Formation of fluorescence bands of polyatomic organic molecules in the gaseous phase upon excitation by electrons

Information on paths in absorption and deactivation of energy gained by molecules in their excitation by electron impact to low-lying singlet states has been obtained from an analysis of changes in the fluorescence spectra of these molecules. It is shown that there is a significant difference in the formation of fluorescence spectra when free molecules are excited by optical radiation and by electrons. It contrast to optical excitation, the interaction of an electron with a molecule is nonselective in character. All electronic states have a chance to be excited, which results in ensembles of emitting molecules with a different store of vibrational energy, and these ensembles each contribute to the fluorescence spectrum. Deceased. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 216–223, March–April, 1998.     

Interaction of Electrons with Indole,Tryptophan, and their Derivatives in the Gas Phase

The electron energy loss spectra (EELS) of indole, 3-indolyl propionic acid, 3-indolealdehyde, 3-dimethylaminomethylindole, tryptophan, and N-acetyl-L-tryptophan in the gas phase upon excitation by monokinetic electrons with an energy of E₀ = 11–50 eV are obtained. The structure of EELS is determined in the main by the indole chromophore; the side groups, except for the C=O group of 3-indolealdehyde, exert an insignificant influence. The energy of the lower triplet level ³L_a is 3.3 eV for indole and its derivatives and 3.2 eV for tryptophan and N-acetyl-L-tryptophan. Four singlet transitions in the region of 4.4–7.2 eV have been identified. The molecules studied, except for tryptophan, fluoresce in the gas phase on excitation by electrons. At low values of E₀ (10–25 eV), the fluorescence spectra are similar and are due to the indole fluorophore. Just as in the case of optical excitation, fluorescence on excitation by electrons is associated with the ¹L_b-S₀ transitions. An increase in the energy E₀ up to 60–80 eV leads to dissociation of a portion of the indole molecules and to the appearance of additional bands in the fluorescence spectrum. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 468–472, July–August, 2005.     

Phonons and electrons at metal surfaces

Some aspects of the coupling between phonons and electrons and the interaction between electrons at metal surfaces are reviewed. Surface science techniques as diverse as electron energy loss spectroscopy, angle-resolved photoelectron spectroscopy, and scanning tunnelling microscopy are employed to study these interactions. Electron–phonon and electron–electron coupling are discussed in terms of renormalized phonon dispersion relations, and increased decay rates of electronic excitations. PACS 63.20.Kr; 68.35.Ja; 68.37.Ef; 68.43.Pq; 72.10.Fk; 72.15.Qm; 73.20.At     

Fullerenes as polyradicals

We present the investigation of the electronic structure of X₆₀ molecules (X=C, Si), containing 60 odd electrons with spin-dependent interaction between them. Conditions for the electrons to be excluded from the covalent pairing are discussed. A computational spin-polarized quantum-chemical scheme is suggested to evaluate four parameters—energy of radicalization, exchange integral, atom spin density, and squared spin— to characterize the effect quantitatively. A polyradical character of the species, weak for C₆₀ and strong for Si₆₀, is established.     

强耦合的动力学Jahn-Teller效应理论

本文讨论了强耦合情况的、即电子的静态畸变能量大于晶格或分子振动量子能量的情况的动力学Jahn-Teller效应。我们发展了一种适用于强耦合情况的微扰方法,在其中将本征值及本征函数依照电子振动耦合系数的倒数或振动量子能量与静态畸变能量之比展开成幂级数,具体讨论了O_h点群中的Г₈态,求得了电子振动能级。区别于弱耦合情况的特点主要在于:1)虽然并不发生静态畸变,但是振动模的简并性及频谱却都发生了变化;2)如果与电子相耦合的振动模是“调谐”的,则电子及晶格振动的集体运动模将出现,这个理论能被用来解释Weinstock等人所做的关于TcF₆及ReF₆的红外光谱以及Raman光谱的实验结果。     

The spectra of characteristic energy loss by fast electrons and optical functions for different transmitted wave vectors in graphite

Based on the experimental spectra of energy loss by fast electrons in graphite in the range 0–40 eV, the spectra of complete complexes of optical functions, including the dielectric constant, which are decomposed into components, are calculated for the Г – P directions and four values of the transmitted electron wave vector |q|. The data obtained are compared with the known theoretical bands. It is found that in the indirect interband transitions of graphite excited by fast electrons with |q| > 0, a quasigap is opened in the neighborhood of point P, which increases with |q|.     

Radiation-induced defects in thin Cu(In,Ga)Se<Subscript>2</Subscript> films on exposure to high-energy electron irradiation

At 4.2 K, the photoluminescence spectra of Cu(In,Ga)Se₂ films irradiated by electrons with an energy of 5 MeV displayed the 0.93-and 0.79-eV bands that owe their origin to the radiative recombination of nonequilibrium charge carriers on radiation-induced defects. The position of the energy levels of the defects is determined and their nature is discussed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 805–808, November–December, 2005.     

Density functional study of structural and electronic properties of Al<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>P (2 ≤ <Emphasis Type="Italic">n</Emphasis> ≤ 12) clusters

Low-lying equilibrium geometric structures of Phosphorus-doped aluminum cluster Al _n P (n = 2–12) clusters obtained by an all-electron linear combination of atomic orbital approach, within spin-polarized density functional theory, are reported. The binding energy, dissociation energy, and stability of these clusters are studied within the local spin density approximation (LSDA) and the three-parameter hybrid generalized gradient approximation (GGA) due to Becke-Lee-Yang-Parr (B3LYP). Ionization potentials, electron affinities, hardness, and static polarizabilities are calculated for the ground-state structures within the GGA. It is observed that symmetric structures with the P atom occupying a peripheral position are lowest-energy geometries of Al _n P (n = 2, 4–11), while the P impurities of Al₃P and Al₁₂P prefer to occupy internal sites in the aluminum clusters. Generalized gradient approximation extends bond lengths as compared to the LSDA lengths. The odd-even oscillations in the dissociation energy, the second differences in energy, the HOMO–LUMO gaps, the ionization potential, the electron affinity, and the hardness are more pronounced within both GGA and LSDA. The stability analysis based on the energies clearly shows the clusters with an even number of valence electrons are more stable than clusters with odd number of valence electrons.     

On the theory of quantum interference between inelastic and elastic electron scattering events

The mechanism of weak localization of relatively fast electrons scattered with a fixed energy loss from disordered media is examined. The main focus of this paper is to put forward an explanation why coherent enhancement of electron scattering in the inelastic-scattering channel takes place at angles which differ from π. A simplified kinematic model is proposed to determine the basic properties of the weak localization of electrons in the inelastic scattering channel. The model reproduces easily the range of scattering angles typical of the weak localization of electrons with a fixed energy loss. The procedure does not require calculation of the contribution from the crossed diagrams. The results agree with those based on the dynamical theory associated with the calculation of the crossed and ladder diagrams. It is possible to follow the transition from the new type of weak localization to the ordinary weak localization with decreasing energy loss. The new-type weak localization is in agreement with the regular weak localization if the energy loss is approximately equal to the energy of an optical phonon. Zh. éksp. Teor. Fiz. 111, 1001–1015 (March 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

Absorption spectra of benzene-isooctane mixtures in the wavelength range 1620–1820 nm

Experimental absorption spectra of benzene, isooctane, and their mixtures are obtained in the wavelength range λ = 1620–1820 nm in which the first overtones of vibrational frequencies of CH, CH₂, and CH₃ hydrocarbon groups are located. Positions of fundamental absorption bands of benzene are refined. Absorption spectra of benzene-isooctane mixtures are shown to intersect in a narrow area near λ ≈ 1695 nm. The main maximum of benzene absorption at λ = 1671.5 ± 0.5 nm, where the influence of isooctane absorption is practically absent, is proposed for determining the content of benzene in benzene-isooctane mixtures. A linear calibration curve for λ = 1671.5 nm that encompasses the full range of benzene concentrations (0–100%) is presented. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 631–634, September–October, 2008.     

Temperature evolution of electronic and lattice configurations in highly ordered trans-polyacetylene

We have studied the temperature evolution of the optical absorption and Raman scattering spectra of a trans-cis blend of “soluble” polyacetylene in a polyvinylbutyral/butanol mixture. On decreasing temperature a reversible restructuring of the electronic and lattice systems of trans-(CH)_x occurs: the electronic energy gap contracts at the high rate of 0.4 meV/K, and the vibrational modes are modified. The experimental data obtained are interpreted in terms of a peculiar interaction of π-conjugated electrons with lattice fluctuations. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 613–617 (10 November 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Extraction of cross-sections of inelastic scattering from energy spectra of reflected atomic particles

REELS spectra of the electrons reflected off niobium are measured with energy resolution <0.5 eV within the 5–40 eV energy range of the probing beam. The measurements were performed for the scattering angles θ = 45° and θ = 120° by means of two electron guns. The process of energy losses is described within the framework of a model with three different energy loss laws: surface, intermediate, and bulk layers are considered. Differential cross-sections of inelastic scattering are represented in the form of simple equations.     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.     

Anregung von Kristallgitterschwingungen durch Elektronenstrahlen

The interaction of electrons with lattice vibrations in thin films of LiF, NaF, MgO, and Al₂O₃ has been studied by means of high resolution electron energy spectroscopy. Just above the excitation energy of the transversal optical modes small bands in the energy loss spectrum are observed. According to the dielectric theory of an unbounded medium only longitudinal optical modes should be excited. The observed energy loss spectrum may be explained by the excitation of surface lattice vibrations. A theoretical energy loss spectrum of LiF has been computed taking into account the surface effects. It is in excellent agreement with the experimental spectrum. Apart from electrons with energy loss also electrons with energy gain were observed. The intensity distribution of the energy gain spectrum is equivalent to the energy loss spectrum. Energy loss and gain spectra of fast electrons by excitation of lattice vibration are corresponding to the Stokes- and Antistokes lines of spectroscopy of light optics.     

Modification of electronic properties during adsorption of conjugate organic molecules on the surface of polycrystalline SnO<Subscript>2</Subscript>

The modification of the electronic structure during adsorption of ultrathin copper phthalocyanine (CuPc) and 3, 4, 9, 10 perylene-tetracarboxylic-dianhydride (PTCDA) coatings on the surface of polycrystalline tin dioxide is traced. Auger electron spectroscopy is employed to find changes in the atomic composition of the surface. It is found with the help of low-energy electron total current spectroscopy using a testing beam of electrons with energies up to 30 eV that the total current spectra typical of organic films are formed when the thickness of the coating being deposited is 2–7 nm. The formation of an interface layer 1.5–2.0 nm in thickness is detected, in which the intensity of the structure of the total current spectra decreases and the effect of interaction of PTCDA molecules with the SnO₂ surface is manifested.     

Ion bombardment of amorphous silicon films during plasma-enhanced chemical vapor deposition in an rf discharge

The characteristics of ion and electron fluxes to the surface of a growing silicon film are investigated in various rf discharge regimes in silane at frequencies of 13.56 MHz and 58 MHz in plasma-enhanced chemical vapor-deposition (PECVD) apparatus. The energy spectra of the ions and electrons bombarding the growing film are measured. The electronic properties of films grown under various degrees of ion bombardment are studied. The correlation of these properties with the ion parameters in the rf discharge plasma during film growth is discussed. Zh. Tekh. Fiz. 68, 52–59 (February 1998)     

Electroluminescent properties of dibenzoxazolyl biphenyl molecules

We have measured the energy loss spectra of 4,4′-bis[(E)-1-(1,3-benzoxazol-2-yl)-2-ethenyl]-2-n-hexyloxy biphenyl for interaction with electrons with energies 17.5 eV and 50 eV. We used time-dependent density functional theory to calculate spectra of the singlet transitions, which match the experimental data well. We have shown that the cross section for the long-wavelength transitions is greater than the cross section for the short-wave transitions, which is attractive for efficient excitation by low-energy electrons. Electroluminescence was achieved for the studied compound. The threshold voltage was 3.5 V. Introducing an additional layer of copper phthalocyanine increases the brightness of the luminescence several-fold. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 684–687, September–October, 2007.     

Secondary electron emission from thin carbon films

For the purpose of investigating how secondary electrons are produced in carbon, the correlation between energy-loss events and secondary electrons was studied experimentally by using the coincidence method. If a secondary electron is detected in coincidence with an electron transmitted through a thin film which has lost an amount of energy E, then the process causing this energy loss results in the production of secondary electrons. We established the existence of these coincidences and have taken inelastic and coincidence spectra for films of different thickness. We found that in carbon secondary electrons are predominantly produced as a result of energy losses of about 20 eV, with an efficiency of about 5%. The escape depth of secondary electrons was also estimated to be approximately 30 Å.