Effective gaunt factors for electron impact excitation of multiply-charged nitrogen and oxygen ions

Electron impact excitation Gaunt factors and thermally averaged Gaunt factors are obtained for the resonance and several nonresonance transitions for various ionization stages of nitrogen and oxygen. In those cases where comparisons can be made, the agreement is good. In addition, a simplified expression for the Gaunt factor is suggested by the data.     

Electron impact excitation collision strengths for neon-like Ni XIX calculated using the relativistic R-matrix method

In a recent paper [Pramana — J. Phys. 64, 129 (2005)] results have been presented for electron impact excitation collision strengths for transitions among the fine-structure levels of the 2s²2p⁶ and 2s²2p⁵3s configurations of Ni XIX. In this paper we demonstrate through an independent calculation with the relativistic R-matrix code that those results are unreliable and the conclusions drawn are invalid.     

Collision strengths for the N = 2 excitation of hydrogenic ions

An empirical formula is given for the estimation of the collision strengths for electron impact excitation of the 2s and 2p states of hydrogenic systems based on close coupling calculations. The coefficients in this formula are given as functions of the nuclear charge, Z, thereby making a possible estimation of the collision strength for any system between H and Fe²⁵⁺.     

Unimolecular dissociation of singly- and multiply-charged fullerene ions

The formation of fragment ions from C₆₀ due to electron impact ionization leading to a bimodal fragmentation pattern with a minimum around the cluster size of 30 is investigated with help of mass spectrometric techniques. Based on a detailed analysis of the energetics and kinetics of the singly- and multiply-charged fragment ions by two-sector field mass spectrometry (MIKE-scans) we can conclude that on the one hand some of the dissociation reactions proceed via evaporation of an intact C₆ ⁺ or C₄ ⁺ unit (and not via the sequential loss of smaller molecular carbon units). On the other hand, measured appearance energies and the direct observation of sequential C₂-loss reactions confirm that the sequential loss of neutral C₂ units is the dominant production mechanism for C_n ^z+ fragment ions with sizes 40 < n < 60. This revised version was published online in August 2006 with corrections to the Cover Date.     

类氢He+、Li2+离子的电子碰撞强度的R矩阵计算

利用R矩阵方法计算了电子与He+离子碰撞的1s-2s和1s-2p跃迁的碰撞强度,结果表明包括10个靶态的计算可以得到精确的类氢离子的n=1→n=2各能级跃迁的碰撞强度,并首次利用R矩阵方法计算了电子与Li2+离子碰撞的1s-2s和1s-2p跃迁的碰撞强度.     

The excitation mechanism of rare-earth ions in silicon nanocrystals

A detailed investigation on the excitation mechanisms of rare-earth (RE) ions introduced in Si nanocrystals (nc) is reported. Silicon nanocrystals were produced by high-dose 80-keV Si implantation in thermally grown SiO₂ followed by 1100 °C annealing for 1 h. Subsequently some of the samples were implanted by 300-keV Er, Yb, Nd, or Tm at doses in the range 2×10¹²–3×10¹⁵ /cm². The energy was chosen in such a way to locate the RE ions at the same depth where nanocrystals are. Finally an annealing at 900 °C for 5 min was performed in order to eliminate the implantation damage. These samples show intense room-temperature luminescence due to internal 4f shell transitions within the RE ions. For instance, luminescence at 1.54 μm and 0.98 μm is observed in Er-doped nc, at 0.98 μm in Yb-doped nc, at 0.92 μm in nc and two lines at 0.78 μm and 1.65 μm in Tm-doped nc. Furthermore, these signals are much more intense than those observed when RE ions are introduced in pure SiO₂ in the absence of nanocrystals, demonstrating the important role of nanocrystals in efficiently exciting the REs. It is shown that the intense nc-related luminescence at around 0.85 μm decreases with increasing RE concentration and the energy is preferentially transferred from excitons in the nc to the RE ions which, subsequently, emit radiatively. The exact mechanism of energy transfer has been studied in detail by excitation spectroscopy measurements and time-resolved photoluminescence. On the basis of the obtained results a plausible phenomenological model for the energy transfer mechanism emerges. The pumping laser generates excitons within the Si nanocrystals. Excitons confined in the nc can either give their energy to an intrinsic luminescent center emitting at around 0.85 μm nor pass this energy to the RE 4f shell, thus exciting the ion. The shape of the luminescence spectra suggests that excited rare-earth ions are not incorporated within the nanocrystals and the energy is transferred at a distance while they are embedded within SiO₂. Rare-earth excitation can quantitatively be described by an effective cross section σ_eff taking into account all the intermediate steps leading to excitation. We have directly measured σ_eff for Er in Si nc obtaining a value of ≈2×10⁻¹⁷ cm². This value is much higher than the cross section for excitation through direct photon absorption (8×10⁻²¹ cm²) demonstrating that this process is extremely efficient. Furthermore, the non-radiative decay processes typically limiting rare-earth luminescence in Si (namely back-transfer and Auger) are demonstrated to be absent in Si nc further improving the overall efficiency of the process. These data are reported and their implications. Received: 9 April 1999 / Accepted: 10 April 1999 / Published online: 2 June 1999     

Ultraviolet oscillator strengths for carbon,nitrogen and oxygen ions

Oscillator strengths were calculated for u.v. transitions in carbon, nitrogen and oxygen ions. Single and multiconfiguration numerical Hartree-Fock wavefunctions were used to represent initial and final states of the various transitions, and both dipole length and dipole velocity transition matrix elements were calculated. A careful choice of a few important configurations leads in most cases to multiconfiguration results which are in good agreement with experiment, and with other theoretical calculations.     

Line strengths and lifetimes for Kr II

Relative line strengths have been calculated for transitions between states of the 4p ⁴ 5p and 4p ⁴ 5s configurations in Kr II. The wavefunctions are obtained in intermediate coupling, including an effective correction for configuration interaction. Absolute values for the line strengths are determined using the coulomb approximation. Lifetimes for the upper 4p ⁴ 5p levels are given together with available experimental results for comparison.     

Line strengths and lifetimes for Kr II

Relative line strengths have been calculated for transitions between states of the 4p ⁴ 5p and 4p ⁴ 5s configurations in Kr II. The wavefunctions are obtained in intermediate coupling, including an effective correction for configuration interaction. Absolute values for the line strengths are determined using the coulomb approximation. Lifetimes for the upper 4p ⁴ 5p levels are given together with available experimental results for comparison.     

Soft X-ray gain in hydrogenic ions: Line pumping,ionization pumping and transient excitation

In a plasma containing hydrogenic ions which is irradiated by appropriate X-ray radiation, gain on the n=3n=2 (Balmer-) transition can be generated. Depending on the spectrum of the pump radiation, there are two different ways of obtaining the inversion: direct excitation of the upper laser level by line radiation, or ionization of the hydrogenic ions by broad band radiation with subsequent population of the upper level by recombination.In the first part of this paper, we present numerical calculations which compare the two pumping methods. The gain is evaluated under idealized conditions, i.e., with a pump spectrum containing only radiation useful for generating an inversion and under more realistic conditions, with a pump spectrum containing radiation at other frequencies as well.In the second part of the paper the possibility of obtaining Balmer- gain in hydrogenic ions under highly transient conditions is investigated, assuming a pump pulse with a duration comparable to the decay time of the lower laser level. Considerable gain is predicted even for a black-body pump spectrum.     

Kinetic parameters,collision rates,energy exchanges and transport coefficients of non-thermal electrons in premixed flames at sub-breakdown electric field strengths

The effects of an electric field on the collision rates, energy exchanges and transport properties of electrons in premixed flames are investigated via solutions to the Boltzmann kinetic equation. The case of high electric field strength, which results in high-energy, non-thermal electrons, is analysed in detail at sub-breakdown conditions. The rates of inelastic collisions and the energy exchange between electrons and neutrals in the reaction zone of the flame are characterised quantitatively. The analysis includes attachment, ionisation, impact dissociation, and vibrational and electronic excitation processes. Our results suggest that Townsend breakdown occurs for E/N = 140 Td. Vibrational excitation is the dominant process up to breakdown, despite important rates of electronic excitation of CO, CO₂ and N₂ as well as impact dissociation of O₂ being apparent from 50 Td onwards. Ohmic heating in the reaction zone is found to be negligible (less than 2% of peak heat release rate) up to breakdown field strengths for realistic electron densities equal to 10¹⁰ cm^?3. The observed trends are largely independent of equivalence ratio. In the non-thermal regime, electron transport coefficients are insensitive to mixture composition and approximately constant across the flame, but are highly dependent on the electric field strength. In the thermal limit, kinetic parameters and transport coefficients vary substantially across the flame due to the spatially inhomogeneous concentration of water vapour. A practical approach for identifying the plasma regime (thermal versus non-thermal) in studies of electric field effects on flames is proposed.     

Oscillator strengths for O(II) ions

Oscillator strengths between various doublet states of O(II) ions are calculated with extensive use of multi-configuration wave functions. The lower levels for the transition are in the 2p^{3 2}D⁰ and 2p^{3 2}P⁰ states, and the upper levels are 2p⁴, 3s, and 3d states. The results, which are estimated to have errors of less than 10% for individual transitions, agree quite well with beam-foil experiments, as well as with calculations made by the use of the non-closed-shell many-electron theory (NCMET). The present ratios of transition probabilities are estimated to have errors of less than 20%. A comparison with recent branching ratio measurements indicates that agreement between theory and experiment is within the overlap of the error bars except for the $\text{538}\text{581}$ Å pair. Agreement with rocket measurements is only within the overlap of the error bars for the $\text{718}\text{797}$ Å pair.     

类Na离子振子强度的相对论效应

 采用相对论和非相对论原子自洽场方法,计算了类Na离子（原子序数为11~92）的3s-np,3p-ns和3p-nd跃迁过程的激发能和光学振子强度;通过比较相对论与非相对论的结果,研究了M壳层电子的相对论效应随原子序数的变化规律。结果表明：对n=3的跃迁过程,相对论效应主要是影响激发能,对线强度(或跃迁矩阵元)的影响不重要;对于n>3的跃迁过程,相对论效应同时影响线强度和激发能,通常对线强度的影响更大。这些结果对惯性约束聚变和X射线激光研究中需要的不透明度参数计算有重要的参考意义。     

类Na离子振子强度的相对论效应

采用相对论和非相对论原子自洽场方法,计算了类Na离子（原子序数为11~92）的3s-np,3p-ns和3p-nd跃迁过程的激发能和光学振子强度;通过比较相对论与非相对论的结果,研究了M壳层电子的相对论效应随原子序数的变化规律。结果表明：对n=3的跃迁过程,相对论效应主要是影响激发能,对线强度(或跃迁矩阵元)的影响不重要;对于n>3的跃迁过程,相对论效应同时影响线强度和激发能,通常对线强度的影响更大。这些结果对惯性约束聚变和X射线激光研究中需要的不透明度参数计算有重要的参考意义。     

New efficient mechanism of excitation of electroluminescence from erbium ions in crystalline silicon

In p–n junctions based on c-Si : Er we have realized highly efficient excitation of erbium electroluminescence at 1.54 μm with an efficiency close to unity. A possible mechanism is Auger recombination of electrons occupying the upper subband of the conduction band with free holes in the valence band whereas the energy of the recombination process is transferred by Coulomb interaction to 4f-electrons of an erbium ion transmitting it to the second excited state ⁴I_11/2 (excitation energy 1.26 eV). The observed three-level excitation of erbium ions is promising for development of a Si : Er laser.     

Oscillator strengths of Cs and Rb-like ions

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 56, No. 1, pp. 13–17, January, 1992.     

Photoluminescence of Er3+ ions in amorphous silicon layers under intensive laser excitation

Transients of the photoluminescence (1.54 μm) of Er³⁺ ions embedded in an amorphous silicon matrix excited with intensive laser pulses are simulated using a phenomenological model which takes into account both the defect-related excitation mechanism and stimulated optical transitions in the ions. The simulated transients are compared with the experimental ones observed in Er-doped amorphous silicon layers under pulsed laser excitation. The modeling and the experimental results demonstrate a possibility to realize a regime of superradiance in the system of Er³⁺ ions pumped via an electronic excitation of the amorphous matrix. Received: 7 August 2001 / Revised version: 1 November 2001 / Published online: 17 January 2002     

Oscillator strengths and excitation cross sections of samarium atoms

It is shown that the available theoretical data on the composition of odd parity levels of the samarium atom lead to a distribution of oscillator strengths and electron impact excitation cross sections which differs significantly from the experimental data.     

Calculation and extrapolation of oscillator strengths in Rb-like,multiply charged ions

The oscillator strengths of dipole transitions between energy levels of the lowest configurations of ions of the Rb I isoelectronic sequence with nuclear charge Z=41–46 are calculated on the basis of the model potential method using an energy approach. For certain ions the reported data are published here for the first time. It is shown that the contribution of core polarization to the oscillator strength attains 15% for some transitions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 89–92, January, 1996.     

Classical binary-encounter collision theory for electron-impact ionization of ions

We calculated electron-ion impact ionization using classical binary-encounter collision theory. A differential cross section per unit energy and momentum transfer is derived for two-electron scattering. We obtain the total ionization cross section by integrating this differential cross section over the appropriate energy and momentum transfer and the bound electron-velocity distribution. To simplify the computation, we calculated the bound electron-velocity distribution in the Thomas-Fermi approximation. Comparison with other theoretical calculations shows that our results agree within 50% with the best quantum qpproximations. Our cross sections and rate coefficients, which are sufficiently accurate for many practical applications, are expressed in computationally simple form.