Probing electron transfer dynamics at MgO surfaces by Mg-atom desorption

Ultraviolet excitation of high surface area MgO films using 4.7 eV femtosecond pulses results in neutral Mg-atom desorption with hyperthermal kinetic energies in the range 0.1-0.4 eV. The Mg-atom hyperthermal energies and power dependences are similar to those previously observed using nanosecond pulsed excitation. Femtosecond two-pulse correlation measurements reveal the existence of different dynamical paths for Mg-atom desorption. One mechanism displays a sub-150 fs time scale and involves the simultaneous or near-simultaneous transition of two electrons to a 3-coordinated Mg(2+) site. Other paths display picosecond time scales that we associate with dynamics following electron trapping at 3-coordinated Mg(2+) surface sites.     

On the electron impact induced hydroxyl loss from o-nitrostyrene

Unimolecular hydroxyl (OD^˙) loss from regio- and stereo-specifically labelled o-nitrostyrenes 1a, 1c and 1d results in the formation of an ion which upon collisional activation gives identical mass spectra. Suggestions are made which aim at explaining: (i) the loss of stereochemical integrity of the diastereotopic methylene hydrogens in the course of hydroxyl elimination; and (ii) to account for the collision induced losses of CO and HNC from the [M—hydroxyl]⁺ ion.     

Auger and electron energy loss spectroscopies of small palladium particles supported on (111) oriented MgO films

The most common tool used to characterize supported metal clusters is the transmission electron microscope. The main advantage of TEM is the combination of high (lateral) resolution imaging with electron diffraction. However the TEM observations are usually made ex-situ i.e. UHV deposited clusters have to be exposed to the atmosphere during transfer to the TEM. This could be a severe limitation for very small reactive clusters. This paper demonstrates that electron spectroscopies can provide in-situ information on the cluster growth (AES), on the electronic structure (ELS) and on the local atomic order (SEELFS) of the clusters. These techniques were applied to Pd clusters of varying size (～10–200 Å) vapor-deposited on thin (111) MgO support films under UHV conditions. An expansion of the lattice and a shift of the loss peaks towards higher energies are observed with decreasing particle size.     

Analysis of Fe-Si layered structures by reflected electron energy loss spectroscopy and inelastic scattering cross-section

This paper reports on our study of the formation of an interface of layered structures in the Fe-Si system by reflected electron energy loss spectroscopy (REELS). Quantitative element analysis was performed using the product of the mean length of the inelastic free path by the inelastic scattering cross-section of electrons. It is shown that the Fe-Si interface is quite uniform.     

High-resolution electron energy loss spectroscopy of anions chemisorbed on electrode surfaces: The effect of counter cations

In the course of experiments that included Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS) on cation exchange at benzoquinone sulfonate chemisorbed on a Pd(111) electrode, it was found that, whereas the AES spectra remained invariant as the counter cation was varied from H⁺ to K⁺ to Cs⁺, profound changes occurred in the HREELS spectra. Specifically, the intensity of the spectral features decreased noticeably when H⁺ was replaced with K⁺. And, when the K⁺ ions were exchanged with Cs⁺, nothing but a flat-line (dead) spectrum was observed; even the elastic peak was completely attenuated. When the Cs⁺ ions were displaced by protons, the initial undiminished spectrum was fully restored. This outcome, while unrelated to cation-exchange selectivity, is of exceptional significance in surface electron spectroscopy. It appears that the positive ions on the surface attracted the low-energy incident electrons such that backscattering towards the energy analyzer was hindered; partially by K⁺ but totally by the larger Cs⁺ ion. The use of HREELS to examine the molecular integrity of chemisorbed anionic species must thus take cognizance of the possibility that the counter cation chosen to preserve interfacial-layer electroneutrality can have a profound effect. To circumvent such complication, low-valent and small-radii cations will have to be employed. In addition, although subject to instrument limitations, higher incident-electron energies could be adopted. AES, with incident-electron energies in the kV range, is impervious to the presence of counter cations.     

On non-adiabatic potential energy surfaces

The concept of a Born–Oppenheimer (BO) potential energy surface (PES) has been extended to non-adiabatic wavefunctions by Hunter and by Wilson. A Hunter non-adiabatic PES corresponding to an excited vibrational state has a set of spikes superimposed on a BO-like PES. It was believed that Wilson PESs were spike-free. We show that it is not the case and that the Wilson PES value at a given nuclear configuration is not the expectation value of a quantum observable but a quotient of such expectation values. Consequently, BO PESs have the quantum interpretation of quotients of approximate expectation values of observables.     

On the origins of intersecting potential energy surfaces

Techniques developed for investigating nonadiabatic processes in molecular systems are adapted to study the structure and properties of holomorphic and meromorphic functions of a complex variable, \(f(z)=\mathfrak {R}(f)+i\,\mathfrak {I}(f)\). The connection is that \(\mathfrak {R}(f)\) and \(\mathfrak {I}(f)\) are correlated two-dimensional scalar functions, interrelated by the Cauchy–Riemann equations. Exploiting this fact, it is demonstrated that \(\mathfrak {R}(f)\) and \(\mathfrak {I}(f)\) of f can be envisaged in Euclidean \({\mathbb {R}}^{3}\) space as a two-state set of constrained, intersecting two-dimensional potential energy surfaces (PESs), called the graph of f. Importantly, the analytic and algebraic properties of f dictate the geometric structure evinced in the graph of f. This parallels multi-state sets of higher-dimensional, constrained, intersecting PESs linked with correlated electronic eigenstates of the parameterized molecular Hamiltonian operator. In view of this association, the language and mathematical infrastructure devised by chemists for discussing and analyzing intersections in higher-dimensional PESs are suitably modified for f. Notably, an algorithm capable of optimizing roots and poles of f through analysis of the real, two-dimensional \(\mathfrak {R}(f)\) and \(\mathfrak {I}(f)\) functions is derived, which is based on intersection-adapted coordinate and constrained Lagrangian methodologies. As constrained, intersecting PESs are indispensible for conceptualizing and characterizing the physics governing nonadiabatic phenomena, f represents a foundational bridge to these more abstract constructions.     

Momentum transfer effects in near surface electron energy loss

We examine two formulations for the differential surface excitation parameter (DSEP): one provided by Tung et al. and the other given by the Chen–Kwei position‐dependent differential inverse inelastic mean free path integrated over the electron trajectory. We demonstrate that the latter converges to the former provided that the dielectric function of the solid does not depend on the momentum transfer or it depends on just the momentum transfer component parallel to the surface. Tung's DSEP represents therefore an approximation to the Chen–Kwei DSEP calculated for a dielectric function with no restrictions on the momentum dependence. The approximation is shown to work in the limit of small momentum transfer and to imply an error of 4%–5% for electrons traveling through the solid with energy E = 1 keV. Copyright © 2015 John Wiley & Sons, Ltd.     

Identification of defect sites on oxide surfaces by metastable impact electron spectroscopy

In this review, thin films of SiO₂ on Mo(1 1 2) and MgO(1 0 0) on Mo(1 0 0) have been characterized using metastable impact electron and ultraviolet photoelectron spectroscopies (metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy). The electronic and chemical properties of the thin films are identical to those of the corresponding bulk oxides. For different prepared defective SiO₂ surfaces, additional features are observed in the band-gap region. These features arise from vacancies or excess oxygen and are consistent with theoretical predictions of additional occupied states in the band-gap due to point defects. Extended defect sites on SiO₂ and MgO are identified using MIES by a narrowing of the O(2p) features with a reduction in the density of extended defect sites. MIES of adsorbed Xe (MAX) is also used to estimate the density of extended defect sites. Furthermore, it is shown that CO is an appropriate probe molecule for estimating the defect density of MgO surfaces. Upon Ag exposure, the change in the work function of a low defect MgO(1 0 0) versus a high defect surface is markedly different. For a sputter-damaged MgO(1 0 0) surface, an initial decrease of the work function was found, implying that small Ag clusters on this surface are electron deficient. In contrast, for SiO₂ no significant change of the work function upon Ag exposure with increasing defect density was observed. On MgO(1 0 0), the presence of defect sites markedly alter the electronic and chemical properties of supported Ag clusters. Such a strong influence of defect sites was not found for Ag clusters on SiO₂.     

Optical constants of organic insulators in the UV range extracted from reflection electron energy loss spectra

Reflection electron energy loss spectroscopy (REELS) spectra were measured for seven insulating organic compounds (DNA, Irganox 1010, Kapton, polyethylene [PE], poly(methyl methacrylate) [PMMA], polystyrene [PS] and polytetrafluoroethylene [PTFE]). Optical constants and energy band gaps were extracted from the measured REELS spectra after elimination of multiple electron scattering via a deconvolution and fitting the normalised single scattering energy loss spectra to Drude and Drude–Lindhard model dielectric functions, constrained by the Kramers–Kronig sum and f-sum rules. Satisfactory agreement is found for those optical constants for which literature data exists. For PTFE, the observed features in the optical data correspond to its electronic structure.     

Interaction of oxide surfaces with water: environmental transmission electron microscopy of MgO hydroxylation.

Environmental transmission electron microscopy (ETEM) is opening an important window for in situ studies of interaction of water with oxides. Studies of MgO smoke nanocrystals under partial pressures of water ranging from 10 mTorr to 10 Torr found their {100} neutral surfaces to be extremely resistant to dissociative adsorption of water and hydroxylation, in agreement with recent theoretical predictions. ETEM observations of electron irradiation driven MgO smoke nanocrystal hydroxylation displayed the anticipated volume expansion, but revealed complex shape changes with elongations toward oxide corners. The reaction rate was found to increase with electron flux at constant water pressure. In situ selected area diffraction studies of MgO single crystals showed that the hydroxide grows with its basal (0001) plane parallel to the polar MgO (111) planes. This is the same crystallographic relationship as in dehydroxylation experiments, but with four variants. Electron energy loss spectroscopy found oxygen K-edge changes consistent with bulk hydroxylation.     

High resolution electron energy loss spectroscopy of clean and hydrogen covered Si(001) surfaces: First principles calculations

Surface phonons, conductivities, and loss functions are calculated for reconstructed (2×1), p(2×2) and c(4×2) clean Si(001) surfaces, and (2×1) H and D covered Si(001) surfaces. Surface conductivities perpendicular to the surface are significantly smaller than conductivities parallel to the surface. The surface loss function is compared to high resolution electron energy loss measurements. There is good agreement between calculated loss functions and experiment for H and D covered surfaces. However, agreement between experimental data from different groups and between theory and experiment is poor for clean Si(001) surfaces. Formalisms for calculating electron energy loss spectra are reviewed and the mechanism of electron energy losses to surface vibrations is discussed.     

A multichannel electron energy loss spectrometer for low-temperature condensed films

We describe a wide-gap multichannel cylindrical deflection electron energy analyzer suitable for measuring the weak signals characteristic of electronically inelastic electron energy loss spectra. The analyzer has nearly ideal fringing field termination, and its resolution and energy dispersion were characterized as a function of energy by solving numerically the equation of motion of electrons in an ideal cylindrical electric field. The numerical results for the radial location of the electrons at the detector as a function of the entrance location, angle, and energy are closely approximated by a second order polynomial, and match closely with those observed. The detection efficiency of the analyzer is 100-150 times better than that of an equivalent single-channel instrument, but limited energy transmission of the zoom lens system used in our case reduced it by a factor of about 2. The performance of the new instrument was demonstrated by measuring the (3)E(1u) electronic spectrum of benzene in only 2 min and the spectrum of endo-benzotricyclo[4.2.1.0(2.5)]nonane.     

Electronic structure of lithium nickel oxides by electron energy loss spectroscopy

The electronic structures of NiO, LiNiO2, and NiO2 are studied by the electron energy loss spectroscopy at Ni L(2,3), Ni M(2,3), and O K edges. The Ni L(2,3) edge spectra suggest that the formal charge of nickel is +2 in NiO, +3 with a low-spin state in LiNiO2, and +4 with a low-spin state in NiO2. This is well confirmed by first-principles calculations. The Ni M(2,3) edge spectra show similar chemical shifts to those of the Ni L(2,3) edge. Superposition of the Li K edge spectrum, however, hinders further analysis. Although the formal charge of oxygen is -2 in all the three phases, the O K edge spectra indicate a more remarkable difference in the electronic structure of the oxygen in NiO2 than that in either NiO or LiNiO2. The spectra suggest that lithium extraction from LiNiO2 reinforces the covalent bonding between the oxygen and nickel atoms and causes a notable reduction in electron density at the oxygen atoms.     

Synthesis of electron energy loss spectra for the quantification of detection limits.

We describe a method for predicting detection limits of minority elements in electron energy loss spectroscopy (EELS), and its implementation as a software package that gives quantitative predictions for user-specified materials and experimental conditions. The method is based on modeling entire energy loss spectra, including shot noise as well as instrumental noise, and taking into account all the relevant experimental parameters. We describe the steps involved in modeling the entire spectrum, from the zero loss up to inner shell edges, and pay particular attention to the contributions to the pre-edge background. The predicted spectra are used to evaluate the signal-to-noise ratios (SNRs) for inner shell edges from user-specified minority elements. The software also predicts the minimum detectable mass (MDM) and minimum mass fraction (MMF). It can be used to ascertain whether an element present at a particular concentration should be detectable for given experimental conditions, and also to quickly and quantitatively explore ways of optimizing the experimental conditions for a particular EELS analytical task. We demonstrate the usefulness of the software by confirming the recent empirical observation of single atom detection using EELS of phosphorus in thin carbon films, and show the effect on the SNR of varying the acquisition parameters. The case of delta-doped semiconductors is also considered as an important example from materials science where low detection limits and high spatial resolution are essential, and the feasibility of such characterization using EELS is assessed.     

A study of the sodium tungsten bronzes by high-resolution electron energy loss spectroscopy

Effective masses of conduction electrons in the sodium tungsten bronzes have been determined from the frequency of surface plasmon features in high-resolution electron energy loss spectra. Good agreement is found with effective masses from optical experiments. The results support the view that there is no major depletion in the carrier concentration at the surface of the bronzes.     

High-resolution electron energy loss spectroscopy study of O-Cu(410)

We have investigated oxygen adsorption on Cu(410) by high-resolution electron energy loss spectroscopy, dosing O2 with a supersonic molecular beam at different surface temperatures and for different angles of incidence and beam energies or by backfilling. In the investigated crystal temperature range (127 < T < 570 K), adsorption is always dissociative. Depending on T, impact energy, and angle of incidence, the oxygen atoms end up in different adsorption configurations, characterized by different vibrational signatures. In particular, at grazing incidence when only the step edge is exposed to O2, the adatoms end up initially preferentially at the step edge. An ordered overlayer forms at half monolayer coverage when the adsorbate is mobile. Oxide patches develop eventually for large exposures performed by backfilling and at high crystal temperature.     

Richardson–Lucy deconvolution of reflection electron energy loss spectra

A procedure for deconvolving the energy spread introduced by the primary beam and the analyzer in a reflection electron energy loss spectrum (REELS) has been developed. The procedure is based on the Richardson–Lucy (RL) algorithm. The approach has been successfully tested on experimental spectra by comparison with spectra with an inherent high‐energy resolution. As a typical result, it was found that the effective energy resolution of spectra with a full width half maximum (FWHM) of the elastic peak of ～1.5 eV in the raw experimental data can be reduced to ～0.7 eV in the deconvoluted spectra. Copyright © 2009 John Wiley & Sons, Ltd.