Electronic Excitation Threshold Research Articles

Enhanced ultraviolet aging resistance of epoxy resins through surface enrichment achieved by fluorinated graphene oxide@CeO2

Fluorinated graphene oxide@CeO2 (FGO@CeO2) was synthesized using an in-situ growth method and incorporated as a filler into epoxy resins to improve their UV aging resistance. On UV irradiation, it facilitated the surface enrichment of the modified epoxy resin (MEP), forming a UV-blocking layer. Incorporating FGO@CeO2 induced n→π* electronic transitions within the MEP and lowered the electronic excitation threshold, expanding the UV spectral absorption band and enhancing the UV absorption intensity. A 785% increase in the UV absorption capacity was achieved in the MEP compared with that in the pristine epoxy resin. After 300 h of UV exposure, the photothermal temperature rise in the MEP was 13.67 ± 2 °C higher than that in pure epoxy resin.

Elastic and inelastic low-energy electron scattering from pyridine.

A comprehensive investigation of elastic and inelastic electron scattering from molecular pyridine is reported using the ab initio R-matrix method with the static exchange plus polarization and close-coupling approximations for incident energies up to 10eV. The two well-known low-lying 1 2B1 and 1 2A2 shape resonances as well as a 2 2B1 mixed-character resonance compare well with the theoretical and experimental results. We also detect five core-excited resonances (1 2A1, 1 2B2, 3 2B1, 2 2A2, and 4 2B1), which lie above the first electronic excitation threshold. The total elastic cross sections and momentum transfer cross sections agree reasonably with previous reference data. Comparisons of the differential elastic cross sections of pyridine with those measured for benzene, pyrazine, and pyrimidine show remarkable agreement at scattering angles above 40° but behave differently for forward scattering below 40° below 6eV, due to the dominant effect of the permanent dipole moment on the differential cross section in the low energy region with narrow scattering angles. Inelastic electronic excitation cross sections are presented, showing the influence of core-excited resonances below the ionization threshold for the first time.

Photoexcitation-Induced Nonthermal Ultrafast Loss of Long-Range Order in GeTe.

Nonadiabatic quantum molecular dynamics is used to investigate the evolution of GeTe photoexcited states. Results reveal a photoexcitation-induced picosecond nonthermal path for the loss of long-range order. A valence electron excitation threshold of 4% is found to trigger local disorder by switching Ge atoms from octahedral to tetrahedral sites and promoting Ge-Ge bonding. The resulting loss of long-range order for a higher valence electron excitation fraction is achieved without fulfilling the Lindemann criterion for melting, therefore utilizing a nonthermal path. The photoexcitation-induced structural disorder is accompanied by charge transfer from Te to Ge, Ge-Te bonding-to-antibonding, and Ge-Ge antibonding-to-bonding change, triggering Ge-Te bond breaking and promoting the formation of Ge-Ge wrong bonds. These results provide an electronic-structure basis to understand the photoexcitation-induced ultrafast changes in the structure and properties of GeTe and other phase-change materials.

Solvent effects on the π* shape resonances of uracil

We have investigated the effect of microsolvation on the π* shape resonances of uracil, referred to as π1 * and π2 * in the order of increasing energy. Our study considered uracil-water aggregates with six solvent molecules obtained from Monte Carlo simulations in the liquid phase. To explore the ensemble statistics, we combined scattering calculations, performed in the static exchange and static exchange plus polarization approximations, with linear regressions of virtual orbital energies to the scattering results. In general, the solvent molecules stabilize the anion states, and the lower lying π1 * resonance becomes a bound state in most of the solute-water clusters. We also discuss how the strength of the H bonds can affect the energies of the anion states, in addition to the number and donor/acceptor characters of those bonds. The thermal distributions for the vertical attachment energies, obtained from 133 statistically uncorrelated solute-solvent clusters, are significantly broad in the energy scale of the autoionization widths. The distributions for the π1 * and π2 * anion states slightly overlap, thus giving rise to a quasi-continuum of attachment energies below ≲2.5 eV, in contrast to the gas phase picture of electron attachment to well separated resonances below the electronic excitation threshold. Both the stabilization of the anion states and the spread of attachment energies could be expected to favor the dissociative electron attachment processes believed to underlie the electron-induced damage to biomolecules.

Vibrational excitation induces double reaction.

Electron-induced reaction at metal surfaces is currently the subject of extensive study. Here, we broaden the range of experimentation to a comparison of vibrational excitation with electronic excitation, for reaction of the same molecule at the same clean metal surface. In a previous study of electron-induced reaction by scanning tunneling microscopy (STM), we examined the dynamics of the concurrent breaking of the two C-I bonds of ortho-diiodobenzene physisorbed on Cu(110). The energy of the incident electron was near the electronic excitation threshold of E0=1.0 eV required to induce this single-electron process. STM has been employed in the present work to study the reaction dynamics at the substantially lower incident electron energies of 0.3 eV, well below the electronic excitation threshold. The observed increase in reaction rate with current was found to be fourth-order, indicative of multistep reagent vibrational excitation, in contrast to the first-order rate dependence found earlier for electronic excitation. The change in mode of excitation was accompanied by altered reaction dynamics, evidenced by a different pattern of binding of the chemisorbed products to the copper surface. We have modeled these altered reaction dynamics by exciting normal modes of vibration that distort the C-I bonds of the physisorbed reagent. Using the same ab initio ground potential-energy surface as in the prior work on electronic excitation, but with only vibrational excitation of the physisorbed reagent in the asymmetric stretch mode of C-I bonds, we obtained the observed alteration in reaction dynamics.

Positron hydrogen molecule scattering under Ro‐vibrational LFCC scheme using PCOP potential below the electronic excitation threshold

AbstractRecently we have presented a Laboratory Frame Ro‐Vibrational Close Coupling (LFCC) calculation for positron hydrogen molecule scattering process using Positron Correlation Polarization (PCOP) Potential and discussed the results below positronium formation threshold (Mukherjee and Sarkar, J. Phys. B 41 125201 (2008) [1]). In the present article we have reported total (integrated) cross sections as well as vibrational excitation cross sections in higher positron energy region below the first electronic excitation threshold of hydrogen molecule for same system with same coupling scheme. The results are compared with the existing elaborate theories and the measured values. In the reported energy region the present total cross sections agree well with one of the measured values but differ with other. However, the present theoretical values for 0 → 1 and 0 → 2 vibrational excitation cross sections vary appreciably with the other results. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization effects on electronic excitation of molecules by low-energy electron impact: Study one−-furan scattering

The Schwinger multichannel method is applied to study the influence of polarization effects on the electronic excitation of the furan molecule by low-energy electron impact. We discuss the importance of inclusion of these effects through the comparison of theoretical results for the electronic excitation of the ${^{3}B}_{2}$ state of furan obtained with and without the proper treatment of the polarization of the target. The electron-furan scattering presents two prominent shape resonances in the ${^{2}A}_{2}$ and ${^{2}B}_{1}$ symmetries at around the electronic excitation threshold of the ${^{3}B}_{2}$ state (3.7 eV). At this low-energy, the inclusion of polarization effects in the calculation moves to lower energies the resonances positions obtained either in the close-coupling or in the static-exchange level of approximation. This phenomenon strongly influences the electronic excitation process. The present results show that a simple close-coupling calculation cannot be applied for molecular systems with low-energy electronic excitation thresholds around misplaced resonances.

Construction of the undulator beamline equipped with a UHV-STM for observations of synchrotron-radiation-stimulated surface reaction

An undulator beamline equipped with a UHV-scanning tunneling electron microscopy (STM) system has been designed and constructed at the UVSOR facility to investigate synchrotron-radiation-stimulated reactions. Using this undulator beamline, we have observed irradiation effects on the hydrogen terminated-(H-) Si(1 1 1) surfaces in atomic scale. The small protrusions, which are assigned to the rest-atom with missing H, appeared on the monohydride surface after irradiation. The density of them monotonically increased with irradiation dose. This phenomenon has been observed almost independent on the Si 2p core electron excitation threshold, indicating the significant contribution of the valence electron excitations to the Si H bond dissociations

Thymine excision from DNA by subexcitation electrons

We show the ability of slow electrons to effectively decompose gas phase thymidine into the sugar and thyminyl moiety, at comparable intensities. The reaction proceeds via dissociative electron attachment at energies below the threshold for electronic excitation (<3 eV), with the excess charge localized on the respective fragments. Within nucleic acid, this reaction may excise thymine from the DNA strands. Since ballistic low energy electrons are the predominant radiolytic species, the present findings represent an essential reaction in the understanding of the initial events in radiation damage of DNA.

Electron attachment to uracil: effective destruction at subexcitation energies.

We demonstrate that electrons at energies below the threshold for electronic excitation (<3 eV) effectively decompose gas phase uracil generating a mobile hydrogen radical and the corresponding closed shell uracil fragment anion (U-H)(-). The reaction is energetically driven by the large electron affinity of the (U-H) radical. This observation has significant consequences for the molecular picture of radiation damage, i.e., genotoxic effects or damage of living cells due to the secondary component of high energy radiation.

Formation and evolution of negative ion resonances at surfaces

The behaviour of molecular negative ion resonances formed by free electrons at the surfaces of multilayer molecular films is studied by means of electron stimulated desorption (ESD) of negative ions. In the low energy region ESD usually proceeds via dissociative electron attachment (DEA) to an individual molecule at or near the surface. Three prototypical cases are discussed in light of their gas phase analoga. (A) In the ESD cross-section of F − from a multilayer NF 3 film features (resonances) are considerably narrower with respect to the corresponding gas phase DEA analogues. This is a mirror of the particular dynamics of the unimolecular reaction in relation to the energy constraints operative in ion desorption. (B) The absolute DEA cross-section can appreciably be enhanced by the medium due to an increase of the autodetachment lifetime of the negative ion resonance. This is demonstrated in CHF 2Cl for an electronically excited resonance. (C) The possibility of specific and effective bond cleavage in DEA is used to selectively synthesize Cl 2 from a chlorofluoroethane compound (1,2-C 2F 2Cl 4). It is shown that by selecting the electron energy to values below the threshold for electronic excitation, the extend of the reaction can be pushed completely onto the product side.

Resonance-enhanced dissociation of a molecular ion below its electronic excitation threshold.

A mechanism for dissociative excitation (DE) of molecular ions via autoionizing resonances is discussed. This mechanism produces dissociation at collision energies below the excitation threshold for the ion's dissociative states and can alter the dissociation products. Calculated DE cross sections for $^{4}\mathrm{HeH}^{+}$ and $^{3}\mathrm{HeD}^{+}$ between 5 and 25 eV contain contributions from several resonances and agree well with the recent measurements by Str\"omholm et al. [Phys. Rev. A (to be published)]. In this completely ab initio study the electronic scattering parameters came from complex Kohn variational calculations and a quantum wave-packet treatment was used for the dissociation dynamics. \textcopyright{} 1996 The American Physical Society.

Breaking individual chemical bonds via STM-induced excitations

We present experimental and theoretical results on the STM-induced SiH bond-breaking on the Si(100)-(2 × 1):H surface. First, we examine the character of the STM-induced excitations. Using density functional theory we show that the strength of chemical bonds and their excitation energies can be decreased or increased depending on the strength and direction of the field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from this excited site can be suppressed, and localized excited state chemistry can take place. Our experiments show that SiH bonds can be broken when the STM electrons have an energy >6 eV, i.e. above the onset of the σ→σ∗ transition of SiH. The desorption yield is ∼2.4 × 10 −6 H-atoms/electron and is independent of the current. We also find that D-atom desorption is much less efficient than H-atom desorption. Using the isotope effect and wavepacket dynamics simulations we deduce that a very fast quenching process, ∼10 15 s −1, competes with desorption. Most of the desorbing atoms originate from the “hot” ground state produced by the quenching process. Most interestingly, excitation at energies below the electronic excitation threshold can still lead to H atom desorption, albeit with a much lower yield. The yield in this energy range is a strong function of the tunneling current. We propose that desorption is now the result of the multiple-vibration excitation of the SiH bond. Such excitation becomes possible because of the very high current densities in the STM, and the long SiH stretch vibrational lifetime. The most important aspect of this mechanism is that it allows single atom resolution in the bond-breaking process — the ultimate lithographic resolution.

Positron-molecule scattering at low and medium energies

This review surveys the recent developments carried out by us on positron–molecule scattering at low and intermediate energies. The relative merits between the laboratory frame rotational close-coupling (rotational LFCC) and the adiabatic nuclei rotation models near the electronic and rotational excitation threshold energies are discussed. The importance of the use of the rotational LFCC model for polar molecules is also reviewed. The rovibrational LFCC and the adiabatic nuclei rovibration models are used to calculate the scattering cross sections for e+–H2 scattering. The results demonstrate the necessity of the inclusion of the vibrational motion dynamically in the theory.

Electronic excitation of H2 by e+ impact using adiabatic nuclear rotation model.

The adiabatic nuclear rotation (ANR) model has been employed to obtain rotational excitation cross sections for electronically elastic and electronic excitation processes in ${\mathit{e}}^{+}$-${\mathrm{H}}_{2}$ scattering. The present results are compared with the more accurate laboratory-frame rotational close-coupling approximation (LFCCA) predictions. The electronically inelastic rotational excitation results using the ANR model differ from the corresponding LFCCA results near the electronic excitation threshold energies. \textcopyright{} 1996 The American Physical Society.

Photoemission study of CeNi single crystals

Resonant photoemission from the (010) surfaces of CeNi single crystals has been observed around the Ni 3p and Ce 4d core electron excitation threshold. In addition to the two-peak profile in the overall aspect of the 4f spectrum, two additional features are resolved.

Resonant photoemission in CeNi single crystals.

The valence-band photoemission of the (010) surfaces of CeNi single crystals has been observed. The features occurring in angle-resolved normal-emission spectra do not show an appreciable amount of dispersion. The energy distribution curves are squeezed and narrower than those suggested by an existing energy-band calculation. Resonant photoemission has been observed both near the 3p core electron excitation threshold of Ni and the 4d threshold of Ce. An aspect of the super-Coster-Kronig final state with a post-collision interaction is found near the onset of the resonance in the two-hole bound-state satellite. In addition to the two-peak profile in the overall spectral aspect of the 4f spectrum, two additional features are resolved. The origin of these features is discussed. The ratio of Fano's q parameters between the 3p-3d resonance in Ni ions and the 4d-4f resonance in Ce ions agrees with a result of a simple theoretical analysis.

A Monte Carlo calculation of subexcitation and vibrationally-relaxing electron spectra in irradiated liquid water

An energy spectrum of “subexcitation electrons” produced in liquid water by electrons with initial energies of a few keV is obtained by using a Monte Carlo transport simulation calculation. It is found that the introduction of vibrational-excitation cross sections leads to the appearance of a sharp peak in the probability density function near the electronic-excitation threshold. Electrons contributing to this peak are shown to be more naturally described if a novel energy spectrum, that we propose to name “vibrationally-relaxing electron” spectrum, is introduced. The corresponding distribution function is presented, and an empirical expression of it is given.

Low-energy e+-N2 scattering calculations using the Kohn variational method

The authors present preliminary calculations of low-energy e+-N2 elastic scattering using the Kohn variational method. They present results for the Sigma g+ and Sigma u+ scattering symmetries below the first electron excitation threshold at 8.59 eV incident position energy. The trial functions for the calculations include separable correlation functions of Sigma g+, Sigma u+, Pi u and Pi g electronic symmetries and the results agree with R-matrix calculations by Danby and Tennyson (1991). The calculations are carried out in a similar way to previous successful low-energy e+-H2 scattering calculations. This paper shows that it is possible to apply the method used in the e+-H2 scattering calculations to positron scattering by a target molecule, N2, containing 14 electrons.

Subexcitation electrons: An appraisal of our understanding

The transport of subexcitation electrons, i.e. electrons having kinetic energies T below the first electronic excitation threshold E 1 of a medium, is a subject of considerable importance to the understanding of the earliest processes in radiation chemistry and biology. In recent years, relevant cross section values have been determined so that serious transport analysis is warranted, at least for gases composed of the commonly occurring molecules such as H 2, O 2, CO 2 and H 2O. Results for each gas elucidate a particular point, e.g. the role of resonances. For H 2O, transport analysis has been carried out for both gas and ice. The scope of the discussion is limited to kinetic energies much higher than the thermal energy, i.e. T ⪆ 0.1 eV at room temperature. The behavior of electrons at lower energies is treated by Shizgal in the present Symposium.