Energy dependence of the ionization probability of sputtered Cu and Ni

Despite its great sensitivity, the usefulness of secondary ion mass spectrometry (SIMS) for many applications has been limited by an inadequate understanding of the probability of sputtering an atom in an ionized state. To determine this ionization probability for clean Cu and Ni surfaces, I have measured the energy distribution of sputtered neutrals and ions by quadrupole mass filtering and retarding potential analysis using potential modulation differentiation. Analysis of sputtered neutrals was accomplished by electron impact ionization. Because the neutrals outnumber the ions by at least two orders of magnitude, the ratio of sputtered ions to neutrals is an accurate measure of the ionization probability. For energies below 20 eV the dependence of the ionization probability on energy goes as P(E) α Eⁿ, where n = 0.65 for clean Cu. The absorption of oxygen on the Cu surface increases the total ion yield while causing a reduction in the value of the exponent n. Similar results are found for nickel, where n = 0.54 for the clean surface.     

Probability of the two-electron mechanism of the formation of doubly charged barium ions as a function of laser radiation intensity

The formation of singly and doubly charged ions upon nonlinear ionization of barium atoms is studied as a function of the frequency and intensity of a 8800-8920-cm^?1 IR color center laser. Barium atoms are ionized via the four-photon resonance with the strongly perturbed bound 6p ^{2 1} D ₂ state. Doubly charged barium ions are produced by a two-electron mechanism. It is found that the probability of the formation of doubly charged ions under these conditions linearly depends on the laser radiation intensity.     

Determination of energy dependent ionization probabilities of sputtered particles

We present a novel method to determine the spectral ionization probability of sputtered species as a function of their emission velocity or energy. The technique is based on detection of neutral and ionic species in a reflectron time-of-flight mass spectrometer under otherwise identical experimental conditions. Using a pulsed ion extraction scheme in combination with sufficiently short primary ion pulses, the spectral ionization probability α⁺(v) can be determined without knowledge of possible energy discrimination effects in instrument transmission. Comparing the measured ionization probability with theoretical predictions, we find that none of the prevailing ionization models is capable of describing the experimental data over the whole velocity range studied.     

Impact parameter dependence of the probability for K-shell ionization by nuclei

The probability W for the atomic ionization by nuclei as a function of the impact parameter x is expressed through the ionization matrix element M_if. Values of W(x) for K-shell ionization calculated with the first Born M_if are compared with the experimental data and other calculations.     

Coupled channel analysis of inner-shell vacancy formation in superheavy quasimolecules

We calculate the ionization probability of the quasimolecular 1sσ-state in collisions of very heavy ions with (Z ₁+Z ₂) α?1. Multistep excitation processes between bound and continuum states are investigated. Due to the multistep processes the number of created 1sσ-vacancies is typically increased by a factor 2–5 over that obtained within time-dependent perturbation theory (i.e. the one-step processes). Some implications of the many-electron problem and the Pauli principle are discussed. The δ-electron distribution is compared with recent experimental data.     

Spontaneous Raman scattering upon resonant three-photon ionization of a samarium atom

The manifestation of spontaneous Raman scattering upon the three-photon ionization of a samarium atom has been investigated. The dependence of the Sm⁺ yield on the laser frequency, along with the peaks due to the two-photon excitation of bound states from different levels of the 4f ⁶6s ^{2 7} F ground term, exhibits a strong peak due to single-photon excitation of the 4f ⁵5d6s ^{2 7} D°₃ state from the 4f ⁶5d6s ⁹ H ₂ excited state. The 4f ⁶5d6s ⁹ H ₂ state was populated as a result of spontaneous Raman scattering. The large amplitude of the observed peak indicates a high efficiency of this process. The estimations performed indicate that the probability of resonant ionization through an excited state populated due to spontaneous Raman scattering can be comparable with the probability of three-photon ionization through the two-photon intermediate resonance. The necessary conditions for implementing this channel of resonant three-photon ionization are formulated.     

Impact parameter dependence of the probability for removal of the 2s and 2p atomic electrons by nuclei

The probability W(x) of inner shell ionization by nuclei as a function of the impact parameter x is expressed through the ionization matrix element M_if. Values of W(x) for removal of the 2s and 2p electrons are calculated with the first Born M_if and compared with the SCA result.     

Transport properties of conduction electrons in n-type inversion layers in (100) surfaces of silicon

The conductivities of n-type inversion layers in (100) surfaces of p-type silicon were measured extensively as functions of electron density in the inversion layer, the ambient temperature and the applied magnetic field. Measurements were made on the carefully fabricated four “classes” of MOS field-effect transistors whose maximum mobilities at 4·2K were 14,000, 8000, 6800 and 1500 cm²/V·sec, respectively. From the temperature dependence of the mobility, dominant momentum scattering was reasonably ascribed to surfon at 100 ～ 300 K. and degenerate or non-degenerate coulomb scattering at lower temperatures as treated by Stern and Howard. From the curves of conductivity vs temperature at low temperatures and low electron concentration for specimens with high mobilities, an activation energy of 1·2 meV, relating to the shallow bound states associated with the lowest electrin sub-band, was observed. The conductivity σ_xx of the inversion layer in a strong transverse magnetic field showed behaviors like those of completely free electrons without effects belonging to its material in its oscillation pattern. That is, the peak value of σ_xx as a function of the gate voltage V_R dependend only on the Landau index. The σ_xx as a function of the magnetic field H at a constant V_R showed a similar Shubnikov-de Haas (SdH) type oscillation to that of three dimensional one. The SdH oscillation gave an “apparent” g-value g* which ranges from 2 to 5 depending on the surface carrier density n_s, due to the change in the ratios of the widths of the Landau levels to the level separation. The “reasonable” g-value of the conduction electrons in the inversion layer has been determined using a modified tilted magnetic field method. The g-value at the fixed magnetic field was independent of surface carrier density n_s and tended to 2 in the extreme strong magnetic field.Discussion is made of the g-value relating to the Landau level width and the energy gaps in the density of states under strong magnetic field.     

Correlation function of ultrahigh-energy cosmic rays favors point sources

Closed analytical expressions for the probability of multiphoton ionization of atoms and ions by a time-varying electric field ?(t) are obtained by the imaginary time method. These expressions apply for arbitrary values of the Keldysh parameter γ. The dependence of the ionization probability and the photoelectron momentum spectrum on the shape of an ultrashort laser pulse is considered.     

Oxidation of cleaved InAs(1 1 0) surfaces at room temperature: Surface band-bending and ionization energy

The interaction of unexcited, molecular oxygen with cleaved InAs(1 1 0) surfaces was investigated at room temperature by using a Kelvin probe and photoemission spectroscopy excited by HeI radiation. Exposures up to 10⁵ L of O₂, which result in an oxygen uptake up to only a few percent of a monolayer, cause the formation of an inversion layer on the specimen doped p-type and of an accumulation layer on those doped n-type. Between 10⁵ and 10⁸ L of O₂ the Fermi level is found to be pinned at 0.13 eV above the bottom of the conduction band on samples doped p- as well as n-type. This energy position of the Fermi level agrees with the charge- neutrality level of the virtual gap states as calculated by Tersoff. Since the ionization energy remains unchanged up to exposures of 10⁸ L of O₂ it is concluded that the oxygen is incorporated into rather than adsorbed on cleaved InAs surfaces.     

Ionization of Rydberg atoms by the kicks of half-cycle pulses

We present a quantum mechanical model to study the ionization of quasione-dimensional Rydberg atoms interacting with half-cycle pulses (HCPs) and use it to demonstrate the inadequacy of semiclassical approaches to calculate ionization probabilities of such atoms subject to the impact of more than one HCP. For a single-kicked atom both models correctly reproduce the experimentally observed ‘s-curve’ as can be seen by plotting the ionization probability P as a function of momentum transfer q₁. We demonstrate that for a twice-kicked atom, the semiclassical model yields numbers for P which are not physically realizable. For fixed values of momentum transfers q₁ and q₂, in a twice-kicked atom, the ionization probability as a function of time delay between the kicks exhibits periodic decay and revival. The results of the semiclassical approach appear to agree with the quantum mechanical values at the times of revival of P, else these show considerable deviation. We attempt to provide a physical explanation for the limitation of the semiclassical approach.     

Is ionization adiabatic near threshold?

Time-dependent perturbation theory is applied to a two-level model of neon to predict the energy dependence of 2p → 3p shakeup accompanying ionization. In this picture, the outgoing electron is treated as a classical particle moving in the potential of the nucleus and the other electrons. The time dependence of the perturbation is calculated numerically and integrated to give the shakeup probability. For core ionization this model predicts the probability for shakeup at the shakeup threshold to be about 79% of the value at the sudden limit. For 2p ionization the corresponding ratio is 67%. Thus, even at the threshold the process is closer to the sudden limit than to the adiabatic limit. Experimental results are in agreement with these theoretical predictions.     

Electron-electron interactions in the surface inversion layer of a semiconductor

The effective unscreened interaction between a pair of electrons in the inversion layer of an MIS structure is estimated by using classical electrostatics together with a knowledge of the quantum mechanical wave function of the inversion layer electrons. The change in the effective mass $\text{m}{}^1$ and effective g-value g¹ are evaluated in the random phase approximation.     

Calculation of cross sections of simultaneous ionization in ion-atom collisions by the generalized geometrical model

The geometrical model (GM) of ionization in ion—atom collisions [8, 9] was generalized to describe ionization of both colliding particles (simultaneous ionization) due to electron—electron interaction. The generalized GM (GGM) allows calculation of the cross sections for electron loss by an incident particle with simultaneous target ionization at collision velocities higher than characteristic electron velocities, accurate within a factor of two with respect to the Born or impulse approximation. An advantage of the GGM, except for its simplicity, is easy calculation of p(b) (p is the ionization probability and b is the impact parameter), which makes it possible to include the contribution of simultaneous ionization into more general approximate schemes for calculating cross sections of multielectron ionization of atoms or ions.     

An observation by photoconductivity of strain splitting of shallow bulk donors located near to the surface in silicon mos devices

Photoconductivity is observed in n-channel inversion Si MOSFETS at 4.2 K at infrared frequencies up to 700 cm^?1. Two groups of sharp lines of opposite sign are observed in the regions of 300 and 650 cm^?1 together with a continuum of transitions starting at ～ 350 cm^?1. The sharp lines are interpreted as bound transitions from shallow neutral phosphorous donors and boron acceptors on either side of the depletion layer. The angular dependence of the Zeeman splitting of the sharp lines demonstrates that the normal 90° symmetry of the 100 surface is lifted for devices with thin metal gates due to the presence of a strong unixial stress component.     

Probability of ionization of sputtered particles as a function of their energy: Part I: Negative Si ions

In this study we have investigated how the probability of ionization of sputtered Si atoms to form negative ions depends on the energy of the atoms. We have determined the ionization probability from experimental SIMS energy distributions using a special experimental technique, which included de-convolution of the energy distribution with an instrumental transmission function, found by separate measurements.We found that the ionization probability increases as a power law ∼E^0.677 for particles sputtered with energies of 0-10 eV, then becomes a constant value (within the limits of experimental error) for particles sputtered with energies of 30-100 eV. The energy distributions of Si⁻ ions, measured under argon and cesium ion sputtering, confirmed this radical difference between the yields from low and high-energy ions.To explain these results we have considered ionization mechanisms that are different for the low energy atoms (<10 eV) and for the atoms emitted with higher energy (>30 eV).     

Lorentz ionization of atoms in a strong magnetic field

Lorentz ionization emerges due to the motion of atoms or ions in a strong magnetic field. We use the semiclassical approximation to calculate the probability w _L of Lorentz ionization. We also find the stabilization factor S, which takes into account the reduction by the magnetic field of the probability of ionization decay of the bound s state. We estimate the probabilities w _L in magnetic-cumulation experiments and in astrophysics. We also qualitatively examine the dynamics of the magnetic cumulation process with allowance for the conductivity of the shell. Finally, we discuss a paradox related to the use of the quasistationary solution at the shell expansion stage. Zh. éksp. Teor. Fiz. 115, 1642–1663 (May 1999)     

Comparative study of He(23S)-penning ionization and He(I) photoionization of CF4, CCl4, and the chlorofluoromethanes by electron-ion coincidence spectroscopy

We present electron—ion coincidence spectra of the chlorofluoromethanes obtained after He(I)-photoionization and Penning ionization by He(2³S).Remarkable differences between both modes of ionization exist for CF₃Cl. Our tentative interpretation suggests the existence of a strongly bound interaction potential of ionic character between He(2³S) and CF₃Cl, in addition to the essentially flat covalent potential.