On the energy spectra of secondary ions emitted from silicon and graphite single crystals

Secondary ion emission from silicon and graphite single crystals bombarded by argon ions with energies E ₀ varied from 1 to 10 keV at various angles of incidence α has been studied. The evolution of the energy spectra of C⁺ and Si⁺ secondary ions has been traced in which the positions of maxima (E _max) shift toward higher secondary-ion energies E ₁ with increasing polar emission angle θ (measured from the normal to the sample surface). The opposite trend has been observed for ions emitted from single crystals heated to several hundred degrees Centigrade; the E _max values initially remain unchanged and then shift toward lower energies E ₁ with increasing angle θ. It is established that the magnitude and position of a peak in the energy spectrum of secondary C⁺ ions is virtually independent of E ₀, angle α, and the surface relief of the sample (in the E ₀ and α intervals studied). Unusual oscillating energy distributions are discussed, which have been observed for secondary ions emitted from silicon (111) and layered graphite (0001) faces. Numerical simulations of secondary ion sputtering and charge exchange have been performed. A comparison of the measured and calculated data for graphite crystals has shown that C⁺ ions are formed as a result of charge exchange between secondary ions and bombarding Ar⁺ ions, which takes place both outside and inside the target. This substantially differs from the ion sputtering process in metals and must be taken into account when analyzing secondary ion emission mechanisms and in practical applications of secondary-ion mass spectrometry.     

Features of the energy distribution of secondary particles upon the ion bombardment of graphite

The secondary emission of carbon atoms from the (0001) plane of graphite nanocrystallites bombarded with argon ions with energies of 1 and 10 keV and the incidence angle α = 45° is investigated. The unusual oscillating energy distributions of secondary C⁺ ions with main maxima E _max in the range of 40–60 eV and peaks corresponding to the energies E ₁ ≈ 20, 30, 70, 80, and 100 eV have been revealed. The C⁺ ion yield decreases, the energy spectrum increases, and the maximum E _max shifts to larger energies E ₁ with increasing emission angle (with respect to the normal to the surface). The secondary-ion emission from the (0001) face of graphite is numerically simulated with allowance for the charge exchange of secondary ions to obtain a qualitative explanation of the observed results.     

Anisotropy of secondary ion emission from a Ni4Mo single crystal

The azimuthal and polar angle distribution of Ni⁺ and Mo⁺ ions emitted from an ordered Ni⁴ Mo single crystal irradiated with 10-keV Ar⁺ ions was studied. Different azimuthal distributions for Ni⁺ and Mo⁺ ions emitted from the (001) Ni₄Mo face were detected; emission maxima were observed in 〈 011 〉 and 〈 001 〉 directions for Ni⁺ and Mo⁺ ions, respectively. It was shown that polar distributions of nickel’s secondary ions vary with its energy. The observed systematic features were explained by correlated collisions in the upper layers of a Ni₄Mo single crystal.     

Energy distributions of atomic and molecular ions sputtered by C60 projectiles

In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C₆₀⁺ bombardment. These measurements have been conducted using our TRIFT™ spectrometer, recently equipped with the C₆₀⁺ source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C⁺(E^−0.4) > H⁺(E^−1.5) > Ag⁺(E^−3.5). In particular, the distribution of Ag⁺ is not broader than those of Ag₂⁺ and Ag₃⁺ clusters, in sharp contrast with 15 keV Ga⁺ bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C₆₀⁺ instead of Ga⁺ as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag⁺ and Ag_n⁺ clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS⁺ ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C₇H₇⁺ and C₁₅H₁₃⁺ and polycyclic fragments, such as C₉H₇⁺ and C₁₄H₁₀⁺, the high energy decay is steep (E⁻⁴ − E⁻⁸). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles.     

Secondary ion emission from Si under nitrogen ion bombardment

The yield and energy distribution of positive secondary ions emitted from Si under N₂⁺ ion bombardment were measured. The obtained mass peaks correspond to three types of secondary ion species, that is, physically sputtered ions (Si⁺, Si₂⁺), chemically sputtered ions (SiN⁺ Si₂N⁺) and doubly charged ions (Si²⁺). The dependence of secondary ion emission on the primary ion energy was studied in a range of 2.0–20.0 keV. The yields of physically and chemically sputtered ions were almost independent of the primary ion energy. The yield of the doubly charged ion strongly depended on the primary ion energy. The energy distribution of secondary ions of the three types showed the same dependence on the primary ion energy. The most probable energy of the distribution increased with the primary ion energy. On the other hand, for the energy distribution curves of sputtered ions, the tail factors N in E^?N were constant and showed a m/e dependence.     

Bounds on charged lepton mixing with exotic charged leptons

We examine the effects of mixing induced light heavy charged lepton neutral currents on the partial wave amplitude for the process l⁺l⁻ →ZZ (withl = e,μ or τ). By imposing the constraints that the amplitude should not exceed the perturbative unitarity limit at high energy (√s = Λ), we obtain bounds on light heavy charged lepton mixing parameter sin²(2θ _L ^a ) where θ _L ^a is the mixing angle of the ordinary charged lepton with its exotic partner. For Λ = 1 TeV, no bound is obtained on sin² (2θ _L ^a ) form _E < 0.69 TeV. However, sin² (2θ _L ^a ) ≤ 1.52×10⁻⁵ form _E = 5 TeV, sin² (2θ _L ^a ) ≤ 2.41 ×10⁻⁷ form _E = 10 TeV. Similarity for Λ = ∞ no bound is obtained on sin² (2θ _L ^a ) for m_E < 1.97 TeV and sin² (2θ _L ^a ) ≤ 0.15 form _E = 5 TeV and sin² (2θ _L ^a ) ≤ 3.88×10^-2 form _E = 10 TeV.     

Beta-gamma-gamma directional correlation in103Rh

β-γ-γ directional correlation studies for the cascades (i)β-rays ofE _max=0.12 MeV,γ-rays of 557 keV andγ-rays of 53 keV and (ii)β-rays ofE _max=0.21 MeV,γ-rays of 444 keV andγ-rays of 53 keV have been made. The triple correlation functionsW(θ) were obtained to beW(θ)=1+(−0.153±0.031)P ₂(cosθ)+(0.004±0.035)P ₄(cosθ) forβ-rays ofE _max 0.12 MeV→557→53 keV cascade andW(θ)=1+(0.163±0.042)P ₂(cosθ)+(−0.035±0.058)P ₄(cosθ) forβ rays ofE _max=0.21 MeV→444 keV→53 keV cascade. Spins and parities of the 650, 537 and 93 keV levels of¹⁰³Rh are deduced by triple angular correlation and the internal conversion coefficient studies. Multipolarities of the transitions are also determined.     

Study of optical absorption and EPR spectra of Mn<Superscript>2+</Superscript> ion in cadmium maleate dihydrate

The optical absorption and EPR spectra of Mn²⁺ ion doped in cadmium maleate dihydrate have been theoretically investigated by diagonalizing the complete energy matrices for a d⁵ configuration ion in a trigonal ligand-field. According to the suggestion of the optical absorption studies, we assume that the Mn²⁺ ion enters the host lattice interstitially and the distorted octahedral symmetry for the impurity ion is trigonal. Moreover, the local lattice structure parameters of the system are determined. The results show that the six oxygen ions around the Mn²⁺ ion are at the same distance R=2.115 ?, and there are three Mn-O bonds forming an angle θ₁ of 66.26° with the C₃-axis and three others forming an angle θ₂ of 43.40°.     

Emission of quasi-thermal ions from a solid bombarded by cluster ions

Comparative studies of the emission of quasi-thermal atomic and cluster ions from V, Nb, Ta, Au, and In targets bombarded by cluster ions Au _m ⁻(m = 1–9), as well as from Si and Bi targets bombarded by cluster ions Au _m ⁻(m = 1–9) and Bi _m ⁻(m = 1–5), with energy E ₀ ranging from 6 to 21 keV are carried out. In the case of bombardment by heavy cluster ions, the fraction of the quasi-thermal component in the energy spectra of sputtered atomic ions reaches 50 (for V, In, and Au), 70 (Nb), or more than 90% (Ta). In addition, quasi-thermal ions play a considerable part in the emission of small cluster ions Au₂⁺, In₂⁺, In₃⁺, and Bi _n ⁺(n = 2–7). The results of the generalizing investigation favor the presence of thermal spike conditions at cluster bombardment and their appreciable contribution to the emission of atomic and small cluster ions.     

Energy distributions and emission coefficients of secondary ions ejected from AIIIBV group semiconductors

The energy distributions of Ga⁺ and Ga₂ ⁺ secondary ions sputtered from the surface of single crystals of GaAs and GaP by Ar⁺ (E = 18 keV, j = 1·10^–6 A/cm²) ions were studied. Integral emission coefficients for secondary ions from four semiconductors (GaAs, GaP, InAs, and A1_0.35Ga_0.65As) bombarded by Ar⁺ ions (E = 2 keV, j₀ = 1·10^–6 A/cm²) were also measured. It is shown that the energy distribution of the Ga⁺ secondary ions are broad and qualitatively similar to distributions of secondary ions sputtered from metal oxide surfaces. Very high integral emission coefficients (up to 20–30%) for secondary ions are also observed, exceeding analogous coefficients for clean metal surfaces. These facts (the significant number of low energy secondary electrons and high secondary ion emission coefficients) are due to the lower density of free electrons in comparison with metals, which ensures a high degree of survival of sputtered ions, and the partially ionic character of the bonds in the semiconductors under study.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 88–92, November, 1990.     

Electron yield per ion charge-state correction for an ion collector with unsuppressed secondary electron emission

The electron yield per ion charge-state γ/q was measured for emission of electrons from clean polycrystalline gold induced due to impact of Ta ^q+ (11≤q≤41) ions with kinetic energy per chargeE _i/q from 15 keV/q to 150 keV/q. The dependence of γ on angle of incidence was analyzed with use of relation γ(ϑ)=γ₀ cos^−f ϑ. The fitting of experimental data gives a range of γ₀/q from 1 to 1.75 for Ta¹³⁺ and from 1.5 to 1.73 for Ta³⁹⁺. The dependence of γ₀/q onq andE _i is discussed with respect to measurement of ion currents emitted from laser-produced plasmas with an ion collector with unsuppressed secondary electron emission. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy, and by grant A1010819 from the Grant Agency of the Academy of Sciences of the Czech Republic.     

Angular-resolved energy distribution of secondary ions emitted from a silicon target sputtered by a xenon ion beam

This paper reports preliminary results obtained on an experimental apparatus dedicated to the study of angular resolved energy distribution of particles emitted from a sputtered target. Secondary ions emitted during the bombardment of a silicon target by xenon ions at a primary energy of 10keV have been studied. In its low energy part the distribution reaches a maximum around 8eV, and then decreases according to an E ^–1 law. In the range 200eV to 1000eV, a second maximum appears whose height depends on the emission angle. Apart from this range, the angular distributions have a cosine square-like shape. On the contrary, the angular distribution of ions with energy between 200eV and 1000eV is pointed in a forward direction near the specular reflection direction of the ion beam. It is assumed that the measured ions correspond to two ionic populations: secondary ions sputtered according to the linear cascade theory and recoil silicon target ions.     

Delayed ionisation of C70

A delayed ionisation experiment has been carried out on laser excited C₇₀ molecules. Ions and electrons are detected in coincidence to distinguish the ionisation of C₇₀ from that of C₆₈ molecules created by the dominant process, emission of C₂. From the time dependence of the C₇₀⁺ yield and the ratio to the C₆₈⁺ yield, we can deduce both the dissociation energy, E_d = 9.7 ±0.3 eV, and the pre–exponential factor, A_d = 1.7 ×10²⁰ s^-1, in the Arrhenius decay law for C₂ emission. The power of photon emission from neutral C₇₀ is also determined.     

The peculiarities of the low-energy ion scattering by polycrystal targets

In the present work, experimental and computer simulation studies of low-energy (E₀ = 80-500 eV) Cs⁺ ions scattering on Ta, W, Re target surfaces and K⁺ ions scattering on Ti, V, Cr target surfaces have been performed for more accurate definition of mechanism of scattering, with a purpose of evaluation of an opportunity of use of slow ions scattering as a tool of surface layers analysis. The choice of the targets was based on the fact that the ratios of atomic masses of target atoms and ions μ = m₂/m₁ were almost the same for all cases considered and greater than 1 (direct mass ratio) however, the difference of binding energies of target atoms in the cases of Cs⁺ and K⁺ scattering was almost twice as much. It has been noticed that the dependencies of the relative energy retained by scattering ions at the maximum of energy distribution versus the initial energy E_m/E₀ (E₀) have a similar shape in all cases. The relative energy retained by scattering ions increases while the initial energy of incidence ions decreases. The curves are placed above each other relative to the binding energies of target atoms, to show what this says about the influence of binding energy on a process of scattering of low-energy ions. The correlation between value of energy change maintained by an ion for different values of E₀ in the case of scattering by targets with different masses of atoms and its binding energies is experimentally established. The contrary behavior of the E_m/E₀ (E₀) dependencies concerning the target atom binding energy quantity E_b for cases with direct (μ > 1) and inverse (μ < 1) mass ratio of colliding particles is established. The comparison of experimental energy distributions with calculated histograms shows that the binary collision approximation cannot elucidate the abnormally great shift in the maxima of relative energy distributions towards greater energy retained by scattering ions.     

Electron screening in backward elastic scattering

Abstact: The elastic scattering cross sections, σ (E,θ), for the systems He+Ta and He+W have been measured at θ_lab=165° and E _lab=76.1 keV to 3.988 MeV using targets with a thickness of a few atomic layers. The results are smaller than the results given by the Rutherford scattering law, σ_R(E,θ), due to the effects of electron screening and can be described by σ(E,θ)/σ_R(E,θ)=(1+U_e/E)⁻¹, where U _e is an atomic screening potential energy. The deduced average value, U _e=28 ± 3 keV, is consistent with the Moliére- and Lenz-Jensen-models as well as electron binding energies. Received: 25 May 1998     

Probing noncommutative space-time in the laboratory frame

The phenomenological investigation of noncommutative space-time in the laboratory frame is presented. We formulate the apparent time variation of noncommutativity parameter θ_μν in the laboratory frame due to the earth’s rotation. Furthermore, in the noncommutative QED, we discuss how to probe the electric-like component θ _E =(θ₀₁,θ₀₂,θ₀₃) by the process at future e^-e⁺ linear collider. We may determine the magnitude and the direction of θ _E by detailed study of the apparent time variation of the total cross-section. If no signal is observed, the upper limit on the magnitude of θ _E can be determined independent of its direction.     

Continuum light emission from sputtered species of graphite during ion beam irradiation

Light emission during sputtering of graphite targets with 1–10 keV Ne⁺, Kr⁺ and Xe⁺ beams has been investigated in the 180-600 nm wavelength range. Beside the characteristic lines of sputtered C₁ and C₁ ⁺, a continuum superimposed with a number of broad structures was observed in the 250-520 nm range, and having a maximum at 386 nm. Mass analysis of the sputtered flux confirmed the presence of negative carbon clusters C _m ^- , C₂ ^- being the dominant one. Ion beam parameters i.e. ion mass, energy, current density and ion dose were varied to identify the origin of the continuum emission. On the basis of the experimental results, it is suggested that the continuum is predominantly due to the overlapping of various band systems of sputtered C₂ with a small contribution from the heavier sputtered carbon clusters C_m (m>2). Received 24 September 1999 and Received in final form 11 February 2000     

Secondary ion emission under the bombardment of Si by multiply charged Si q+ ions

The spectra of secondary ion emission under the bombardment of a B-doped Si target by multiply charged Si ^q+ ions (q = 1?C5) have been studied in the energy range of 1 to 10 keV per unit of charge. A multifold increase in the yield of secondary cluster Sk _n ⁺ ions, multiply charged Si ^q/+ ion (q = 1?C3), and H⁺, C⁺, B⁺, Si₂N⁺, Si₂O⁺ is observed as the charge of the multiply charged ions increases. The increase in the yield of secondary ions with increasing charge of the multiply charged-ion charge is most significant for ions with relatively high ionization potentials.     

Cross-section and rate coefficient calculation for electron impact excitation, ionisation and dissociation of H2 and OH molecules

The weighted total cross-section (WTCS) theory is used to calculate electron impact excitation, ionisation and dissociation cross-sections and rate coefficients of OH, H₂, OH⁺, H₂ ⁺, OH^- and H₂ ^- diatomic molecules in the temperature range 1500–15000 K. Calculations are performed for H₂(X, B, C), OH(X, A, B), H₂ ⁺(X), OH⁺(X, a, A, b, c), H₂ ^-(X) and OH^-(X) electronic states for which Dunham coefficients are available. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. One and two temperature (θ_e and θ_g respectively for electron and heavy particles kinetic temperatures) results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θ^b)exp (-c/θ).     

Molecular ion emission from single large cluster impacts

We investigated the emission of the secondary ions stimulated by single impacts of 136 keV Au₄₀₀⁴⁺ projectiles. The study was carried out on targets of glycine, phenylalanine, and C₆₀. In addition, a target of C₆₀ was examined with 18 keV C₆₀⁺ projectiles. The experiments were performed in the event-by-event bombardment/detection mode. The secondary ions were identified with linear time-of-flight mass spectrometer equipped with an 8-anode detector. The Au₄₀₀⁴⁺ projectile induces abundant multi-ion emission, for instance the average number of detected ions (atomic, fragment, molecular and cluster ions) emitted per event from glycine target is 12.5. The glycine intact molecular ion (Gly⁻) yield is 1.14. The bombardment of a C₆₀ target results in the efficient emission of multiple intact C₆₀⁻ (total yield is 0.15).