Compton scattering and charge transfer in Er substituted DyAl2

A unique applicability of Compton spectroscopy in probing the electronic states of rare earth aluminides using high energy (662 keV) γ-rays is reported. We have measured first-ever Compton profiles of Dy_1-xEr_xAl₂ (x=0, 0.2) using 20Ci ¹³⁷Cs Compton spectrometer. The charge reorganization in Dy_1−xEr_xAl₂ (x=0, 0.2), on the formation of compound, has been discussed using the valence band Compton profile data. The experimental Compton profile data unambiguously establish charge transfer from Al to Dy (Dy and Er) on formation of x=0.0 (0.2) compound, which is in tune with spin polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) calculations. A reasonable agreement between SPR-KKR based Compton profiles and the experimental data show applicability of the Green function method in predicting the electronic properties of rare earth compounds.     

Electron correlation effects in the momentum distribution of highly oriented pyrolytic graphite

Absolute Compton profiles of highly oriented pyrolytic graphite have been determined using ¹⁹⁸Au 412 keV gamma radiation. The anisotropy in the electron momentum density between Compton profiles measured parallel to the c-axis and averaged over the basal plane have been compared with a recent pseudopotential calculation of Chou et al. Excellent agreement is found over the entire momentum range studied. Differences between absolute Compton profiles J_theory(p_z − J_exp (p_z) vindicates a correlation correction to take account of the electron-electron Coulomb interactions.     

Electronic properties of the magnetocaloric compound GdAl2: A Compton scattering study

We present Compton profiles of the GdAl₂ compound and its constituents using a 20Ci ¹³⁷Cs Compton spectrometer. The experimental Compton data have been analysed using theoretical data obtained from the spin polarised relativistic Korringa–Kohn–Rostoker (SPR-KKR) method and also the charge transfer on the formation of the compound. Both the experimental and the SPR-KKR theoretical Compton data support a charge transfer from Al→Gd in GdAl₂, which is in accordance with the conclusions drawn from the partial, total and integrated density of states of GdAl₂ and its constituents.     

Calculation of the Compton profiles of vanadium,niobium and their dihydrides

We have calculated the Compton profiles of V, Nb, VH₂ and NbH₂ using the self-consistent augmented plane wave (APW) method within the local-density approximation of Hedin-Lundqvist. The results are compared with other theoretical works and available experiments. In going from the pure metals to the metal dihydrides we observe significant changes in the directional Compton profiles due to their different band structures.     

Investigation of electronic properties of crystalline arsenic chalcogenides: Theory and experiment

The electronic structure of crystalline As₂S₃ and As₂Se₃ has been calculated in this paper. We present the energy bands, density of states (DOS) and the Compton profiles using the linear combination of atomic orbitals (LCAO) scheme based on the density functional theory (DFT). From the calculated total and partial density of states it is seen that the lone-pair p-states of sulphur/selenium contribute closest to the Fermi energy level. To interpret the theoretical data on the Compton line shape, we have measured the Compton profiles on a 100 mCi ²⁴¹Am spectrometer. It is seen that the density functional theory within generalised gradient approximation gives a slightly better agreement with the experimental momentum densities. The nature of chemical bonding in arsenic chalcogenides is studied using Mulliken's population analysis and the experimentally measured equal-valence-electron-density profiles; As₂S₃ is found to be more ionic compared to As₂Se₃.     

Electron momentum distributions and compton profiles of some molecules with FSGO model

The electron momentum distributions and the Compton profiles (within the impulse approximation) of H₂, LiH, methane, water, acetylene, ethylene, ethane cyclopropane and cyclobutane have been calculated using the floating spherical Gaussian orbital (FSGO) wavefunctions. The agreement of the single-FSGO Compton profiles with the corresponding experimental or the Hartree-Fock (HF-SCF) theoretical ones is fairly good in most of the cases examined. The advantages and drawbacks of using the FSGO model for the calculation of Compton profiles are discussed.     

Compton scattering study of orthorhombic sulphur

The Compton profiles of orthorhombic sulphur crystals have been measured the scattering vector perpendicular to the crystal planes (110) and (001) using 60 keV radiation from a ²⁴¹Am source. Significant anisotropy was observed in the electron momentum distribution. Fourier transformed Compton profiles were compared with theoretical autocorrelation functions for a S₈ ring molecule. It is shown that the observed anisotropy can be only partially interpreted in terms of intermolecular interactions. The strong correlation, observed in the direction perpendicular to the plane of the S₈ ring indicates a meaningful ring — ring interaction in the orthorhombic sulphur crystal.     

Ab initio Compton maps of small molecules

A closed form expression for the evaluation of directional Compton profiles (DCP) from the position space one-electron reduced density matrix expanded in Cartesian Gaussian basis functions is presented. The Compton map, i.e. a two-dimensional contour plot of the DCP function J(q) in a selected plane, is introduce as a convenient method for describing the DCP Examples of theoretical Compton maps are given for the hydrogen molecule ion (both ground and excited states), water, ammonia, ethylene and ¹A₁ methylene molecules in their ground states. Compton maps generated with Hartree-Fock, configuration interaction and density functional theory methods are compared.     

Analytical performance of Compton/Rayleigh signal ratio by total reflection X-ray fluorescence (TXRF): A potential methodological tool for sample differentiation

The high sensitivity Compton and Rayleigh X-ray scattering signals can be used to gain valuable information on the chemical composition of various matrices, by exploiting the ratio of those signals as a function of the effective atomic number (Z_eff). Neither total reflection X-ray fluorescence (TXRF) nor the effect of the experimental setup, including sample preparation, X-ray excitation source selection, and band deconvolution procedure, has been assessed in this kind of approach. Here, a Compton/Rayleigh ratio and Z_eff-based TXRF method was set up and tested as an analytical tool for milk samples differentiation. The method was developed using a 90° scattering angle and assessed using different X-ray excitation sources: a molybdenum tube (Mo Kα 17.5 KeV) and a tungsten tube (W Lα 8.5 KeV and W-Brems 35 KeV). The evaluation of independent Compton and Rayleigh signals was performed by non-Gaussian and Gaussian curve resolution methods, and both height and area-based calculations were evaluated. Different sample preparation conditions were assessed. By using 11 standard materials, a calibration curve for Compton/Rayleigh ratio versus Z_eff was established. The method was tested to determine the Z_eff of milk samples, which enabled its use as a parameter to differentiate them. Good precisions were obtained with the Mo excitation source and the area-based calculations, which allowed to differentiate undiluted milk samples by species, treatment, and fat content according to their Compton/Rayleigh ratio. This simple and rapid method has the potential to be used for the differentiation of various types of samples, including liquids, solids, and aerosols.     

Electronic properties and Compton profiles of molybdenum dichalcogenides

We present the first ever experimental Compton profiles of molybdenum dichalcogenides (MoX₂; X=S and Te) using 20 Ci ¹³⁷Cs Compton spectrometer. To interpret our experimental data, we have computed the theoretical profiles, energy bands and density of states using linear combination of atomic orbitals method in the framework of density functional theory and its hybridisation with Hartree Fock. The energy bands and density of states using full potential linearised augmented plane wave method have also been computed. Both theories show the existence of the indirect band gap. In addition, the relative nature of bonding is explained in terms of equal-valence-electron-density profiles and valence band charge densities.     

Derivation of the radial distribution function from experimental Compton profiles

The analysis of experimental Compton profiles in position rather than momentum space provides a useful method of interpreting Compton data. A density matrix approach is employed to establish the relationship between the Compton profile J(p), and the Fourier transform of the momentum density B(r), and, for a homogeneous system, the radial distribution function g(r). An earlier Compton profile measurement on sodium provides the data for a demonstration of the long range charge correlations in a metal, in analogy with the Friedel oscillations in a screening charge.     

Compton profile study of V3Ge and Cr3Ge

In this paper the results of a Compton profile study of two polycrystalline A15 compounds, namely, V₃Ge and Cr₃Ge, have been reported. The measurements have been performed using 59.54 keV γ-rays from an ²⁴¹Am source. The theoretical Compton profiles have been computed for both the compounds using ab-initio linear combination of atomic orbitals (LCAO) method employing CRYSTAL98. For both the A15 compounds, the isotropic experimental profiles are found to be in good overall agreement with the calculations. The comparison points out residual differences in V₃Ge whereas for Cr₃Ge the differences are within experimental error. The behaviour of valence electrons in the two iso-structural compounds has been examined on the scale of Fermi momentum. The valence electron distribution seems to be dominated by the metallic constituents rather than Ge and two compounds show covalent nature of bonding which is larger in V₃Ge compared to Cr₃Ge.      

Electronic density distorsion of NiO2 due to intercalation by Li

High resolution measurements of Compton profiles of Li_xNiO₂ have been carried out using 60 keV photons at ESRF (Grenoble, France). Theoretical profiles are obtained using the plane wave expansion of wave functions calculated using ab-initio SCF method. Theory and experiment are in good agreement. We have further decomposed the charge density modification into a rigid charge transfer from lithium to e_g¹ states of NiO₂ and the distorsion of NiO₂ charge density mainly due to Ni–O bond changes resulting from lattice parameter change and Jahn-Teller effect.     

Compton line profile component of atomic hydrogen in the profile of NbH1.2

The Compton profile of polycrystalline NbH_1.2 has been measured at room temperature using a -ray Compton spectrometer with a 1.85×10¹⁰ Bq(0.5 Ci)²⁴¹ Am annular source and an intrinsic Germanium detector. This is compared to the spherical average profile of NbH_0.3 single crystals oriented along [100], [110] and [111]. The difference between these profiles bears a striking similarity to the Compton profile of atomic hydrogen. This can be understood assuming that a large part of the dilute hydrogen forms atomic clusters at the grain boundaries.     

Molecular compton profiles from inelastic atom-electron scattering

Recent measurements of the electron loss peak from H ₂ ⁺ ions impinging on Ar have been analyzed in terms of projectile Compton profiles (CP). It is shown that projected momentum distributions, i.e. CP's, can be determined for the molecular ion H ₂ ⁺ .     

Constraints on the axion-electron coupling constant for solar axions appearing owing to bremsstrahlung and the compton process

The axio-electric effect in silicon atoms is sought for solar axions appearing owing to bremsstrahlung and the Compton process. Axions are detected using a Si(Li) detector placed in a low-background setup. As a result, a model-independent constraint on the axion-electron coupling constant |g _Ae | ≤ 2.2 × 10⁻¹⁰ has been obtained, which leads to the bounds m _A ≤ 7.9 eV and m _A ≤ 1.4 eV (at 90% C.L.) for the mass of the axion in the DFSZ and KSVZ models, respectively.     

Calculation of valence electron momentum densities using the projector augmented-wave method

We present valence electron Compton profiles calculated within the density-functional theory using the all-electron full-potential projector augmented-wave method (PAW). Our results for covalent (Si), metallic (Li, Al) and hydrogen-bonded ((H₂O)₂) systems agree well with experiments and computational results obtained with other band-structure and basis-set schemes. The PAW basis set describes the high-momentum Fourier components of the valence wave functions accurately when compared with other basis set schemes and previous all-electron calculations.     

Anisotropy in the Compton profiles of Nb and NbH0.76

The Compton profiles of Nb and NbH_0.76 single crystals have been measured along [100] and [110] using 412 keV gamma-radiation from a¹⁹⁸Au source. The changes in the band electron profile of the metal following the introduction of hydrogen are interpreted in terms of the protonic model of the hydride. Qualitative changes in the directional profiles can be understood within the rigid band approximation.     

Compton profiles of electron momentum distribution inβ-Ga

We report for the first time the Compton profiles of electron momentum distribution inβ-gallium calculated along the crystallographic directions (100), (110) and (111). The conduction electron states for this purpose are determined in the energy band calculations using model potential. The core states, on the other hand, are represented each by a single tight-binding function. The results show that the Compton profiles are nearly isotropic with very little directional dependence, which is suggestive of a free-electron-like distribution of the conduction electrons in this system. The latter conclusion is in close confirmity with similar conclusions drawn in augmented plane wave (APW) calculation of energy bands and the derived Knight-shift results inβ-Ga.     

Compton profiles of multiply ionized oxygen atoms

We have calculated Compton profiles of multiply ionized oxygen atoms with electronic configurations 1s^m2sⁿ2p^q, m=1−2,n=0−2,q=0−4. The values of the Compton profiles from the present calculation can be used to determine the doubly differential electron production cross sections in recent ion-atom collision experiments with oxygen ions in the rest frame of the target atoms. The calculations have been performed in impulse approximation using numerical Hartree-Fock wave functions. Compton profiles of neutral oxygen atoms, available in the literature, are in excellent agreement with the present calculation. The variation of Compton profile with the degree of ionization is investigated.