Effects of high pressure on the luminescent properties of nanocrystalline and bulk Y2O3:Eu3+.

High pressure-induced spectral changes in a 20-nm cubic nanocrystalline yttria doped with europium and its corresponding bulk were studied in the range of 550-750 nm, corresponding to the 5D0 --> 7Fd (J = 0-4) transitions. The results demonstrate that the bulk Y2O3 underwent phase transition from the cubic phase to the monoclinic phase as the pressure increased to 15 GPa, while the 20-nm nanocrystals did not. This can be concluded from the fact that the 5D0 --> 7F0 line and the three 5D0 --> 7F1 sublines originating from the cubic phase disappeared, while another group of 5D0 --> 7F0 and 5D0 --> 7F1 lines appeared. In addition, the relative intensity of the peak around 630 nm to that around 611 nm varied obviously as the pressure surpassed 15 GPa. The variations in the nanocrystals were more sluggish in comparison to those in the bulk, indicating that the nanocrystalline yttria had improved compressibility, which is attributed to an increased surface energy in nanocrystals. The local environment surrounding luminescent Eu3+ in the nanocrystals and the bulk both became more disordered with the increase of the pressure. The phase transition from the cubic to the monoclinic is irreversible.

New developments for an electron impact (e,2e)(e,3e) spectrometer with multiangle collection and multicoincidence detection.

We describe new developments aimed to extend the capabilities and the sensitivity of the (e,2e)(e,3e) multicoincidence spectrometer at Orsay University [Duguet et al., Rev. Sci. Instrum. 69, 3524 (1998)]. The spectrometer has been improved by the addition of a third multiangle detection channel for the fast "scattered" electron. The present system is unique in that it is the only system which combines three toroidal analyzers all equipped with position sensitive detectors, thus allowing the triple coincidence detection of the three electrons present in the final state of an electron impact double ionization process. The setup allows measurement of the angular and energy distributions of the ejected electrons over almost the totality of the collision plane as well as that of the scattered electron over a large range of scattering angles in the forward direction. The resulting gain in sensitivity ( approximately 25) has rendered feasible a whole class of experiments which could not be otherwise envisaged. The setup is described with a special emphasis on the new toroidal analyzer, data acquisition hardware, and data analysis procedures. The performances are illustrated by selected results of (e,2e) and (e,3e) experiments on the rare gases.

Investigation of valence orbitals of propene by electron momentum spectroscopy.

The binding energy spectra and momentum distributions of all valence orbitals of propene were studied by electron momentum spectroscopy (EMS) as well as Hartree-Fock and density functional theoretical calculations. The experiment was carried out at impact energies of 1200 eV and 600 eV on the state-of-the-art EMS spectrometer developed at Tsinghua University recently. The experimental momentum profiles of the valence orbitals were obtained and compared with the various theoretical calculations. Moreover, the experiment with a new analysis method presents a strong support for the correct ordering of the orbital 8a' and 1a'', i.e., 9a' < 8a' < 1a'' < 7a'.

Direct observation of distorted wave effects in ethylene using the (e,2e) reaction.

We report here the direct measurements of electron momentum distributions for ethylene using the (e,2e) reaction at different impact energies from 400 to 2400 eV. The "turn up" effects in the (e,2e) cross sections of the 1b(3g) orbital compared with the plane-wave impulse approximation calculations were observed at low and high momentum regions, and such discrepancies become smaller with the increase of the impact electron energies. It is suggested that the observed discrepancies are due to the distorted-wave effects in molecules, while appropriate theoretical calculations using distorted waves in molecules could not be achieved until now.

An investigation of valence shell orbital momentum profiles of difluoromethane by binary (e,2e) spectroscopy.

The electron binding energy spectra and momentum profiles of the valence orbitals of difluoromethane, also known as HFC32 (HFC-hydrofluorocarbon) (CH(2)F(2)), have been studied by using a high resolution (e,2e) electron momentum spectrometer, at an impact energy of 1200 eV plus the binding energy, and by using symmetric noncoplanar kinematics. The experimental momentum profiles of the outer valence orbitals and 4a(1) inner valence orbital are compared with the theoretical momentum distributions calculated using Hartree-Fock and density functional theory (DFT) methods with various basis sets. In general, the shapes of the experimental momentum distributions are well described by both the Hartree-Fock and DFT calculations when large and diffuse basis sets are used. However, the result also shows that it is hard to choose the different calculations for some orbitals, including the methods and the size of the basis sets employed. The pole strength of the ionization peak from the 4a(1) inner valence orbital is estimated.

The outer valance orbital electron densities of cyclopentane by binary (e,2e) spectroscopy.

The binding energy spectra and electron distributions in momentum space of the valence orbitals of cyclopentane (C(5)H(10)) are studied by Electron Momentum Spectroscopy (EMS) in a noncoplanar symmetric geometry. The impact energy was 1200 eV plus binding energy and energy resolution of the EMS spectrometer was 1.2 eV. The experimental momentum profiles of the outer valence orbitals are compared with the theoretical momentum distributions calculated using Hartree-Fock and density functional theory (DFT) methods. The shapes of the experimental momentum distributions are generally quite well described by both the Hartree-Fock and DFT calculations when the large and diffuse basis sets are used.