Near-threshold electron-impact ionization of magnesium

The effective electron-impact ionization cross section of the magnesium atom is studied in detail in the energy range from the ionization threshold to 16 eV. A large number of particular features caused both by atomic autoionization states and by the formation and decay of a short-lived state of the negative ion of the magnesium atom are revealed. These particular features are identified using experimental and theoretical data on the photoionization (photoabsorption) and on the spectra of ejected electrons.     

The fragmentation of negative ions of fullerene C60 trifluoromethyl derivatives

The formation and decay of gas-phase negative ions of trifluoromethylated fullerenes C₆₀(CF₃) _n (n = 2–10) were studied. The resonance electron capture mass spectra were measured to find that the main fragmentation channel of negative ions was the detachment of trifluoromethyl groups. The degree of fragmentation directly depended on the energy of electrons and reached the complete splitting off of all the CF₃ addends with the formation of C₆₀⁻ ions. The observed metastable ion signals were analyzed to determine the scheme of sequential fragmentation of negative ions.     

Negative molecular ions of azafullerenes and their hydrogenated derivatives

The formation and decay of negative molecular ions of azafullerenes and their hydrogenated derivatives are investigated by mass spectrometry. The mechanisms of resonant electron capture and the lifetimes of negative molecular ions with respect to the electron autodetachment in azafullerene molecules are discussed. A comparative analysis of the data obtained for azafullerenes and hydrogenated fullerene derivatives is carried out.     

Pulsed nanosecond annealing of magnesium-implanted silicon

Single-crystalline silicon is implanted by magnesium ions at room temperature and then subjected to pulsed ion-beam annealing. The surface morphology, crystallinity, and optical properties of the implanted silicon are studied before and after annealing. It is shown that ion implantation makes a near-surface layer of silicon about 0.1 m thick amorphous. Pulsed nanosecond ion-beam annealing results in silicon recrystallization and the formation of crystalline magnesium silicide precipitates. Optimal values of the implantation dose and pulse energy density for the formation of magnesium silicide precipitates in the near-surface layer of silicon are found.     

Formation of positive and negative ions of thymine molecules under the action of slow electrons

The formation of positive and negative molecules of thymine—a base of nucleic acids—under the action of slow electrons is investigated by the method of crossed electron and molecular beams. The method developed makes it possible to measure the molecular beam intensity and determine the energy dependences and absolute values of total cross sections for the formation of positive and negative ions of thymine molecules. It is found that the maximal cross section for the formation of positive ions is reached at an energy of 95 eV and its absolute value is, accordingly, 1.4 × 10^?15 cm². The total cross section for the formation of negative ions is 8.2 × 10^–18 cm² at an energy of 1.1 eV. The mass spectra of thymine molecules are measured and the cross sections of dissociative ionization are determined.     

On the energy dependence of the yield of doubly charged negative ions during the capture of free electrons by C60(CF3)12 trifluoromethylfullerene molecules

Peaks of C₆₀(CF₃) _n ^2? doubly charged negative ions (n = 6–12) have been observed in the mass spectra of the resonance electron capture by trifluoromethylfullerene C₆₀(CF₃)₁₂ molecules. It has been established that these ions are formed owing to the attachment of two free isoenergetic electrons. The autodetachment of an extra electron has been detected for the doubly charged molecular ions (n = 12). It has been established from the observation of the delayed fragmentation of the most abundant ions with n = 8 and 10 that the doubly charged negative ions, like their singly charged analogs, are metastable with respect to the separation of the CF₃ fragment(s). The yield of doubly charged negative ions has been obtained as a function of the electron energy. By comparing them with the analogous dependences for the singly charged ions, the specific features have been revealed which were associated with the presence of the repulsive Coulomb barrier and the regular effect of the doubled energy of two additional electrons on the energy dependence of the dissociative decay of the doubly charged negative ions. The absolute cross section for the formation of the C₆₀(CF₃) ₁₀ ^2? ions has been measured. At the energy of their yield maximum near the 5 eV, it is ～1 × 10^?19 cm².     

Dimer self-organization of impurity ytterbium ions in synthetic forsterite single crystals

Paramagnetic centers formed by impurity Yb³⁺ ions in synthetic forsterite (Mg₂SiO₄) grown by the Czochralski technique are studied by X-band CW and pulsed EPR spectroscopy. These centers are single ions substituting magnesium in two different crystallographic positions denoted М1 and М2, and dimer associates formed by two Yb³⁺ ions in nearby positions М1. It is established that there is a pronounced mechanism favoring self-organization of ytterbium ions in dimer associates during the crystal growth, and the mechanism of the spin–spin coupling between ytterbium ions in the associate has predominantly a dipole–dipole character, which makes it possible to control the energy of the spin–spin interaction by changing the orientation of the external magnetic field. The structural computer simulation of cluster ytterbium centers in forsterite crystals is carried out by the method of interatomic potentials using the GULP 4.0.1 code (General Utility Lattice Program). It is established that the formation of dimer associates in the form of a chain parallel to the crystallographic axis consisting of two ytterbium ions with a magnesium vacancy between them is the most energetically favorable for ytterbium ions substituting magnesium in the position М1.     

Sb3+对Tb3+激活硅酸盐玻璃发光性能的影响

利用高温熔融法分别制备了Sb3 ,Tb3 单掺和共掺的硅酸盐发光玻璃,并分析了它们的光谱性质。根据对Sb3 和Tb3 掺杂硅酸盐玻璃的激发光谱、发射光谱和发光衰减时间等特性的分析,研究了澄清剂Sb2O3的加入对Tb3 激活硅酸盐玻璃发光性能的影响。结果表明,在紫外光激发下,Tb3 激活硅酸盐玻璃中存在Sb3 离子至Tb3 离子的能量传递,但能量传递效率较低,能量传递表现为Sb3 离子的3P1能级与Tb3 离子的5D3能级之间的能量无辐射共振转移。同时Sb3 离子的加入将在Sb3 离子和Tb3 离子的激发重叠区域(200～350nm)对Tb3 离子的激发产生不小的负面影响,尚不足以通过Sb3 离子至Tb3 离子的能量传递得以弥补。因此在使用Sb2O3作为Tb3 激活硅酸盐发光玻璃的澄清剂时,应当注意权衡Sb3 离子对Tb3 离子发光性能的影响。     

Über die Energieabhängigkeit von Ionen-Molekül-Reaktionen mit negativen Ionen

Some ion-molecule reactions with negative ions (OH^? and NH ₂ ^? ) have been studied using a primary ion source, in which the negative ions were produced by electron impacts with electron resonance capture, and a secondary ion source, in which the primary negative ions produced ion-molecule reactions with formation of secondary negative ions. The secondary ions were magnetically analyzed and registered by a multiplier. Cross sections of the reactions in function of the energy of the primary ions were measured in the region of low energies up to about 20 eV. In the low energy region the cross sections rapidly decrease with increasing energy ～E ^?1/2 as it has been the case with the energy dependence of the cross section of charge transfer reactions with negative ions at the same energies.     

First-principles calculations of oxygen vacancies and cerium substitution in lutetium pyrosilicate

Cerium-doped lutetium pyrosilicate (LPS:Ce) has attracted much attention for its extensive applications. However, oxygen vacancies will lower its luminescent efficiency. The charge transfer transition between cerium ions and neighboring oxygen vacancies has a long decay time. First-principles calculations on oxygen vacancies, cerium substitution and their complexes in LPS have been performed to research their influence on luminescence. The bridging-oxygen vacancy has the lowest formation energy among oxygen vacancies. We discuss the process of luminescence quenching due to oxygen vacancies. The cerium substitution is less favorable in the oxygen-rich condition. The defect complex of the cerium substitution and bridging-oxygen vacancy has the lowest formation energy among defect complexes. The charge transfer transition between cerium ions and neighboring oxygen vacancies is not related to it but to other two defect complexes. All defect complexes have high formation energies in oxygen-rich condition. We discuss the density of states of perfect and defective crystals.     

Application of the united atom model for estimating the lifetime of negative molecular ions relative to electron autodetachment

A model proposed for describing the scattering of low-energy electrons (whose energy ranges between thermal energy and several electronvolts) from polyatomic molecules makes it possible to estimate the lifetime of shape resonances. The parameters of the model are determined by specific structural and experimental characteristics of molecules. The results of approximate computations of the lifetimes for negative ions of molecules with different symmetries (diatomic halogens, parabenzoquinone, fullerene C₆₀, benzothiadiazoles, anthraquinone derivatives, and substituted benzene forms) are presented. The obtained data show that the lifetimes are sufficient for the formation of fragment ions observed in the mass spectra of negative ions.     

Simulation of pulsed electropositive and electronegative plasmas

A particle-in-cell (PIC) code with nonperiodic boundary conditions, including ionization and ion motion, is used to simulate the approach to equilibrium and the decay in the postdischarge of model electropositive and electronegative plasmas in a symmetric RF diode. In the electropositive plasma the density decreases to 1/e in ~10 μs. The electronegative ion plasma density is about four times higher and decreases to 1/e in ~50 μs, with the electron temperature and density decreasing to zero in a few microseconds. During this latter time scale, a weak field is set up to drive the negative ions to the boundaries at the same rate as the ion diffusion velocity. In the postdischarge this is close to thermal, hence some hundreds of microseconds are required to remove most of the negative ions from the system. As negative ions are thought to be the precursors for particulate formation, plasmas pulsed at around 1 kHz will severely decrease the lifetime of the negative ions and thereby reduce the possibility of particulate growth     

Electron-impact ionization and dissociative ionization of sulfur in the gas phase

We describe the methods and the results of investigation of the yield of positive ions formed as a result of electron-impact ionization of sulfur. The ionization energy for the basic molecule and the energies corresponding to the emergence of fragment ions are obtained from the ionization efficiency curves. The dynamics of formation of molecular sulfur ions in the temperature range 320–700 K is investigated. The energy dependences of efficiency S _n of the ion formation for n = 1–6 are analyzed, and their appearance energies are determined. The total cross section of sulfur ionization by a monochromatic electron beam is also investigated. Using the linear approximation method, we marked out features on the ionization function curve, which correspond to the ionization and excitation energies for multiply charged ions. The total cross section of the formation of negative sulfur ions is measured in the energy range 0–9 eV.     

Optical spectroscopy of negative ions based on laser assisted electron attachment

The suggestion of the existence of a photoassociative electron attachment process, X = e + hv → X^?**, resulting in excited negative ion state formation is put forward, the energy necessary for the formation of such states being the sum of those of the photon and electron involved. The excited negative ion states can be detected by secondary electrons arising as a result of autodetachment and carrying away an energy well in excess of the incident electrons. The process concerned may provide the basis for a new type of unstable negative ion spectroscopy and constitutes a useful tool for obtaining atomic and molecular negative ions in a specific quantum state for spectroscopic studies.     

Formation of ion damage tracks

The formation of damage tracks in insulators from the passage of energetic (MeV/amu) ions indicates that the energy lost by an ion to electronic excitation is partially transferred to atomic motion. It is known that a track consists of localized regions of extended defects that are separated by lengths that exhibit only point defects. The utility of tracks for selective detection of various types of ions arises because of preferential chemical etching along the track as compared to etching the bulk material. In this letter we propose a new model to explain both the localized damage regions and the preferential etching of damage tracks. The formation of each region of extended defects is initiated by the Auger decay of a vacancy produced in an inner electronic shell of an atom of the insulator by the incident ion. This decay produces an intense source of ionization within a small volume around the decaying atom, which causes decomposition of the material in a manner similar to that observed in pulsed laser irradiation. The resulting chemical or crystalline modification of the material is the latent track, which because of its changed structure can be preferentially etched.     

Untersuchung der unelastischen Wechselwirkung langsamer monoenergetischer Elektronen mit einfachen Kohlenwasserstoff-Molekülen I.Elektronenanlagerungs-Wirkungsquerschnitte in Abhängikeit von der Elektronenenergie und vom Molekülaufbau

Investigation of the Inelastic Interaction of Slow Monoenergetic Electrons with Simple Hydrocarbon Molecules. I. Dependence of the Electron Attachment Cross Sections on Electron Energy and on Molecular Structure The formation of negative ions from a series of simple hydrocarbons was studied experimentally via an interaction of hydrocarbon molecules with monoenergetic electrons in the range of 0–15 eV. The ion formation is characterized by means of the attachment cross section in dependence on electron energy. Negative ions were formed as well by dissociative electron attachment as by direct electron attachment. The electron attachment is depending on electron energy, molecular seize, and on the number of unsaturated C-C-bonds in the molecule. The formation processes are discussed and the role of negative hydrocarbon ions in the plasma is estimated.     

Evolution of the density profiles and flows of charged particles during the diffusive decay of an electronegative gas plasma

Different scenarios of the spatiotemporal evolution of the parameters of the diffusive decay of a pulsed electronegative gas plasma in the absence of plasma chemical processes are studied. It is shown that nonlinear diffusion in a plasma with negative ions occurs in several stages. The rate of electron density decay increases with time and, in the beginning of the second stage, almost all the electrons escape from the discharge volume. On the other hand, the ion density profile is smoothed out due to ion-ion ambipolar diffusion and the flow of negative ions toward the wall is absent in the first stage of decay. In the second stage, the main diffusion mode is first established and then the ion-ion (electronless) plasma decays exponentially with a characteristic time determined by ion-ion ambipolar diffusion.     

Electronic excitations during grazing scattering of hydrogen atoms on KI(001) and LiF(001) surfaces

The energy loss of hydrogen atoms with energies of 400 eV and 1 keV is studied in coincidence with the number of emitted electrons during grazing scattering from atomically clean and flat KI(001) and LiF(001) surfaces. The energy loss spectra for specific numbers of emitted electrons are analyzed in terms of a binary interaction model based on the formation of transient negative ions via local capture of valence band electrons from anion sites. Based on computer simulations we derive for this interaction scenario probabilities for the production of surface excitons, for electron loss to the conduction band of KI, for emission of electrons, and for formation of negative hydrogen ions. The pronounced differences of data obtained for the two surfaces are attributed to the different electronic structures of KI and LiF.     

Energy spectrum of one-particle excitations in liquid dielectrics under high pressures and temperatures

The effect of pressure on the conductivity of molecular liquids (hydrogen, oxygen, and nitrogen) and alkali metals (cesium and rubidium) in the region of the experimentally observed dielectric-metal transition is investigated. It is shown that capture of free electrons by atoms or molecules (resulting in the formation of negative ions) is advantageous from the point of view of energy in liquids under moderately high pressures and temperatures. In spite of the fact that the ionization potential increases with density, the energy of an electron transition to the level of the negative ion decreases and, hence, the forbidden gap also decreases. For high densities, the level of negative ions is broadened and transformed into the conduction band. It is assumed that the exponential dependence of conductivity on density and temperature in the transition region is associated with transfer of quasi-free electrons located at the level of atomic negative ions. The spectrum of negative ions of hydrogen, oxygen, and cesium in the strongly compressed state is determined, and the forbidden gap calculated for these substances is found to be in good agreement with the results obtained for hydrogen and oxygen in single shock-wave experiments.     

On energy transfer and generation of an electric current in the vicinity of an electrode dipped in an electrolyte and strongly heated by a current passing through it

Processes occurring when a metal electrode dipped in an electrolyte is heated by intense evaporation of the electrolyte are considered in terms of a physically rigorous model. Based on the Onsager principle of least energy dissipation rate in nonequilibrium processes, the fractions of thermal energy that are spent on heating and evaporating the electrolyte and on heating the vapor are found. The energy is released within the vapor-gas sheath when an electric current flows between the electrode and electrolyte surface. It is found that the electrolyte vapor temperature exceeds 1300 K. Analytical expressions are derived for the vapor-gas sheath thickness, the electrolyte vapor pressure, and the velocity of the vapor escaping the discharge zone. It is shown that field evaporation of thermally activated negative ions from the electrolyte surface cannot provide an electric current with densities found in experiments but is responsible for the generation of free electrons near the electrolyte surface. These electrons arise when the ions decay via collisions with excited molecules.