Gamma-ray spectra in the positron-annihilation process of molecules at room temperature

In this study, a fully self-consistent method was developed to obtain the wave functions of the positron and electrons in molecules simultaneously. The wave function of a positron at room temperature, with a characteristic energy of approximately 0.04 eV [1], was used to analyse the experimental results of its annihilation in helium, neon, hydrogen, and methane molecules. The interactions between the positron and molecule provide a significant correction in the gamma-ray spectra of the annihilating electron–positron pairs. It was also observed that high-order correlations offered almost no correction in the spectra, as the interaction between the low-energy positron and electrons cannot drive the electrons into excited electronic states. More accurate studies, which consider the coupling of the positron–electron pair states and vibration states of nuclei, must be undertaken.     

Size effects in photoexcitation of triplet states of ammonium tetraphenylborate

This paper reports on the results of complex investigations of photoexcited states of ammonium tetraphenylborate, which are characterized by self-sensitized luminescence. It has been established that the excitation by UV light at 77 K leads to the formation of stable triplet states due to the capture of electrons on electron traps. The EPR and luminescence excitation spectra exhibit the formation of a set of triplet states with different distances between electrons and holes. The performed investigations give grounds to affirm that, in bulk samples, cations in the structure of ammonium tetraphenylborate are electron traps. When the size of the ammonium tetraphenylborate sample is changed to 6 and 3 nm, the capture of excited electrons on sorbed oxygen molecules becomes dominant. In this case, the appearance of the spectrum of O₂⁻ anion radicals has been detected by the EPR method. The proposed interpretation of the observed effects has been confirmed by the thermoluminescence data on the recombination of electron-hole pairs, which correlate with a change in the intensity of the EPR spectra during annealing.     

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.     

Quantum-chemical study of the structure and optical properties of sensitized dyes of an indoline-thiazolidine series

Based on the density functional theory, quantum-chemical calculations of the structure and electronic absorption spectra of the molecules D102, D149, and D205 of an indoline-thiazolidine series, which are used as sensitizers for solar cells, are performed. Circular dichroism spectra are predicted. The mechanisms by which intra- and intermolecular electron transfer occur upon excitation to a triplet state, as well as the relaxation mechanism, are described. The geometric and electronic structures of the molecules under study in the ground singlet and excited triplet states are considered, and the relation between their structure and photochemical properties is discussed.     

Analysis of electronic-vibrational spectra of uracil,thymine, and cytosine

A theoretical analysis of absorption spectra of uracil, thymine, and cytosine—nucleic acid bases— is carried out. Structural dynamic models of these molecules in their electronically excited states are constructed. On the basis of the calculated vibrational structure of the electronic spectra, different tautomeric forms of these molecules are determined. The possibility of modeling the influence of hydrogen bonds on the electronic-vibrational spectra is shown.     

Biography

The mononegative, dinegative and monopositive ions of biphenylene have been formed in solution. The electronic spectra of the negative ions (in tetrahydrofurane and dimethoxyethane solution) have been measured in the range 10 000–44 000 cm^-1, and that of the positive ion (in H₂SO₄ solution) in the range 10 000–35 000 cm^-1. The presence of paramagnetic ions in the solutions of the singly-charged species has been confirmed by measurement of electron spin resonance absorption. The long wavelength electronic spectra of the monopositive and mononegative ions are closely similar, which confirms the essentially ‘aromatic’ character of the biphenylene structure. Assignments are proposed for the principal bands in the spectra of the ions and the neutral molecule. Explicit calculation of the energies of the excited states by an approximate self-consistent field molecular orbital method yields results in reasonably good overall agreement with the experimental energy spectra. There is evidence from relative intensities of some of the bands that the ‘pairing’ of bonding and antibonding sets of molecular orbitals which is characteristic of benzenoid alternant hydrocarbons is partially removed in biphenylene. This may result from a difference of electronegativity of the carbon atoms in the 4-membered ring from that of the remaining carbon atoms.     

Multiquantum scattering processes and transmission electron energy loss spectra

Microscopic many-body theory for electronic properties of solid states is developed with an emphasis on the role of correlation memory effects. Heisenberg equation of motion, fluctuation-dissipation theorem and operators of commutation have been used to calculate multiplasmon transmission electron energy loss spectra. Multiquantum integral kinetic equation for the longitudinal complex dielectric function is derived. Strong interaction between high-energy probe beam electrons penetrating the solid state and plasma of valence electrons is taken into account. It is shown that average number of high-frequency plasmons generated in every collision process is more than one for typical values of metal parameters. It is obtained that excitation of a good few plasmons is simultaneous event. Calculated multiplasmon structure of electron energy loss spectra coincides with experimental.     

Influence of radiation cross section uncertainties on shock wave structure

This numerical study investigates the sensitivity of non-equilibrium shockwave structure to uncertainties in (a) the ground and excited state continuum radiative cross section and (b) the bound-bound radiative cross section in a three level (ground, excited, and free electronic states considered) argon-like gas at Mach 18 and a pressure of 1 cm Hg. Changing the values of the radiative cross sections by an order of magnitude does not significantly influence the relaxation region; however, a large change occurs in not only the magnitude but also the extent of both the electron and excited state precursors. Increasing either the ground state continuum or the bound-bound cross section decreases the number of free electrons in front of the shock wave. These cross sections also influence the extent of the electron precursor through their influence on the ratio of the electrons produced by ground state photoionization to those produced by ground state photoexcitation followed by excited state photoionization. The bound-bound cross section also controls the excited state precursor, because the production of excited states is entirely due to line radiation. Increasing the excited state photoionization cross section increases both the magnitude and extent of the electron precursor, because the cross section influences the ratio of the two processes that compete to produce free electrons, and at the same time it influences the magnitude of the excited state precursor.     

Impact of the Vibrationally Excited States on the Macroscopic Behaviour of the Band-like Electron Beam Discharge Plasma in H/H2-Mixture

The investigation of the impact of the vibrationally excited molecules in the electronic ground state was performed by simultaneously solving a balance equation system for the main charge carriers, the H atoms, the metastable H atoms, the H₂ molecules in the different vibrational states and for the power transfer of the electrons in the beam discharge mixture plasma. The balance equations for the vibrational states include in particular one-quantum step excitation and deexcitation, electronic excitation, dissociation and ionization from each vibrational level in electron collisions as well as the finite life time of these states because of the gas transfer through the band-like plasma. A main finding is that due to the additional impact of vibrationally excited molecules there is a marked enhancement of the resulting dissociation and ionization degree in the beam discharge plasma at medium power input from the turbulent electric field. For discharge parameters of practical interest the ionization and dissociation budget, the population of the vibrational states, the different energy dissipation processes and the energy pumping into the ladder of the vibrational states were calculated and discussed in detail.     

The structure of 16O; A review of the theory

The nucleus ¹⁶ ₈O₈ is the prototype for a large number of developments in nuclear structure theory. It is a doubly magic N=Z nucleus, light enough that an isotopic spin formalism should be a valid approximation. The Brueckner-Hartree-Fock procedure in a spherical basis should be capable of describing the gross properties of the ground state. The excited states of negative parity exhibit the characteristic low-lying ‘octupole vibrational state’ and there is a much studied ‘giant dipole region’ which should be amenable to the analysis of the ‘random phase approximation’. The first excited state is the ‘mysterious second zero’ par excellence and a great deal of work on describing it via the method of ‘deformed state admixtures’ has been carried out. The first excited state and a number of other excited states appear to support spectra reminiscent of rotational bands and the collective character of these states has been extensively studied in both the Bloch-Horowitz and α-cluster model schemes.     

Identification of excited singlet states of chlorophenol isomers

The UV spectra of optical absorption of para-, meta-, and ortho-chlorophenol are recorded in the gas phase. The bands of UV spectra are assigned to the electronic transitions of molecules to definite excited singlet states on the basis of calculations by the TDDFT B3LYP/6-311++G(d, p) method. In each case the electron configuration making the predominant contribution to the particular singlet state is determined. The energies of singlet electronic transitions are shown to depend on the energy spacing between the molecular orbitals involved in these transitions.     

基于电子顺磁共振的ZnTPP激发态及其TEMPO各向异性的研究

为了研究卟啉类敏化剂的光致激发态能量转移和电子转移问题,本文基于紫外可见光谱仪和电子顺磁共振波谱仪,构建了以锌卟啉为研究对象的"锌卟啉-稳态自由基-二甲苯"实验体系.锌卟啉的紫外可见光谱显示,在可见光谱的B带和Q带出现明显的特征峰,是锌卟啉分子中电子由基态能级跃迁至激发态能级产生的.低温条件下受紫外可见光辐照的实验体系的电子顺磁共振波谱,检测到了稳态自由基四甲基哌啶氧化物的增强吸收电子顺磁共振波谱.根据分子激发态相关理论、光化学物理反应理论和化学诱导电子自旋极化理论对实验结果进行了分析,结果表明,四甲基哌啶氧化物稳态自由基电子顺磁共振波谱的增强吸收对应于锌卟啉光致激发态的能量转移和电子转移;四甲基哌啶氧化物在低温(143 K)下的电子顺磁共振波谱表现出的各向异性特征现象来源于氮氧自由基电子与氮核的各向异性超精细相互作用.     

Calculation and interpretation of vibronic absorption and fluorescence spectra of the first electronic nπ* transitions of pyridine and pyrimidine

We have calculated vibronic spectra of the first electronic nπ* transitions of pyridine and pyrimidine in the isolated state using the DFT method in the Franck-Condon approximation. Vibrational spectra for the ground and excited states have been calculated in the anharmonic approximation, which allowed us to refine the assignment of normal vibrations of pyridine and pyrimidine. We have done a complete interpretation of the vibrational structure of the absorption and fluorescence spectra of pyridine and pyrimidine. It has been shown that Fermi resonances between fundamental and combination vibrations and overtones 12 and 16b + 4, 6a and 2 × 16b affect the formation of the vibrational structure of electronic spectra of pyrimidine. Good agreement between calculated and experimental spectra confirms the correctness of the models of the two molecules in their ground and excited states, which makes it possible to use the models in further investigations of various properties of these molecules in electronically excited states, e.g., tautomerism of pyrimidine bases of nucleic acids.     

Quantum theory of electron stimulated desorption

The process of electron stimulated desorption of adsorbates from metal surfaces is investigated within the framework of quantum mechanical scattering theory. The Born-Oppenheimer adiabatic approximation is assumed to be valid for the adsorbate motion. The transition amplitude for desorption via the resonant excitation of excited states of the adsorbate then can be factorized into an electronic excitation amplitude and a Franck-Condon factor. The Franck-Condon factor is more complicated than in molecules. The continuum of substrate excitations coupling to the adsorbate gives rise to an absorptive part of the Born-Oppenheimer potential governing the motion of the adsorbate in the excited state. This absorptive part leads to a considerable reduction of the desorption cross section. Explicit quantum mechanical expressions for the corresponding reduction factor are given.The desorption of neutrals is considered in some detail. It turns out that within the adiabatic approximation this process requires the existence of neutral excited states of the adsorbate. The reneutralization of ionic excited states by electron capture from the substrate back into the ground state of the adsorbate, while possible on purely energetical grounds, occurs with zero probability in the adiabatic approximation and thus cannot be responsible for the large abundance of neutral desorbing particles. Neutral excited states of the adsorbate in principle should show up in inelastic electron scattering. The relation between electron stimulated desorption cross sections and inelastic electron scattering cross sections is discussed briefly.     

Cluster-model study on the K-shell excited states of crystalline lithium under intense laser irradiation

Molecular-orbital calculations are performed to elucidate electronic structures and optical properties of lithium clusters in which several K-shell electrons are simultaneously excited to the valence levels. It is shown that relaxation of valence electrons around localized core holes influences the photoabsorption near-edge spectra significantly. The spectra in the excited state are modified from those in the ground state due to the presence of initial core holes. Potential energy surfaces are calculated for core-ionized Li₉ ^z+ clusters, which exhibit bound states for z≤3. The present cluster calculations would serve as prototypical models of laser-excited hollow atom solids with applications to X-ray optics.     

Transport properties of quantum dots

Linear and nonlinear transport through a quantum dot that is weakly coupled to ideal quantum leads is investigated in the parameter regime where charging and geometrical quantization effects coexist. The exact eigenstates and spins of a finite number of correlated electrons confined within the dot are combined with a rate equation. The current is calculated in the regime of sequential tunneling. The analytic solution for an Anderson impurity is given. The phenomenological charging model is compared with the quantum mechanical model for interacting electrons. The current-voltage characteristics show Coulomb blockade. The excited states lead to additional fine-structure in the current voltage characteristics. Asymmetry in the coupling between the quantum dot and the leads causes asymmetry in the conductance peaks which is reversed with the bias voltage. The spin selection rules can cause a ‘spin blockade’ which decreases the current when certain excited states become involved in the transport. In two-dimensional dots, peaks in the linear conductance can be suppressed at low temperatures, when the total spins of the corresponding ground states differ by more than 1/2. In a magnetic field, an electron number parity effect due to the different spins of the many-electron ground states is predicted in addition to the vanishing of the spin blockade effect. All of the predicted features are consistent with recent experiments.     

电子激发和溶剂效应对芴酮-甲醇分子间氢键增强现象的理论研究 (cited: 1)

理论研究了电子激发和溶剂效应导致的芴酮-甲醇复合体系中分子间氢键增强现象．通过基态和激发态性质的计算,不仅展示了分子间氢键键长的变化以及变化在振动光谱中的影响,而且揭示了导致氢键变化的内在物理机制：溶质分子的电子激发及溶剂化效应引起的电子重新分布,增大了溶质和溶剂分子的偶极矩,导致了它们之间的相互作用的增大,并最终加强了分子间氢键的强度．还分别对处于液相及气相中的复合体的基态和激发态的几何结构、红外谱、复合体及构成分子的偶极矩进行了理论计算,结果阐明了电子激发与溶剂化效应对氢键变化的贡献,同时还发现只有进一步引入溶剂化效应,复合体的基态、激发态的性质才能与实验达到精确一致．所有激发态均采用所开发的基于含时密度泛函理论解析计算一阶、二阶激发态能量导数的方法．     

Electron impact excitation of luminescence of uracil

We have measured the luminescence spectra of isolated uracil molecules in the wavelength range of 200–500 nm that were excited by slow electrons. The spectrum contains more than 20 spectral bands and lines. We show that the luminescence spectrum of uracil is formed by processes of dissociative excitation of molecules, dissociative excitation with ionization, and excitation of electronic states of the initial molecule and molecular ion.     

OPTICAL PROPERTIES OF δ-DOPED GaAs AND GaAs/Al0.1Ga0.9As SUPERLATTICES

Radiative transition in δ-doped GaAs superlattices with and without Al_0.1Ga_0.9As barriers is investigated by using photoluminescence at low temperatures. The experimental results show that the transition mechanism of δ-doped superlattices is very different from that of ordinary superlattices. Emission intensity of the transition from the electron first excited state to hole states is obviously stronger than that from the electron ground state to hole states due to larger overlap integral between wavefunctions of electrons in the first excited state and hole states. Based on the effective mass theory we have calculated the self-consistent potentials, optical transition matrix elements and photoluminescence spectra for two different samples. By using this model we can explain the main optical characteristics measured. Moreover, after taking into account the bandgap renormalization energy, good agreement between experiment and theory is obtained.     

The evaluation of temporal electronic structures of nonresonant Raman excited virtual state of thiourea

An algorithm has been introduced to calculate molecular bond polarizabilities of thiourea, which supply essential electronic information about the nonresonant Raman excited virtual states. The main dynamical behaviour of the excited virtual states of thiourea is that the Raman excited electrons tend to flow to the N-H bonds and C-N bonds from the S-C bonds because of the electronic repulsion effect. The difference in Raman excited electron relaxation time of thiourea under 514.5-nm and 325-nm excitations has been observed, which quantitatively shows that the Raman scattering process is dependent on the wavelength of the pumping laser. Finally, the distribution of the electrons at the final stage of relaxation is given out through the comparison between the bond electronic densities of the ground states and the bond polarizabilities after deexcitation.