Lowest Energy Electronic Absorption Research Articles

Large spin asymmetry in electron absorption and reflection from oxidized single crystal Fe/MgO(001) films

High quality single crystal Fe films to be used in spin polarized electron spectroscopy have been prepared in ultra-high vacuum by epitaxial growth on MgO(001) substrates. Oxygen absorption on the clean surface followed by annealing at 600°C produces a well-ordered p(1×1) overlayer. The low energy electron absorption and reflection from the Fe(001)- p(1×1)O surface display a huge dependence on the polarization of the primary beam. Peak values of the spin asymmetries of the absorbed (reflected) current larger than 13% (45%) around 4–8 eV electron kinetic energy are obtained. The presence of the surface oxide protects the system against contamination, so that an almost negligible asymmetry deterioration is observed after prolonged operation in vacuum. The results are discussed in view of applications in electron spin polarimetry.

Spin dependent electron absorption in Fe(001)-p(1×1)O: A new candidate for a stable and efficient electron polarization analyzer

The low energy electron absorption by a p(1×1) ordered oxygen overlayer on magnetized Fe(001) depends on the spin polarization of the primary beam. The peak value of the spin asymmetry is 12.5% at 6 eV electron kinetic energy, where the percentage absorption is 65%. The presence of the surface oxide prevents the system from contamination, so that an almost negligible asymmetry deterioration is observed after prolonged operation in vacuum. After exposure to atmosphere followed by heat-cleaning a peak value still larger than 7% is achieved. These results suggest spin dependent current absorption from Fe(001)-p(1×1)O as a very promising candidate for an efficient and extremely stable electron polarization analyzer.

Correlation of 95Mo NMR chemical shifts, electronic absorption bands, and solvent donor numbers for some dimeric Mo 2(O 2CR) 4L 2 solvent adducts

The 95Mo NMR chemical shifts are reported for two series of solvent adducts of the dimeric molybdenum(II) complexes Mo 2(O 2CR) 4L 2 (R = CF 3, Bu t ; L - solvent). The large deshielding effect (over 450 ppm for the R = CF 3 series as the solvent changes from chloroform to hexamethylphosphoramide) correlates well with the donor number of the solvent. The lowest energy electronic absorption band corresponding to the δ− δ * transition also moves to longer wavelength with the more strongly coordinating solvents, which is consistent with weakening of the MoMo quadruple bond and the dominance of the ΔE parameter in the paramagnetic term which controls the magnitude of δ( 95Mo).

Pressure-Induced Molecular Dissociation and Metallization in Hydrogen-BondedH2SSolid

Infrared spectra at high pressure and room temperature reveal that molecular dissociation and metallization occur in solid ${\mathrm{H}}_{2}\mathrm{S}$ near 46 and 96 GPa, respectively. The disappearance of SH stretching bands in the $2300--2500{\mathrm{cm}}^{\ensuremath{-}1}$ region and simultaneous appearance of a lattice vibrational mode around $1300{\mathrm{cm}}^{\ensuremath{-}1}$ signal the molecular dissociation. At higher pressures, a low-energy electronic absorption band develops and eventually extends throughout the infrared region studied $(700--6000{\mathrm{cm}}^{\ensuremath{-}1})$. Thus, metallization seems to occur by closing a band gap originating from S-S bond formation in the dissociated phase.

Low-energy electronic absorption of C60-

In light of a recent experiment we reexamine the low-energy optical absorption of ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$. Using a tight-binding model augmented by a Hubbard interaction term we do a single-particle configuration-interaction calculation for ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$. The results of our calculation support the interpretation that the observed first electronic transition is actually the second dipole-allowed transition. It is suggested that for calculations in which the state space is expanded the intensity of the first transition will be greatly suppressed relative to the second transition. Consequently the reason that the first transition is not observed in optical experiments is due to electron-electron correlation.

Electronic absorption spectra in C60- and C60+

We calculate the electronic absorption spectra of ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{C}}_{60}^{+}$ within a simple tight-binding model augmented by a Hubbard interaction term. To fit the existing experimental data, the Hubbard U must be of the order of the molecular bandwidth (\ensuremath{\sim}10 eV). We predict low-energy electronic absorption peaks (0.7 eV) which should be accessible to experiments. The question of the existence of these relatively low-energy peaks addresses important issues regarding the role of electron-electron correlation in fullerene systems.

Luminescent heterobimetallic complexes. Electronic structure and spectroscopy of [MPt(dppm)2(CCPh)2]PF6(M = Au or Ag) and crystal structure of [AuPt(dppm)2(CCPh)2]PF6(dppm = Ph2PCH2PPh2)

The crystal structure of [AuPt(dppm)2(CCPh)2]PF6·H2O (dppm = Ph2PCH2PPh2) has been determined: triclinic, space group P, Z= 2, a= 11.146(4), b= 14.512(2), c= 20.322(3)A, α= 81.55(1), β= 101.27(2), γ= 109.35(2)°. The complex cation consists of a square-planar P–Pt(CCPh)2–P and a linear P–Au–P moiety and the measured intramolecular Pt–Au separation is 2.910(1)A. The absorption and emission spectra of the complex and of [AgPt(dppm)2(CCPh)2]PF6 and trans-[Pt(dppm)2(CCPh)2] have been measured. Both dinuclear complexes possess low-energy intense electronic absorption bands {[AuPt(dppm)2(CCPh)2]PF6, λmax= 387 nm, Iµmax= 1.11 × 104 dm3 mol–1 cm–1; [AgPt(dppm)2(CCPh)2]PF6, λmax= 368, Iµmax= 1.63 × 104 dm3 mol–1 cm–1} which are red-shifted from that of trans-[Pt(dppm)2(CCPh)2]. In solid form, all three complexes exhibit photoluminescence at room temperature. Extended-Huckel molecular orbital calculations have been performed on the model complexes [AuPt(dmpm)2(CCPh)2]+ and trans-[Pt(dmpm)2(CCPh)2](dmpm = Me2PCH2PMe2).

The Electronic Absorption Edge of Petroleum

The electronic absorption spectra of more than 20 crude oils and asphaltenes are examined. The spectral location of the electronic absorption edge varies over a wide range, from the near-infrared for heavy oils and asphaltenes to the near-UV for gas condensates. The functional form of the electronic absorption edge for all crude oils (measured) is characteristic of the “Urbach tail,” a phenomenology which describes electronic absorption edges in wide-ranging materials. The crude oils all show similar Urbach widths, which are significantly larger than those generally found for various materials but are similar to those previously reported for asphaltenes. Monotonically increasing absorption at higher photon energy continues for all crude oils until the spectral region is reached where single-ring aromatics dominate absorption. However, the rate of increasing absorption at higher energies moderates, thereby deviating from the Urbach behavior. Fluorescence emission spectra exhibit small red shifts from the excitation wavelength and small fluorescence peak widths in the Urbach regions of different crude oils, but show large red shifts and large peak widths in spectral regions which deviate from the Urbach behavior. This observation implies that the Urbach spectral region is dominated by lowest-energy electronic absorption of corresponding chromophores. Thus, the Urbach tail gives a direct measure of the population distribution of chromophores in crude oils. Implied population distributions are consistent with thermally activated growth of large chromophores from small ones.

Linear optical properties of a series of linear polyenes: implications for polyacetylene

A homologous series of polyacetylene oligomers having up to 13 double bonds has been prepared, thus allowing, for the first time, an examination of optical properties of polyenes having enough conjugation length to suggest extrapolation to the polymer. Two linear optical properties, UV-Vis absorption spectra in solid and solution form and Raman scattering spectra in solid form, have been measured. Both the lowest energy electronic absorption peak and the frequency of the Raman bands become linear in 1 n for n > 7 . Assuming the validity of the 1 n extrapolation, the results suggest that the familiar forms of polyacetylene have effective conjugation lengths that average approximately 30 double bonds. Raman data are discussed in terms of the competing models for dispersion of the Raman spectra in polyacetylene. Our results strongly suggest that conjugation length disorder is the origin of the dispersion.

Observation of a quantum size effect with unusual long period by experiments of absorption of low energy electrons and by RHEED — a simple physical model

Absorption experiments of electrons of very low energy (few eV) have been done during deposition, at 80 K, of indium on polycrystalline and textured (111) gold films. The target current, for limited range of the energy (E) of the electrons, versus the indium thickness is modulated by damped oscillations with unexpectedly long period of which the value varies with the electronic energy. These results, as those obtained earlier by electrical resistance and work function measurements, can be well explained by a simple physical picture: an effect of vernier (i.e., a stroboscopic effect) between: (i) the quantization of the transverse electronic states in the indium layer in the case of the free electron model (giving a period of half the de Broglie wavelength for electrons of energy E); (ii) the almost perfect atomic layer by atomic layer growth of indium on the gold substrate (d(101) = 0.271 nm). Oscillations of the same type have been found with RHEED experiments by monitoring the variations of the specular intensity in the diffraction pattern.

Absorption and elastic and inelastic reflection of spin-polarized low-energy electrons from Fe(110).

The spin dependence of low-energy electron absorption and reflection from ferromagnetic Fe(110) is investigated using the normalized difference between the absorption (or reflection) of electrons polarized parallel and antiparallel to sample magnetization. The resulting absorption and reflection spin asymmetries are found to be reciprocal to each other, with their magnitudes related in a simple manner. Structure in the spin asymmetries is identified with features in the spin-split bulk band structure and is thus found to be related to the spin-polarization fine structure of secondary electrons emitted from the same surface. The general structure of the spin asymmetry as a function of incident energy is dominated by elastic-scattering events, although inelastic scattering is found to play a major role in determining the sign and magnitude of the absorbed and total (energy-integrated) reflected asymmetries. Furthermore, the existence of nonzero elastic-reflection asymmetries indicates the existence of spin-split unoccupied energy bands up to 50 eV above the vacuum level. Inelastic-scattering spin asymmetries show a 2-eV energy-loss feature that is identified as due to the creation of Stoner excitations in the ferromagnet. This feature is determined to be surprisingly insensitive to the incident electron energy (from 0 to 50 eV) and angle (from 0\ifmmode^\circ\else\textdegree\fi{} to 65\ifmmode^\circ\else\textdegree\fi{} from the normal) in contrast to predictions of a recent theory of spin-polarized electron scattering in ferromagnets.

Metallic carbonyl complexes containing heterocyclic nitrogen ligands—I. Rhenium derivatives

The preparation and spectroscopic properties of monometallic complexes of rhenium(I)tricarbonylbromide coordinated to 6,7-dihydrodipyrido-[2,3-b : 3′,2′-j]-1,10-phenanthroline (2-4N), 7,8-dihydro-6H-cyclohepta-[2,1-b : 3,4-b]-di-1,8-naphthyridine (3-4N), 2,2′-bi-(3-methyl)-1,8-naphthyridine (Me 2O-4N), 2,2′-bi-1,8-naphthyridine (O-4N) and 2,7-di(2′-pyridyl)-1,8-naphthyridine (bpnp) are described. The complexes show a low energy electronic absorption band which is solvent sensitive and has been assigned as a t 2 g → π* rhenium-to-ligand charge-transfer band. The CO stretching frequencies were in accord with facial geometry for the complexes. The NMR properties agreed with the IR results, showing the equivalence of the two naphthyridine fragments in the complexes. Electrochemically all the complexes display two reductions and two oxidations in the potential region from + 2.0 to - 1.6 V. The reduction potential, E red, for the Re I/Re 0 couple shows a good relationship with the lowest energy absorption band.

Photochemical hole-burning spectroscopy of a photosynthetic reaction center mutant with altered charge separation kinetics: properties and decay of the initially excited state

Photochemical hole-burning spectra have been obtained for the lowest energy electronic absorption band of the primary electron donor P of photosynthetic reaction centers (RCs) that exhibit different rates for the initial charge separation step. Wild type (WT) and Tyr(M)210 - Phe mutant [(M)Y210F] RCs from Rhodobacter sphaeroides were studied at 1.5 K in 50% glycerol/buffer glasses. In both types of RC, difference spectra obtained by using narrow-bandwidth excitation on the low-energy edge of the P band display weak, narrow zero-phonon holes coincident with the laser wavelengths, in addition to a broad background bleach of much larger amplitude. The zero-phonon hole width in each type of RC corresponds quantitatively to the broadening expected from the excited-state lifetime. These results are inconsistent with electronic relaxation of the excited state of P prior to electron transfer. The agreement between the time- and frequency-domain experiments may result from vibrational thermalization prior to electron transfer. An alternative mechanism, which does not invoke subpicosecond vibrational thermalization and which is consistent with a wide range of data, is also presented. Quantitative estimates were obtained for the total electron-phonon coupling strength (Huang-Rhys factor) S and for the magnitudes of the homogeneous and inhomogeneous broadening in the P absorption band from self-consistent simulations of the 1.5 K absorption and hole spectra from a linear electron-phonon coupling model. The vibronic and line-width parameters deduced from the simulations are very similar for WT and (M)Y2lOF RCs.

Relationship between the bite size of diphosphine ligands and tetrahedral distortions of "square-planar" nickel(II) complexes: stabilization of nickel(I) and palladium(I) complexes using diphosphine ligands with large bites

Nickel and palladium complexes of the type (M(L{sub 2}){sub 2})(BF{sub 4}){sub 2} and (M(L{sub 2})(L{sub 2}{prime}))(BF{sub 4}){sub 2} (where L{sub 2} and L{sub 2}{prime} are diphosphine ligands) have been synthesized. The lowest energy electronic absorption band for the nickel complexes decreases in energy as the bite size of the diphosphine ligand increases. Similarly, the half-wave potentials for the Ni(II/I) and Pd(II/O) couples become more positive as the bite size increases. Structural studies of (Ni(dppm){sub 2})(BF{sub 4}){sub 2} (where dppm is bis(diphenylphosphino)methane) and (Ni-(dppb){sub 2})(PF{sub 6}){sub 2} (where dppb is 1,2-bis(diphenylphosphino)benzene) show that increasing the bite size of the diphosphine ligands results in larger tetrahedral distortions. The crystal structure of (Ni(Dppm){sub 2})(BF{sub 4}){sub 2}(C{sub 50}H{sub 44}B{sub 2}F{sub 8}NiP{sub 4}) and (Ni(Dppb){sub 2})(PF{sub 6}){sub 2}(C{sub 74}H{sub 64}F{sub 12}NiP{sub 6}) were measured and are reported herein. Calculations made using the extended Huckel theory indicate that the observed distortions may have an electronic as well as a steric component. The calculations also allow rationalization of the electronic absorption spectra, electrochemical data, and the stability of the Ni(I) and Pd(I) complexes (Ni(dppp){sub 2})(BF{sub 4}) (where dppp is 1,3-bis(diphenylphosphino)propane) and (Pd(dppx){sub 2})(BF{sub 4}) (where dppx is {alpha},{alpha}{prime}-bis(diphenylphosphino)-o-xylene). Complexes containing the ligand dppm have a marked tendencymore » to become five-coordinate, as indicated by the structural determination of (Ni(dppm){sub 2}(CH{sub 3}CN))(PF{sub 6}){sub 2}. The crystal structure for the latter complex is reported.« less

Resonance Raman spectroscopy of the acid–base indicator neutral red

AbstractResonance Raman spectra of the acid–base indicator neutral red and its monoprotonated and diprotonated derivatives, excited in the region of the lowest energy electronic absorption bands, are reported. The bands are assigned to vibrations of the phenazine ring system and the substituents. Measurements of the band intensities have permitted an evaluation of the geometric changes attendant upon excitation to the resonant excited state.

Approaches to the Chemical, Electrochemical, and Photochemical Activation of Carbon Dioxide by Transition Metal Complexes

AbstractThe activation of CO2 by chemical, electrochemical, and photochemical means is discussed. Binuclear transition metal complexes mediate oxygen atom transfers from CO2 by three distinct chemical pathways: (i) deoxygenation of CO2, (ii) multiple bond metathesis, and (iii) disproportionation. The complex Ir2 (μ‐CNR)2(CNR)2(dmpm),(dmpm = bis(dimethylphosphino)methane) undergoes double cycloaddition of CO2 to its μ‐CNR ligands. A subsequent reaction produces the bis(carbamoyl) complex [Ir2(μ‐CO)(μ‐H)(CONHR)2(CNR)2(dmpm)2]Cl. Isotope labelling studies show that the μ‐CO ligand results from net deoxygenation of CO2. In contrast, the binuclear nickel complex Ni2(μ‐CNMe)(CNMe)2(dppm)2 (dppm = bis‐(diphenylphosphino)methane) reacts with liquid CO2 to give the tricarbonyl complex Ni2(μ‐CO)(CO)2(dppm)2. Isotope labelling indicates that the carbonyl ligands are not derived from CO2 deoxygenation, but from C/CO triple bond metathesis. The reaction of CO2 with the Ir(0) complex Ir2(CO)3(dmpm)2 leads to CO2 disproportionation by formation of the carbonate, Ir2(CO3)(CO)2(dmpm)2, and tetracarbonyl, Ir2(CO)4(dmpm)2, complexes. The complex Ir2(CO3)(CO)2 (dmpm)2 undergoes reversible O‐atom transfers from its carbonate ligand. The electrochemical activation of CO2 by the binuclear Ni2(μ‐CNMe)(CNMe)2(dppm)2 and trinuclear [Ni3(μ‐CNMe)(μ‐I)(dppm)3][PF6] species is described. The triangular nickel complex [Ni3(μ3‐CNMe)(μ3‐I)(dppm)3][PF6] is an electrocatalyst for the reduction of CO2. The cluster exhibits a reversible single electron reduction at E0(+/0) = −1.09 V vs. Ag/AgCl. In the presence of CO2, the cluster reduces CO2 by an EC' electrochemical mechanism. The reduction products correspond to the disproportionation and H‐atom abstraction products of CO2*−, with a partitioning ratio of 10:1. Isotope labelling studies with 13CO2 indicate that 13CO2*− disproportionation produces 13CO and 13CO32−.Studies of the photochemical activation of CO2 by Ni2(μ‐CNMe)(CNMe)2(dppm)2 are described. The bimolecular photochemical addition of CO2 to the complex was examined by laser transient absorbance spectroscopy. Photolysis at 355nm in the presence of CO2 (1 atm) leads to cycloaddition of CO2 to the μ‐CNMe ligand and the complex Ni2(μ‐CN(Me)C(O)O)(CNMe)2(dppm)2 with Φ355=0.05. The triplet excited state of Ni,(μ‐CNMe)(CNMe)2(dppm)2 was determined to react with CO2 with the bimolecular reaction rate constant k = 1 × 104 M−1 s−1. Bridging ligand substituent effects and solvent dependence of the lowest energy electronic absorption spectral bands of the series of complexes, Ni2(μ‐L)(CNMe)2(dppm)2, L = CNMe, CNC6H5, CN‐p‐C6H4Cl. and CN‐p‐C6H4Me, confirm the assignment of di‐metal to bridging ligand charge transfer (M2→μ‐LCT). This assignment is supported by results of extended Hückel calculations which indicate a LUMO of predominantly μ‐isocyanide π* character. A systematic study of the nature of the lowest excited states of related d10–d10 binuclear complexes of the type Ni2(μ‐L)(CNMe)2(dppm)2, where L = CNMe(Ph)+, CNMe2+, CNMe(C5H11)+, CNMe(H)+, CNMe(CH2C6H5)+, and NO+ reveals dramatic differences in the lowest excited states of the three classes of complexes: μ‐isocyanide, μ‐aminocarbyne, and μ‐nitrosyl. Spectroscopic and extended Hückel MO studies confirm that the μ‐isocyanide complexes are characterized by di‐metal to bridging ligand charge transfer (M2 → μ‐LCT) excited states. However, the μ‐aminocarbyne and μ‐nitrosyl complexes exhibit bridging ligand to metal charge transfer (μ‐L→M2) and intraligand (IL) lowest excited states, respectively.

Bimetallic complexes of rhenium(I). Preparation of Re(BL)(CO)3Cl and [Re(CO)3Cl]2(BL) (BL = 2,3-bis(2-pyridyl)pyrazine, 2,3-bis(2-pyridyl)quinoxaline, and 2,3-bis(2-pyridyl)benzo[g]quinoxaline)

Mono- and bimetallic complexes of Re(CO) 3 Cl containing the bidentate, bridging ligands 2,3-bis(2-pyridyl) pyrazine (dpp), 2,3-bis(2-pyridyl) quinoxaline (dpq), and 2,3-bis(2-pyridyl) benzo [g] quinoxaline (dpb) have been prepared and characterized. The synthesis and characterization of the new bidentate bridging ligand dpb are reported. The lowest energy electronic absorption band has been assigned as a metal-to-ligand charge-transfer (MLCT) process, where the acceptor state is localized primarily on the bridging ligand

Spin-dependence of absorbed and reflected current on Fe(110) and a comparison with secondary electron polarization fine structure

Measurements of the normalized difference in the absorption and reflection of low-energy electrons with opposite spin-polarization, and the spin-polarization of low-energy secondary electrons, from Fe(110) are found to be influenced by the spin-split bulk band structure. Maxima and minima in the absorbed and reflected current asymmetries are interpreted in terms of band structure effects and are shown to be directly related to the spin-polarization fine structure in low-energy secondary electron emission from ferromagnets. The feasibility of using spin-dependent low-energy electron reflection from ferromagnets as an electron spin-polarization detector is discussed.

Improvements to the image bandpass filter for electron and positron spectromicroscopy

The design and construction of a low-pass electron energy mirror with controllable image rotation and pass energy to operate in a magnetic field is described. Image rotation is radially uniform and rotations of up to approximately 40 degrees are demonstrated. A low-pass filter is described incorporating a virtual reflecting surface and a novel electron trap also operating in a magnetic field. This reduces the proportion of elastically and inelastically scattered high-energy electrons escaping the filter. Both the above elements are demonstrated working in an image bandpass filter operating in a photoelectron spectromicroscope. Low-energy (0-35 eV) electron absorption and reflection from a number of electrode surfaces is investigated and recommendations made on possible structures. A scheme is proposed whereby azimuthally averaged axial emission angles can be obtained from electrons emitted in a photoelectron spectromicroscope using an image bandpass filter. The devices considered are equally suited to the manipulation of low-energy positrons and will allow simultaneous positron-energy analysis and imaging.

Energy-dependence of inner potential in Fe from low-energy electron absorption (target current)

For unpolarized low-energy electrons (0–60 eV) incident on a Fe(0 0 1)_ surface, the measured current I absorbed by the crystal (target current) displays a considerable amount of fine structure depending on the angle of incidence. Dynamical calculations of the elastic reflection coefficient R e show that - d 2 R e /d E 2 is in good agreement with d 2 I/d E 2 for suitably chosen real and imaginary parts of the inner potential. The energy dependence of the latter is thereby determined. Comparison with a bulk band structure calculated using the same real potential reveals a correpondence between target current minima and band gaps as well as current maxima and critical points.