Measured Electron Affinities Research Articles

Electron affinity of indium and the fine structure of In−measured using infrared photodetachment threshold spectroscopy

Binding energies of the fine-structure levels of the indium negative ion In${}^{\ensuremath{-}}$ are measured using infrared photodetachment threshold spectroscopy. The relative cross section for neutral atom production is measured with a crossed ion-beam--laser-beam apparatus over selected photon energy ranges between 300 and 700 meV. An $s$-wave threshold is observed due to the opening of the In${}^{\ensuremath{-}}$ ($5{p}^{2}$${}^{3}{P}_{0}$) to In($5p$${}^{2}{P}_{1/2}$) ground-state-to-ground-state transition, which determines the electron affinity of In to be 383.92(6) meV. The present result is in good agreement with previous theoretical calculations, but it differs substantially from the previously measured electron affinity and reduces the uncertainty by a factor of 150. $s$-wave thresholds are also observed for detachment from the excited fine-structure levels of In${}^{\ensuremath{-}}$, permitting accurate determination of the fine-structure intervals of 76.06(7) meV for $J=0--1$ and 170.6(6) meV for $J=0--2$, which are in good agreement with the previous measurements and substantially reduce the uncertainties.

Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n- (n = 1−10) Using Photoelectron Spectroscopy

TiO2 is a wide-band-gap semiconductor, and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)n clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)n- clusters for n = 1-10 have been obtained at 193 nm (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)n- clusters for n > 1 appears to be localized in a tricoordinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanistic understanding of TiO2 surface defects and photocatalytic properties.

Chemical formation of neutral complexes from charged metal clusters: reactions of pre-formed aluminum cluster anions with methyl iodide

Aluminum cluster anions were reacted with methyl iodide vapor in a fast-flow tube apparatus, yielding primarily I − and clusters of the type Al n I −. Based on thermodynamic considerations, the presence of I − is taken to indicate a neutralization reaction pathway whereby clusters of the type Al n CH 3 are formed. Comparison of relative reaction rates to previously measured electron affinities suggests that the Al n –CH 3 interaction is covalent, rather than ionic, in nature. It is suggested that the neutral cluster complexes reported here may hold potential as high energy fuel dopants.

Photoelectron spectroscopy of Tin− clusters (n=1–130)

Photoelectron spectra of cold Tin− anion clusters for n=1–130 were investigated at four detachment photon energies: 532, 355, 266, and 193 nm. Improved spectral resolution provides well-resolved electronic structures of the clusters, and the spectral evolution as a function of cluster size was probed systematically. Narrow and well-resolved spectral features were observed at n=13, 19, and 55, consistent with the high symmetry icosahedral structures proposed for these clusters. The measured electron affinities as a function of size in the studied size range do not extrapolate to the bulk work function, indicating that Ti clusters with n=130 may not assume the bulk structure.

Vibration and vibronic coupling ofC20isomers: Ring, bowl, and cage clusters

Three ${\mathrm{C}}_{20}$ isomers ring, bowl, and cage were studied according to hybrid density-functional theory. We determined the stable geometries of neutral and negatively charged clusters, and then calculated their infrared light, Raman, and anion photoelectron spectra. The neutral cage clusters of ${\mathrm{C}}_{i}$ and ${\mathrm{C}}_{2h}$ symmetries are found to have close energies and the calculated photoelectron spectra corresponding to both symmetries resemble experimental spectrum; thus, we give clear assignment of the observed spectrum. On the contrary, we found that there is some difference between the calculated and measured spectra of the bowl, implying that the electron correlation effect plays an important role in this isomer. The measured electron affinities are well reproduced by theoretical calculations. Moreover, a calculation on the vibronic coupling suggests that the solids synthesized from cage ${\mathrm{C}}_{20}$ are candidates for superconductors. To evaluate the stability of the isomers, we estimated the free energy by taking into account the vibrational effect.

In search of theoretically predicted magic clusters: Lithium-doped aluminum cluster anions

Lithium-doped aluminum cluster anions, LiAln− were generated in a laser vaporization source and examined via mass spectrometry and anion photoelectron spectroscopy (n=3–15). The mass spectrum of the LiAln− series exhibits a local minimum in intensity at n=13. The electron affinity vs cluster size trend also shows a dip at n=13. Agreement is quite good between our measured electron affinity values and those calculated by Rao, Khanna, and Jena, suggesting that their predictions about the structure and bonding of LiAl13 and other clusters in this series are also largely valid.

Electronic structure and chemical bonding between the first row transition metals and C2: A photoelectron spectroscopy study of MC2− (M=Sc, V, Cr, Mn, Fe, and Co)

Vibrationally resolved photoelectron spectra of MC2− (M=Sc, V, Cr, Mn, Fe, and Co) are reported at two detachment photon energies, 532 and 355 nm. All the spectra showed a well resolved vibrational progression in the ground state detachment features. Electron affinities, vibrational frequencies, and information about the low-lying electronic states were obtained for the first row transition metal dicarbide molecules. The measured electron affinities for the MC2 species show strong metal-dependence with a minimum at VC2 and a maximum at MnC2. The ground state vibrational frequencies were observed to decrease from ScC2 to a minimum in CrC2 and then increases slightly in MnC2 and FeC2. The trends of the electron affinities and vibrational frequencies for the MC2 species were found to correlate well with the corresponding monoxides, suggesting that the chemical bonding in M–C2 is analogous to that in M–O. The M–C2 bonding was thus interpreted to be quite ionic, and MC2 can be qualitatively viewed as M2+C22−, analogous to M2+O2−.

Study of the low-lying electronic states of CCO by photoelectron spectroscopy of CCO− and ab initio calculations

The low-lying electronic states of CCO have been investigated by photoelectron spectroscopy of CCO− at wavelengths of 266 and 355 nm in conjunction with ab initio calculations. Photodetachment is observed to occur to the X̃ 3Σ−, Ã 3Π, ã 1Δ, and b̃ 1Σ+ electronic states of CCO. This marks the first observation of the low-lying singlet states. A revised value for the electron affinity of CCO is found to be 2.289±0.018 eV. These results are compared with CASPT2 ab initio calculations of the energetics and structure of the ground and excited states of CCO and CCO−. Using the measured electron affinity of CCO, the heats of formation Δf H0298(CCO)=3.99±0.20 eV and ΔfH0298(CCO−)=1.67±0.20 eV are determined. In addition, the C–C bond dissociation energies in CCO and CCO− are determined, as well as the H–CCO bond energy in HCCO.

Spin -Orbit Coupling in the O- 2 Anion

Abstract We could resolve the spin-orbit splitting of 160 ± 4 cm-1 in the 2 II ground state of O2 - anions by high resolution photodetachment photoelectron spectroscopy. The observed splitting is in good agreement with the theoretically derived value. Our directly measured electron affinity of O 2 is 450 ± 2 meV and deviates within experiment errors from previous values. kHz repetition rate was applied to avoid space charge and improve electron energy resolution in a time-of-flight electron energy analyzer.

Observation of laser excitation of rhombic C4 using the coulomb explosion method

The combination of laser photodetachment of C 4 − and the Coulomb Explosion Imaging method was applied for the investigation of the structure of several C4 isomers and was correlated with their measured electron affinities.

Negative ion photoelectron spectroscopy of chromium dimer

The 476-514-nm photoelectron spectra of Cr 2 - , recorded at 4-meV resolution, show vibrationally resolved transitions to the ground state and to one excited electronic state of neutral Cr 2 . The measured electron affinity of Cr 2 is 0.505±0.005 eV. Observed vibrational energies in the 1 Σ g + ground state of Cr 2 up to υ=9, 3040 cm -1 above the zero point level, fit a Morse potential with ω c =480.6±0.5 cm -1 and ω e x e =14.1±0,3 cm -1

Electron affinities and Marcus reorganization energies. A correlation between gas phase electron affinities and solution phase redox potentials

Redox potentials determined by cyclic voltammetry were used in conjunction with published electron affinities to determine the solvation energies for series of three classes of compounds: substituted benzoquinones, substituted nitrobenzenes, and polynuclear aromatic hydrocarbons. Excellent linear correlations were obtained between the measured electron affinities and the E0 of the substrates. The calculated electron affinities, EA (E0), were computed using the average solvation energies for the three classes of compounds, and were found to be in excellent agreement with the measured values. The nitrobenzenes and quinones had one solvation energy, while the aromatic hydrocarbons were correlated with a significantly different value. The solvation energy of a variety of compounds could also be related to their Marcus reorganization energiesλ(0), by a linear plot with a high correlation coefficient. From simple electrochemical measurements of similar compounds, either electron affinities or Marcus λ(0) values can be estimated. Keywords: electron affinities, Marcus reorganization energies, cyclic voltammetry.

Laser photodetachment spectroscopy of Se − and S −

The photodetachment cross sections of Se − and S − ions have been studied. The photodetachment technique used in these studies employed an ion trap of the Penning design. A tunable cw dye laser was used to observe photodetachment from negative selenium ions near the Se(4 3P 2) threshold. The cross section exhibits magnetic field dependent structure which is due to the excitation of the detached electrons to discrete cyclotron levels. The measured electron affinity was found to be 16297.8(2) cm −1. A tunable pulsed dye laser was used to observe photodetachment from negative sulfur ions near the first excited threshold S (3 1D). No resonance structure was observed. The ratio of the cross section for the 1D channel to the cross section for the 3P channel was measured to be 0.19(3) at an energy of 300 cm −1 above the 1D threshold.

One- and two-photon detachment of the negative chlorine ion

The authors report the first observation of tunable non-resonant two-photon photodetachment of an atomic negative ion and compare the two-photon detachment cross section of Cl- to the single-photon detachment one. In each case, Wigner law behaviour is observed, with s-wave detachment for the single-photon process and p-wave detachment for the two-photon process. The measured electron affinity of chlorine is 29 138.3 (5) cm-1, improving the precision of that number by more than an order of magnitude over previous measurements. The order of non-linearity of the two-photon detachment process appears to be less than two. Possible explanations for this anomaly are discussed.

Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of CH−2 and CD−2

Photoelectron spectra are reported for the CH2(X̃ 3B1)+e−←CH−2 (X̃ 2B1) and CH2(ã 1A1)+e−←CH−2 (X̃ 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to &amp;lt;1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 cm−1 above the zero point vibrational level of the 3B1 states, revealing a previously unexplored region of the quasilinear potential surface of triplet methylene. Approximately 20 new vibration-rotation energy levels for CH2 and CD2 are measured to a precision of ∼30 cm−1 in the v2=2–7 region (bent molecule numbering). Bending vibrational frequencies in the methylene anions are determined to be 1230±30 cm−1 for CH− and 940±30 cm−1 for CD−2, and the ion equilibrium geometries are bracketed. The measured electron affinity also provides values for the bond strength and heat of formation of CH−2, and the gas phase acidity of CH3. A detailed description of the new flowing afterglow photoelectron spectrometer is given.

Nitrobenzenes: a comparison of pulse radiolytically determined one-electron reduction potentials and calculated electron affinities

The electron affinities of a number of substituted nitrobenzenes have been calculated by a semiempirical quantum mechanical method HAM/3. Good correlation is found between the calculated electron affinities and one-electron reduction potentials measured by the pulse radiolysis technique. A comparison is also made between published experimentally measured electron affinities and the electron affinities calculated in this paper.

A program for calculating Regge trajectories in potential scattering

The robust Regge-pole methodology has been used to probe for long-lived metastable anionic formation in Cn (n = 20, 24, 26, 28, 44, 70, 92 and 112) through the calculated electron elastic scattering total cross sections (TCSs). All the TCSs are found to be characterized by Ramsauer-Townsend minima, shape resonances and dramatically sharp resonances manifesting metastable anionic formation during the collisions. The energy positions of the anionic ground states resonances are found to match the measured electron affinities (EAs). We also investigated the size-effect through the correlation and polarization induced metastable resonances as the fullerene size varied from C20 through C112. The C20 TCSs exhibit atomic behavior while the C112 TCSs demonstrate strong departure from atomic behavior attributed to the size effect. Surprisingly C24 is found to have the largest EA among the investigated fullerenes making it suitable for use in organic solar cells and nanocatalysis.

Aromatic hydrocarbon-carbonium ion molecular complexes

Molecular complexes of aromatic hydrocarbons and carbonium ions are described according to Mulliken's donor — acceptor theory. Estimations of electron affinities of several cations are deduced from charge-transfer transition energies and the indepently measured electron affinity of the tropylium ion.

Experimental determination of electron affinities. Part 11.—Electron capture by some cyanocarbons and related compounds

A number of cyanocarbons and related compounds have been studied by the magnetron technique and observed to capture electrons directly. These molecules have electron affinities in the range 1–3.5 eV and show evidence for an additive relationship between the number of CN subsituents and the electron affinity. The measured electron affinities are related to the energy of the charge-transfer band in charge-transfer complexes with pyrene and to the reversible one electron-polarographic reduction potentials. The electron affinities measured are related to the work done in bringing the electron up to the molecule against the external field of the dipolar substituents.

Extension of electron affinities and ionization potentials of aromatic hydrocarbons.

The absolute molecular electronegativities for 14 aromatic hydrocarbons are reported. For the hydrocarbons considered, a linear relationship exists between the electron affinity and the energy of the 1La transition. The molecular electronegativity for some eight aromatic hydrocarbons is approximately constant. On this basis, it is possible to predict some ionization potentials which are in excellent agreement with recent experimental values. It does not appear reasonable to relate the intercept resulting from extrapolation of the aforementioned plot to hν=0 to the work function of graphite. Based on the measured electron affinities, it is possible to deduce a relationship with methyl affinities and calculate ΔEsol between neutral hydrocarbons and their mononegative ions. Certain theoretical approaches can be successfully employed to estimate electron affinities of aromatic hydrocarbons.