The atom-surface interaction potential for He-NaCl: A model based on pairwise additivity

The recently developed semi-empirical model of Fowler and Hutson is applied to the He-NaCl atom-surface interaction potential. Ab initio self-consistent field calculations of the repulsive interactions between He atoms and in-crystal Cl⁻ and Na⁺ ions are performed. Dispersion coefficients involving in-crystal ions are also calculated. The atom-surface potential is constructed using a model based on pairwise additivity of atom-ion forces. With a small adjustment of the repulsive part, this potential gives good agreement with the experimental bound state energies obtained from selective adsorption resonances in low-energy atom scattering experiments. Close-coupling calculations of the resonant scattering are performed, and good agreement with the experimental peak positions and intensity patterns is obtained. It is concluded that there are no bound states deeper than those observed in the selective adsorption experiments, and that the well depth of the He-NaCl potential is 6.0 ± 0.2 meV.     

Indirect exciton dispersion in III–V semiconductors: “Camel's back” in GaP

A new perturbation approach to exciton dispersion in indirect gap semiconductors is developed. For GaP and AlSb existence of the “camel's back” in exciton dispersion is confirmed, and a precise value of the “camel's back” parameter for X^c₁-minima in GaP is reported: E(X^c₁)?E_min(Δ^c₁)=3.5±0.3 meV. At the X-point the 21.44 and 19.48 meV exciton binding energies in GaP are obtained. The corresponding valley-anisotropy splitting is 1.96 meV.     

Evidence for a deep bound-state level in the 4HeNaCl(001) potential

Through a systematic high-resolution study of the elastic bound state resonances observed in the inelastic background of the angular distributions of ⁴He scattering from the NaCl(001) surface, carried out at 90 K, we obtained clear and conclusive evidence that a deep level in the ⁴HeNaCl(001) interaction potential exists at −7.07 meV. Important deviations from the free-atom dispersion are observed, as expected for deep bound-state levels. We discuss the physical reasons why deep-level resonances, which are observed in inelastic scattering, may not be detectable in elastic scattering experiments. This finding appears to be crucial with respect to the theory of atom-surface potentials since former quantum-mechanical and close-coupling calculations excluded the existence of a deep level in ⁴HeNaCl(001) in agreement with elastic scattering studies but at odds with phenomenological predictions.     

Resonant excitation of nitrogen bound excitons in Ga As1−xPx alloys

The luminescence spectra at 18 K of nitrogen-bound exciton in Ga As_1−xP_x; x = 0.59 excited with the energy 2.540 eV show bands that lay in an energy range of about 136 meV. Luminescence spectra obtained with resonant excitation energies show sharp lines that appear on the band's energy range. They are attributed to TA^X, LA^X, 2TA^X, LO^loc, TO^Γ_p and LO^Γ_p modes. These lines have different relative-intensities. In this paper, we identify phonon peaks and we give a possible interpretation of different recombination processes.     

The nature of the predominant acceptors in high quality zinc telluride

We report very sharp bound exciton luminescence spectra in high quality melt-grown very lightly compensated ZnTe, p-type with N_A-N_D in the low 10⁺¹⁵ cm^-3. Bound exciton localisation energies at seven shallow neutral acceptors with E_A between ~55 and ~150 meV are very insensitive to E_A. Optical absorption and dye laser luminescence excitation spectroscopy were necessary to obtain a full separation of the transitions due to different acceptors, together with a study of certain ‘two-hole’ luminescence satellites in which the acceptor is left in a series of orbital states after bound exciton decay. Two shallow acceptors are P_Te and As_Te, a third possibly Li_Zn while a fourth, relatively prominent in our best undoped crystals, may be a complex. A deeper, 150 meV acceptor, frequently reported in the ZnTe literature and electrically dominant in most of our undoped crystals has the Zeeman character of a point defect. We present clear evidence from our spectra that this energy does not represent the binding of a single hole at a doubly ionized cation vacancy, a popular attribution since 1963. This acceptor may be covered by another impurity, possibly Cu_Zn. We also report bound phonon effects, lifetime broadening of excited bound exciton states and observe a single unidentified donor with E_D ~18.5 meV. This energy is determined using selective dye laser excitation at the weak neutral donor bound exciton line and from the onset of valence band to ionized donor photo-absorption.     

Determining potential energy constants for atom- and molecule-surface interactions

Methods developed for diatomic molecule spectroscopy are adapted for use in analysing the energies of atom-surface bound states in order to determine certain features of the surface-averaged potential energy function. In cases for which appropriate data are available, these simple graphical methods can yield a model-independent estimate of the potential well depth e, a value for the coefficient (C₃) of the long-range z^?3 term in the atom-surface potential, and estimates of both the total number of bound states and the energies of any unobserved levels lying near the dissociation limit. Application of these techniques to the data for atomic hydrogen on (001)LiF and NaF and for atomic helium on (001)LiF yielded: ? = 18.6(±1.0), 18.2 (±3.6) and 8.6(±0.8) meV, and C₃ = 250(±90), 180(±110) and 95(±40) meV ų, respectively. Application of this approach to the data for molecular hydrogen on (001)LiF led to a new set of vibrational assignments and showed that ? = 37(±4) meV, and that the H₂ and D₂ data of O'Keefe et al. and the H₂ binding energies which Tsuchida obtained from the data of Frisch and Stern are all internally consistent.     

Nanostructural analysis of ZnO:Al thin films for carrier-transport mechanisms

The carrier mobility of sputter-deposited Al-doped ZnO transparent-conducting (ZnO:Al) thin films was controlled between 22 and 48 cm²/Vs by varying the ZnO:Al seed layer. The statistical distribution of the [001] grain misorientation was characterized from the X-ray diffraction rocking curve in the range from 0.043 (2.5°) to 0.179 rad (10.2°). The grain-boundary energy barriers (E_b) from Seto's model [1] clearly exhibit linear dependence on the grain-boundary misorientation angle (ω) according to the equation E_b = 78 ± 4 + 173 ± 32 ω meV.     

Local density valence band offset in GaAs/AlAs: Role of interface dipole

The valence band offset, ΔE_V ,at the lattice-matched GaAs/AlAs(001) interface is derived from highly precise self- consistent all-electron local density band structure calculations of the (GaAs)_n(AlAs)_n(001) superlattices (with n ⩽ 3). Using the core levels as reference energies, we find that ΔE_V = 0.50 ± 0.05 eV, in very good agreement with recent experimental results (ΔE_V = 0.45 − 0.55 eV). The dependence of ΔE_V on the superlattice thickness is studied and related to the interface charge redistribution which produces an interface dipole potential estimated to be ∼ 0.14 eV.     

Spectroscopic search for the free carrier screening of the excitonic binding

A spectroscopic determination of the energy gap E_g and the exciton energy E_x in highly excited Ge at T = 5–20K is presented. Within 0.05 meV we observe no shift of E_g and E_x up to electron-hole densities of 10¹⁴?10¹⁵ cm^?3. In this range all previous theories predict a sizeable band renormalization (ΔE_g ≈?0.3 meV to ? 2 meV).     

Metastable helium atom scattering from Ni(1 1 0) surface

The survival probability (SP) of metastable helium atoms (He¹) during scattering from the clean, alkalated and oxygen-adsorbed Ni(1 1 0) surfaces has been examined in the kinetic energy range of 50–400 meV. The measurements were carried out using a time-of-flight technique and a pulsed-discharge type metastable helium atom source. The SP is nearly constant for a kinetic energy (E_kin) of 50–100 meV and decreases exponentially with the increase in E_kin at 100–400 meV. It has been shown that the SP at E_kin=100–400 meV depends on the repulsive part of the He¹-surface interaction potential.     

Bound state resonances and related interaction potential for atomic hydrogen on NaCl(001)

Bound state resonances in the coherent elastic scattering of a monoenergetic H atom beam (48.5 meV) on a NaCl(001) surface are measured and analysed, consistently yielding three bound state energies. These energy levels fit into a Morse potential (well depth D = 20.8 meV) and range parameter k = 9.3 nm^?1) which compares well to those of similar interaction systems (as H/NaF and H/KCl) but differs from results reported recently in literature.     

Spectroscopy of 13Be

Six quasi-stationary states of ¹³Be populated in the ¹⁴C(¹¹B,¹²N) ¹³Be reaction at E_lab = 190 MeV are reported. A Q-value = −39.60(9) MeV and a mass excess, M.E.= 33.95(9) MeV, have been found for the lowest observed spectral line. The ground state is unstable with respect to one-neutron emission by 0.80(9) MeV. Excitation energies of 1.22(10), 2.10(16), 4.14(12), 5.09(14) and 7.0(2) MeV have been obtained for the observed spectral lines.     

Photoconductivity in ion-implanted thin films of Si:In and Si:Tl

The extrinsic photoconductivity of Si:In and Si:Tl is studied on ion-implanted thin films. Additional boron implants are used to compensate the phosphorus substrate doping for obtaining high photocurrents. The spectral photocurrent response and ionisation energies determined are identical in the implanted film and in bulk wafers. The ionisation energiesE _In=157±2 meV andE _Tl=252±2 meV are obtained by a novel evaluation which excludes photothermal excitation via exited bound states.     

Classification of the conditionally observable spectra exhibiting central symmetry

The emergence of upside-down symmetry of the bound-state energies E₁=−EN, E₂=−EN−1, … has been observed, in strong-coupling regime, in several pseudo-Hermitian N-state quantum systems. We show that such a symmetry assumption also simplifies a combinatorial classification of these systems since non-equivalent hierarchies of complexifications beyond the strong-coupling boundaries induce non-equivalent pairwise links between the energy levels.     

Shallow impurity states and the free exciton binding energy in gallium phosphide

A new set of donor and acceptor ionization energies in GaP is deduced from photoexcitation spectra. Energy spectrum of donor states confirms an existence of the “camel's back” with a conduction band minima displacement ≈ 0.08(2π/a) from the X, and the corresponding energy shift ≈ 3meV. The free exciton binding energy in GaP is correctly determined: E_ex = 21 ± 2 meV.     

Excitation of Jπ=2+ resonances in 24Mg by the 23Na(p, 12C)12C reaction

Angular distributions of the ²³Na(p, ¹²C_g.s.)¹²C_g.s. reaction were measured at energies on top of resonance-like structures or close to them in the range E_p=8−18 MeV and were analysed using parametrized S-matrix elements. Partial cross sections proceeding through compound nuclear states of ²⁴Mg with various spin J were derived. In was found that the resulting cross sections for J=2 exhibit correlated structures with the “fissioning resonances” recently observed by the ¹²C(¹²C, λ₀) and ²⁴Mg(α, α') ¹²C_g.s + ¹²C_g.s. reactions.     

Optical properties of the ternary compound Tl Sb S2

We present an investigation of the near band-gap optical properties of TlSbS₂ between 2 and 300 K. We use both transmission and reflectivity measurements. The resolution of the first exciton line permits to obtain an accurate determination of the temperature coefficients of both the direct band-gap E₀ and the second threshold E₁. The absorption curves have been fitted according to the Toyozawa's model. We find a strong interaction with a phonon mode of energy 22 meV for both the E₀ and E₁ thresholds. The low temperature reflectivity spectra reveal clearly several direct transitions in the range 1.5–5.5 eV. All these structures have been identified as transitions between the highest valence band and the lowest conduction band.     

High spin states in205Pb and a precision test of the nuclear shell model for three nucleons

Using the²⁰⁴Hg(α, 3n) reaction withα-particles of about 40 MeV, we have proved by applying nowadays conventionalγ-ray spectroscopy in-beam technique, that there are two isomeric states in²⁰⁵Pb at the excitation energies 5,161.3 and 3,195.5 keV having the half-lives 71±3 and 217±5 ns, respectively. These isomeric states have spins and parities 33/2⁺ and 25/2^? and are mainly due to thei ₁₃ ^2/?3 andi ₁₃ ^2/?2 p ₁ ^2/?1 configurations, respectively. This conclusion is supported by the experimentalg-factors of these states being ?0.159±0.008 and ?0.0676±0.0011, respectively. It is furthermore shown that theE2 effective neutron charge is the same forE2 transitions from the 33/2⁺ state in²⁰⁵Pb and from the 12⁺ state in²⁰⁶Pb as required by the assumption that the²⁰⁸Pb core is responsible for the totalE2 strength of the neutron holes, and that these states are due to thei ₁₃ ^2/?3 andi ₁₃ ^2/?2 configurations. The calculatedB(E3) values ofE3 transitions from isomeric states in²⁰⁵Pb and²⁰⁶Pb agree reasonably well with the experimental values as expected from the assumption that theE3-strength should come from particle coupling to the octupole states of the²⁰⁸Pb core. The energies of the six most well established excited states in²⁰⁵Pb with angular momenta in the region 19/2–33/2 were calculated using empirical single-particle energies, empirical two-particle interactions and angular momentum algebra. The average deviation between experimental and calculated energies is ?3 keV and the root mean square deviation 6 keV as compared to the uncertainty ± 5 keV in the nuclear masses used in the calculation. For the orbits concerned the shell model is thus valid with an extremely high precision. The contribution of effective three-particle interaction in these orbits must consequently be less than about 5 keV.     

The gallium-site donors germanium and silicon in gallium phosphide

The node in the Bloch part of the electron wave function expected for a Ga-site donor in GaP removes the usual valley-orbit splitting and associated chemical shift. However, the T₂ ground state can still show a small spin-valley splitting into Γ₈ and Γ₇ states, as previously verified for the Sn donor. We find that the optical properties of the Ge and Si donors deviate appreciably from this “normal” behaviour. The Ge donor is anomalously deep, E_D ～ 202 meV, yet binds an exciton by ～63 meV consistent with the Haynes rule for neutral donors in GaP. We find that this exciton possesses the large oscillator strength, f～3.5 × 10^-3, Zeeman and piezo-optical splittings characteristic of a Γ₆, 1s(A₁) ground state, like a P rather than Ga-site donor. However, f and the exciton localization energy are consistent with expectation for E_D ～ 200 meV, as measured from the lowest set of X conduction band minima, if we assume a symmetric A₁-like wave function. A possible explanation for this unexpected result is advanced. The much shallower Si donor, E_D～82 meV, binds an exciton by only ～ 14 meV, also consistent with the Haynes rule. By contrast, we find this Ga-site donor to be normal except that our Zeeman and piezo-optical results indicate an inverted spin-valley splitting, about 25% of that for the still shallower Sn donor. We also discuss the numerous low-lying excited states, some anomalous phonon replicas in the Ge and Si donor bound exciton spectra and the magneto-optical properties of a sharp line near 2.24 eV, attributed to the decay of excitons bound to (S)_p-(Ge)_p donor-acceptor associates.     

The p-40Ca and n-40Ca mean fields from the iterative moment approach

The real part V(r; E) of the p-⁴⁰Ca and n-⁴⁰Ca mean fields is extrapolated from positive towards negative energies by means of the iterative moment approach, which incorporates the dispersion relation between the real and imaginary parts of the mean field. The potential V(r; E) is the sum of a Hartree-Fock type component V^HF, (r; E) and a dispersive correction δV(r; E); the latter is due to the coupling of the nucleon to excitations of the ⁴⁰Ca core. The potentials V(r; E) and V_HF(r; E) are assumed to have Woods-Saxon shapes. The calculations are first carried out in the framework of the original version of the iterative moment approach, in which both the depth and the radius of the Hartree-Fock type contribution depend upon energy, while its diffuseness is constant and equal to that of V(r; E). The corresponding extrapolation towards negative energies is somewhat sensitive to the detailed parametrization of the energy dependence of the imaginary part of the mean field, which is the main input of the calculation. Moreover, the radius of the calculated Hartree-Fock type potential then increases with energy, in contrast to previous findings in ²⁰⁸Pb and ⁸⁹Y. A new version of the iterative moment approach is thus developed in which the radial shape of the Hartree-Fock type potential is independent of energy; the justification of this constraint is discussed. The diffuseness of the potential V(r; E) is assumed to be constant and equal to that of V_HF(r; E). The potential calculated from this new version is in good agreement with the real part of phenomenological optical-model potentials and also yields good agreement with the single-particle energies in the two valence shells. Two types of energy dependence are considered for the depth U_HF(E) of the Hartree-Fock type component, namely a linear and an exponential form. The linear approximation is more satisfactory for large negative energies (E < −30 MeV) while the exponential form is better for large positive energies (E > 50 MeV). This is explained by relating the energy dependence of U_HF(E) to the nonlocality of the microscopic Hartree-Fock type component. Near the Fermi energy the effective mass presents a pronounced peak at the potential surface. This is due to the coupling to surface excitations of the core and reflects the energy dependence of the potential radius. The absolute spectroscopic factors of low-lying single-particle excitations in ³⁹Ca, ⁴¹Ca, ³⁹K and ⁴¹Sc are found to be close to 0.8. The calculated p-⁴⁰Ca and n-⁴⁰Ca potentials are strikingly similar, although the two calculations have been performed entirely independently. The two potentials can be related to one another by introducing a Coulomb energy shift. Attention is drawn to the fact that the extrapolated energy dependence of the real part of the mean field at large positive energy sensitively depends upon the assumed behaviour of the imaginary part at large negative energy. Yet another version of the iterative moment approach is introduced, in which the radial shape of the HF-type component is independent of energy while both the radius and the diffuseness of the full potential V(r; E) depend upon E. This model indicates that the accuracy of the available empirical data is probably not sufficient to draw reliable conclusions on the energy dependence of the diffuseness of V(r; E).