Three-body recombination of ions with electrons in cooler-storage rings

Three-body recombination (TBR) of bare ions with free electrons of anisotropic velocity distribution is discussed in the context of recombination experiments in cooler-storage rings. The recombination rates are derived from the electron impact ionization rates using the modified Saha equation. Analytical expressions for the TBR rates and new scaling rules with main quantum number n, atomic number Z, and both transverse and longitudinal electron temperatures are derived for the low n-states regime probed in experiments in cooler-storage rings. In this context, the discrepancies found between measured rates and predictions for radiative recombination for the e + Ne¹⁰⁺ system [Gao et al., Phys. Rev. Lett. 75 (1995) 4381] are discussed. Present results show that TBR rates for the flattened electron beam velocity distribution cannot account for the recombination enhancement observed at very low relative energies. This revised version was published online in August 2006 with corrections to the Cover Date.     

High power non-linear magnetophotoconductivity in n-GaAs using the UCSB free electron laser

The kinetics of electrons bound to shallow donor impurities in n-GaAs was investigated by saturation spectroscopy using the University of California at Santa Barbara free electron laser. The resonant photothermal conductivity from 1s–2p⁺ transitions was measured at intensities greatly exceeding previous studies. Saturation of bound-to-free photoionization transitions was measured from 0 to 4 Tesla. The 1s–2p⁺ resonant photoconductive signal shows a distinct intensity dependence caused by the competing bound-to-free transitions which saturate differently. Evaluation of the electron recombination kinetics allows us to calculate the transition time of electrons from the 2p⁺ level to the ground state, the recombination time of free electrons, and the thermal ionization probability of the 2p⁺ state.     

The energy of thermal electrons in electron beam created helium discharges

We have measured the electron energy of the thermal group of electrons in both longitudinal and transverse electron beam created helium glow discharges. The measurement technique employs the ratio of intensities of spectral lines in the 2s³S?np³P He I series. Values of kT_e between 0.07 and 0.11 eV were obtained. These energies are typical of the beam-generated electric field free plasmas. The competitive loss of helium ions by recombination and by charge transfer in a He?Hg electron beam created plasma is calculated. The results are applied to the Hg⁺ laser pumping scheme using a electron beam created He?Hg plasma.     

Laser-induced electron--ion recombination used to study enhanced spontaneous recombination during electron cooling

Spontaneous recombination of highly charged ions with free electrons in merged velocity matched electron and ion beams has been observed in earlier experiments to occur at rates significantly higher than predicted by theoretical estimates. To study this enhanced spontaneous recombination, laser induced recombination spectra were measured both in velocity matched beams and in beams with well defined relative velocities, corresponding to relative electron-ion detuning energies ranging from 1 meV up to 6.5 meV where the spontaneous recombination enhancement was found to be strongly reduced. Based on a comparison with simplified calculations, the development of the recombination spectra for decreasing detuning energies indicates additional contributions at matched velocities which could be related to the energy distribution of electrons causing the spontaneous recombination rate enhancement. This revised version was published online in August 2006 with corrections to the Cover Date.     

强场非次序双电离中再碰撞动力学的强度依赖

利用三维经典系综模型系统地研究了不同强度线偏振激光脉冲驱动下He原子的非次序双电离.结果表明在非次序双电离中回碰电子的返回次数、两电子的碰撞距离和电子对的关联特性都强烈地依赖于激光强度.对于750 nm,随着激光强度的增加,单次返回诱导的非次序双电离事件逐渐减少,而多次返回事件的比例显著增加.对于1500 nm,随着激光强度的增加,前三次返回诱导的非次序双电离事件都会减少,返回次数大于3的轨道对非次序双电离的贡献逐渐增加.这是因为在高强度下每次返回过程中母核的库仑吸引对返回电子横向偏离的补偿较弱,所以需要更多次的返回来补偿电子的横向偏离以实现再碰撞.轨道分析表明非次序双电离中两电子的碰撞距离随激光波长和强度的增加而逐渐减小.最后讨论了非次序双电离中电子对的关联特性对返回次数的依赖.     

Recombination of electrons with highly charged heavy ions at very low energies

Recombination of highly charged ions with free electrons is studied in merged-beams experiments at the UNILAC accelerator in Darmstadt and at the heavy-ion storage ring TSR in Heidelberg. Unexpected high recombination rates are observed for a number of ions at very low energiesE _cm in the electron-ion center-of-mass frame. In particular, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates of U²⁸⁺ ions nearE _cm=0 eV. The observations point to a general phenomenon in electron ion recombination depending onE _cm, on the ion charge state, and possibly also on electron density, electron beam temperature, and strength of external magnetic fields.     

Strong Enhancement of Electron–Ion Recombination Owing to Free–Bound and Bound–Bound Resonance Transitions

The effect of free–bound and bound–bound resonance nonadiabatic transitions of an electron on electron–ion recombination rates in the plasma of a Ne/Xe and Ar/Xe inert gas mixture has been studied. A kinetic model of recombination has been proposed including energy relaxation in collisions with electrons, resonant electron capture to Rydberg states through three-body collisions of Xe⁺ ions with Ne or Ar atoms and dissociative recombination of NeXe⁺ or ArXe⁺ ions, and n → n' resonance transitions. It has been shown that effective resonance processes occurring in quasimolecular systems sharply increase both the recombination coefficient and the effect of collisions with neutral particles even at quite high degrees of ionization of the plasma.     

The electron energy loss spectrum of EuO(100)

Energy losses of 200 eV to 2 keV electrons reflected from a disordered EuO(100) crystal show a bulk plasmon loss consistent with just less than six “quasi free” electrons per EuO unit, and 5p → nd resonance losses above the 5p threshold. The ratio of intensity of the 4d¹⁰ 4fⁿ⁰ → 4d⁹ 4fⁿ⁺¹ “giant resonance” loss at 142 eV to the corresponding direct recombination feature varies with energy, while the direct recombination and related Auger channels show similar energy dependence.     

Recombination of U 92 + ions with electrons

Recombination of fully stripped U⁹²⁺ ions with electrons has been investigated at the Experimental Storage Ring (ESR) in Darmstadt. Absolute recombination rate coefficients have been measured for relative energies from 0 to 33 eV. For energies greater than 20 meV the experimental result is well described by the theory for radiative recombination (RR). Below 20 meV the experimental rate increasingly exceeds the RR calculation as observed previously in the recombination of light bare ions as well as of Bi⁸³⁺. This low-energy rate enhancement is shown to scale as Z^2.6 for bare ions, where Z is the atomic number of the ion. The U⁹²⁺ recombination rate enhancement is insensitive to changes of the electron density. Variation of the magnetic guiding field strength from 80 mT to 120 mT resulted in oscillations of the recombination rate at 0 eV. The oscillations are partly attributed to changes of the transverse electron temperature accompanying the change of the magnetic guiding field strength; partly they may be caused by uncompensated small changes of the interaction angle between the two beams. Received 1st March 2001 and Received in final form 20 April 2001     

Recombination of Au25+ with free electrons at low energies

The origin of the widely observed enhancement of rates for electron-ion recombination at very low energies is still unknown. We investigated the recombination of Au²⁵⁺ with free electrons in a merged-beams experiment at the UNILAC accelerator of the GSI in Darmstadt. At E _rel= 0 eV we found an enormous enhancement factor of 365 compared to the theory of radiative recombination. An increase of the electron density by a factor of 10 had not much influence on the measured rate coefficient. This revised version was published online in July 2006 with corrections to the Cover Date.     

Recombination of lithium-like bismuth and uranium ions with free electrons at the ESR

In two separate experiments at the ESR recombination of 230 MeV/u Bi⁸⁰⁺ and 170 MeV/u U⁸⁹⁺ ions was studied. The analysis of these experiments has been carried on in order to extract information about radiative recombination (RR) of very highly charged ions exposed to electrons with a density of few 10⁶ cm^-3. In the light of the recombination rate enhancement seen in almost all recent experiments at a relative energy E_rel = 0 eV, a surprising result is found for the very highly charged ions, which could add some confusion to an already puzzling picture: in spite of the high charge state of the investigated ions there is no evidence of enhanced recombination beyond the expected RR rate. This observation appears to be supported by other experimental findings at the ESR as well. We suspect, however, that this is a consequence of excessive transverse magnetic field components in the ESR cooler (which have meanwhile been corrected for) causing variable degrees of angular mismatch between the ion beam and the electron beam occurring when the ring settings are tuned for optimum cooling. This revised version was published online in August 2006 with corrections to the Cover Date.     

Simultaneous luminescence due to excitons,condensation and free electrons phases in silicon

Using electrical injection in silicon P⁺πN⁺ structures at low temperature, we observe simultaneously recombination lines from condensation, free excitons and free electrons phases. The possible simultaneous observation of the three phases (condensation, excitons, free electrons) is therefore demonstrated in silicon. The actual temperature of the sample is discussed with the help of recombination oscillations and current-voltage characteristics.     

A first study of dielectronic recombination with the super-expanded electron beam in CRYRING

A first temperature measurement of the expanded electron beam from a super-conducting magnet in the cooler of CRYRING is reported. It is based on detection of a dielectronic recombination resonance in C²⁺. For this measurement C³⁺ was stored in the ring at 3 rm{MeV/amu} energy. A fit to the resonance with free transverse and longitudinal temperatures gave 3.9 × 10^-3 eV and 4.5 × 10^-5 eV, respectively. The result is also compatible with the values of 10^-3 eV for the transverse and 6.7 ×10-5 eV for the longitudinal component, in good agreement with expected values. This revised version was published online in July 2006 with corrections to the Cover Date.     

Recombination measurements at low energies with Ar16+ and Ar18+ ions in a dense, cold electron target

Recombination of multiply charged ions with electrons at very low relative energies has become a major topic of interest, due to the observation of rates which are enhanced beyond the expectations for radiative recombination. We present results for Ar¹⁶⁺ and Ar¹⁸⁺ ions from systematic measurements along the argon isonuclear sequence using a high density cold electron beam target (n_e = 7 × 10⁹ cm^-3) at the UNILAC of GSI. The transverse and longitudinal temperatures of the electron beam were determined from DR resonance features observed with metastable Ar¹⁶⁺ (2³S) ions. The rate at E_rel = 0 for radiative recombination of completely stripped Ar¹⁸⁺ calculated with electron beam temperatures kT_∥ = 0.002 eV, kT_⊥ = 0.2 eV amounts to α = 10^-9 cm³ s^-1. This is exceeded by nearly a factor of 10 by the rate measured in experiments with Ar¹⁸⁺ ions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Manifestation of collective effects in polarization-resolved recombination spectra of the completely filled zeroth landau level of two-dimensional electrons

The luminescence spectra due to recombination of two-dimensional electrons with optically excited holes have been studied in a wide range of electron filling factors in the transverse magnetic field. A nonmonotonic filling-factor dependence of the energy splitting between different circular polarizations of photoluminescence from the completely filled zeroth Landau level of electrons has been observed. It has been shown that this dependence is associated with collective (excitonic) effects that appear due to the interaction between electrons from partially occupied upper Landau levels and holes remaining on the zeroth Landau level after recombination.     

The Effect of Point Defects on the Activation Processes in Semiinsulating GaAs Irradiated by Si28 Ions

The layered concentration, concentration profile, and mobility of electrons in the Si²⁸ ion-implanted layers (IIL) of semiinsulating GaAs are investigated. The specific resistance of the latter is also studied upon radiation annealing (RA) in the temperature range 590–800 °C using electron energies higher than the threshold energy of defect formation. The IIL are shown to form during RA at much lower temperatures. The layers exhibit high electrical activation of Si²⁸ ions, with the electron-concentration profile corresponding to the calculated one, and a low concentration of residual defects limiting the electron mobility. The radiation annealing increases the resistance of semiinsulating GaAs. The calculations show that these effects are due to the Frenkel pairs (FP) generated by radiation. A high degree of ionization of GaAs atoms significantly reduces energies of potential barriers of diffusion, FP recombination, and electrical impurity activation.     

Magnetic field dependence in electron–deuteron recombination at very low relative energy

The rate coefficient for recombination of D⁺ with low energy electrons has been measured at different magnetic fields in the interaction region of the electron cooler at CRYRING. It is found that the measured recombination rate coefficient is influenced by the magnetic field. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dissociative recombination of 3HeH+: comparison of spectra obtained with 100, 10 and 1 meV temperature electron beams

The heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory at Stockholm University has been used for the study of dissociative recombination of ³HeH⁺. The new adiabatically expanded electron beam at CRYRING, which is achieved by means of a superconducting magnet, was used. The electron-beam expansion factor of 100 gave a transverse electron temperature of about 1 meV. This allowed the observation of several new resonances in the recombination cross-section. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modeling of radiation-induced charge trapping in MOS devices under ionizing irradiation

The numerical model of the radiation-induced charge trapping process in the oxide layer of a MOS device under ionizing irradiation is developed; the model includes carrier transport, hole capture by traps in different states, recombination of free electrons and trapped holes, kinetics of hydrogen ions which can be accumulated in the material during transistor manufacture, and accumulation and charging of interface states. Modeling of n-channel MOSFET behavior under 1 MeV photon irradiation is performed. The obtained dose dependences of the threshold voltage shift and its contributions from trapped holes and interface states are in good agreement with experimental data.     

Elastic and inelastic contributions to the auger electron appearance potential spectrum of titanium

The energy distribution of electrons contributing to the L-shell Auger electron appearance potential spectrum of a polycrystalline titanium surface has been measured. The Auger electron appearance potential spectrum is obtained by differentiating the total secondary electron yield of an electron bombarded sample as a function of incident electron energy. At the threshold for scattering from a core level the secondary yield increases. Most of the electrons contributing to this increase have energies below 30 eV, and result from secondary processes following Auger recombination of the core hole. The elastic yield decreases at the threshold, however, due to opening a new channel for inelastic scattering. A comparison of the elastic yield spectrum (DAPS), the total yield spectrum (AEAPS) and the soft X-ray yield spectrum (SXAPS), shows very similar line shapes, but differences in the relative strengths of the lines.