit appreciable blue spectral shifts at moderate and at high fluid densities, intravalence transitions are practically insensitive to medium effects, while excitations to mixed valence—Rydberg configurations are characterized by a moderate blue spectral shift. New information has been obtained concerning the energetics of molecular ionization processes in a dense fluid. The high n = 2–5 Rydberg states of CH₃l exhibit a large red shift at moderate (? = 0–0.5 cm^?3) Ar densities. The ionization potential E_g and the effective Rydberg constant G for CH₃I in Ar was found to decrease from G = 13.6 eV and E_g = 9.55 eV at ? = 0 and E_g = 9.08 eV and constant G for CH₃l in Ar was found to decrease from G = 13.6 eV and E_g = 9.55eV at ? = 0 and E_g = 9.08 eV and G ≈ 7.15 eV at ? = 0.5 g cm^?3. Experimental evidence was obtained for the identification of n = 2 molecular Wannier impurity states of CH₃I and of CH₂O in liquid Ar. These spectroscopic data result in E_g ≈ 8.6 eV for CH₃I in liquid Ar and E_g ≈ 10.2 eV for CH₂O in liquid Ar.     

High resolution electron attachment to CO₂ clusters

Electron attachment to CO? clusters performed at high energy resolution (0.1 eV) is studied for the first time in the extended electron energy range from threshold (0 eV) to about 10 eV. Dissociative electron attachment (DEA) to single molecules yields O(-) as the only fragment ion arising from the well known (2)Π(u) shape resonance (ion yield centered at 4.4 eV) and a core excited resonance (at 8.2 eV). On proceeding to CO? clusters, non-dissociated complexes of the form (CO?)(n)(-) including the monomer CO?(-) are generated as well as solvated fragment ions of the form (CO?)(n)O(-). The non-decomposed complexes appear already within a resonant feature near threshold (0 eV) and also within a broad contribution between 1 and 4 eV which is composed of two resonances observed for example for (CO?)(4)(-) at 2.2 eV and 3.1 eV (peak maxima). While the complexes observed around 3.1 eV are generated via the (2)Π(u) resonance as precursor with subsequent intracluster relaxation, the contribution around 2.2 eV can be associated with a resonant scattering feature, recently discovered in single CO? in the selective excitation of the higher energy member of the well known Fermi dyad [M. Allan, Phys. Rev. Lett., 2001, 87, 0332012]. Formation of (CO?)(n)(-) in the threshold region involves vibrational Feshbach resonances (VFRs) as previously discovered via an ultrahigh resolution (1 meV) laser photoelectron attachment method [E. Leber, S. Barsotti, I. I. Fabrikant, J. M. Weber, M.-W. Ruf and H. Hotop, Eur. Phys. J. D, 2000, 12, 125]. The complexes (CO?)(n)O(-) clearly arise from DEA at an individual molecule within the cluster involving both the (2)Π(u) and the core excited resonance.     

C~6F~6^+→C~6F~5^++F气相单分子分解及碰撞诱导分解 反应研究

用质量分析离子动能谱(MIKES)研究了C~6F~6^+→C~6F~5^++F的气相单分子分解及其与Ar和He的碰撞诱导分解(CID)反应。实验结果表明,C~6F~6^+在电离室中获得足够能量而被激发到某一长效激发态,而CID是诱导其分解的必要步骤,且该分解过程有两条能量不同的反应途径。当碰撞气体为Ar时,两条途径所对应的能量变化分别为0eV和+9.8eV(将多余能量转化为动能),而当碰撞气体为He时,则分别为0eV和-17eV(将多余能量转化为内能)。CID/Ar诱导该长效激发态在C-F键断裂之前将多余能量转化为动能,而在CID/He中则将多余能量转化为内能。     

Electron microscopy, spectroscopy, and first-principles calculations of Cs2O

Oxides of cesium play a key role in ameliorating the photoelectron emission of various opto-electronic devices. However, due to their extreme reactivity, their electronic and optical properties have hardly been touched upon. With the objective of better understanding the electronic and optical properties of Cs₂O in relationship to its structure, an experimental and theoretical study of this compound was undertaken. First-principles density functional theory calculations were performed. The preferred structural motif for this compound was found to be anti-CdCl₂. Here three Cs-O-Cs molecular layers are stacked together through relatively weak van-der-Waals forces. The energy bands were also calculated. The lowest transition at 1.45 eV, was found to be between the K point in the valence band to the Γ point in the conduction band. A direct transition at 2 eV was found in the center (Γ) of the Brillouin zone. X-ray powder diffraction, transmission electron microscopy and selected area electron diffraction were used to analyze the synthesized material. These measurements showed good agreement with the calculated structure of this compound. Absorption measurements at 4.2 K indicated two optical transitions with somewhat higher energy (indirect one at 1.65 and a direct transition at 2.2 eV, respectively). Photoluminescence measurements also showed similar transitions, suggesting that the lower indirect transition is enhanced by three nearby minima at 1.5 eV in the Brillouin zone.     

Electron scattering in Pt(PF3)4: elastic scattering, vibrational, and electronic excitation

Experimental absolute differential cross sections for elastic scattering, and for vibrational and electronic excitation of Pt(PF(3))(4) by low-energy electrons are presented. The elastic cross sections have a deep angle-dependent Ramsauer-Townsend minimum (E(min) = 0.26 eV at θ = 135°). The angular distributions of the elastic cross section at and above 6.5 eV show an unusually narrow peak at an angle which decreases with increasing energy (it is at 40° at 20 eV). Wavy structure is observed at higher angles at 15 and 20 eV. Vibrational excitation cross sections reveal five shape resonances, at 0.84, 1.75, 3.3, 6.6, and 8.5 eV. The angular distributions of the vibrational cross sections have a strong forward peak and are nearly isotropic above about 60°. Electronically excited states are characterized by electron energy-loss spectra. They show a number of unstructured bands, the lowest at 5.8 eV. They are assigned to Rydberg states converging to the 1st and 2nd ionization energies. The cross sections for electronic excitation have very high forward peaks, reaching the value of 50 A?(2) at 50 eV and 0° scattering angle. Purity of the sample was monitored by the very low frequency (26 meV) Pt-P stretch vibration in the energy-loss spectra.     

Detection of a second triplet state in thiophosgene by electron-impact spectroscopy

The electron-impact energy-loss spectrum of thiophosgene was investigated at incident energies of 25 eV and 40 eV and scattering angles from 0° to 80°. In these spectra we observe a previously unreported triplet state at 3.1 ± 0.1 eV which is tentatively assigned as the 1³A₁ (π, gp^*) state. This state may play a role in intramolecular radiationless transitions in this molecule.     

Surface transient effects in ultralow‐energy O2+ sputtering of silicon

It is known that by lowering the impact energy the sputter rate and surface transient width in SIMS will be reduced. However, few studies have been done at ultralow energies over a wide range of impact angles. This study examines the dependence of sputter rate and transient width as a function of O₂⁺ primary ion energy (E_p = 250 eV, 500 eV and 1 keV) and incidence angles of 0–70°. The instrument used is the Atomika 4500 SIMS depth profiler and the sample was Si with 10 delta‐layers of Si_0.7Ge_0.3. We observed that the lowest transient width of 0.7 nm is obtainable at normal and near‐normal incidence with E_p ～ 250 eV and E_p ～ 500 eV. There is no significant improvement in transient width going down in energy from E_p ～ 500 to ～250 eV. The onset of roughening is also not obvious at E_p ～ 250 eV over the whole angular range studied. Although the sputter rate during the surface transient is normally different from that at steady state, only at E_p ～ 250 eV was it observed that the sputter rate remained fairly independent of depth. We conclude that the best working ranges to achieve a narrow transient width and accurate depth calibration are at E_p ～ 250 eV/0° < θ < 20°and 500 eV/0°< θ < 10°. Copyright © 2005 John Wiley & Sons, Ltd.     

Complex model optical potential for electron scattering from oxygen

We report calculations of the total (elastic plus inelastic) cross-sections for the e-O system over a wide energy range 10–5000 eV. A local complex optical potential is calculated for the system using the atomic wavefunctions at the Hartree-Fock level. The real part of potential is composed of the attractive static, correlation polarisation and exchange potential. The imaginary component of the complex potential is a function of target charge density, incident electron energy and mean excitation energy. The resulting complex potential is treated in variable phase approach to yield complex phase shifts and total cross-sections. The total and elastic cross-sections are compared with the other available results. The agreement is excellent for total cross-sections with other theoretical work at energies greater than 100 eV. We have excellent agreement for elastic cross-sections with the experimental and theoretical results at all the energies. The elastic differential cross-sections are presented at 50 eV and 1000 eV. We fit the absorption cross-section values to Bethe asymptotic formula in high energy range (≥ 5500 eV). Ionisation cross-sections above 1000 eV are also deduced from the theory.     

Electron attachment to higher fullerenes and to Sc3N@C80

We report on attachment of free electrons to fullerenes C(n) (n = 60, 70, 76, 78, 80, 82, 84, 86) and to Sc(3)N@C(80). The attachment cross sections exhibit a strong resonance at 0 eV for all species. The overall shape of the anion yield versus electron energy is quite similar for the higher fullerenes, with a minimum around 1 eV and a maximum which gradually shifts from 6 eV for C(60) to approximately 4 eV for large n. The endohedral Sc(3)N@C(80) exhibits a particularly shallow minimum and a maximum below 4 eV. We model autoionization of the anions with due consideration of the internal energy distributions. The relatively low electron affinity of Sc(3)N@C(80) is reflected in its reduced ion yield at higher attachment energies.