R. Grisenti

Absolute total cross section measurements for intermediate energy electron scattering. II. N2, O2 and NO

For pt.I, see ibid., vol.12, p.3787 (1979). Total absolute cross section measurements are presented for electrons scattered by O2, N2 and NO molecules. The energy range covered spans from 100 to 1600 eV. Neglecting the systematic error introduced by the finite angular resolution of the apparatus, the experimental accuracy has been estimated to be +or-2.5%. This systematic effect could necessitate an upward correction of as much as 13% at 1000 eV in N2. No similar evaluation was possible for O2 and NO. A comparison is given for N2 with existing measurements. The agreement is reasonable. The results for O2 are compatible with the existing data in the low-energy range. No comparison was possible for NO.

On the sensitivity of the x-ray excited optical luminescence to the local structure of the luminescent Si sites of porous silicon

X-ray excited optical luminescence (XEOL) has been recorded in a wide x-ray energy range to obtain the extended x-ray absorption fine structure (EXAFS) at the Si K edge of porous silicon. A comparison between EXAFS measurements carried out simultaneously in photoluminescence yield (PLY) mode and in total electron yield (TEY) mode on red and orange porous silicon samples is presented. Experimental results suggest that TEY provides average structural information on all luminescent and nonluminescent Si sites. On the contrary, PLY is able to probe the local structure near the light emitting sites, and to monitor the modifications induced by current density changes during the sample preparation. PLY–EXAFS shows that the luminescent Si nanostructures are smaller and more disordered than the average structures of the porous layer probed by TEY, suggesting that the luminescent sites are located at the surface of the nanostructures.

Absolute total cross sections for electron-CO2 scattering at energies form 0.5 to 3000 eV

Employing two different experimental techniques, absolute total cross sections for e--CO2 scattering have been measured over a very wide energy range between 0.5 and 3000 eV. The cross section function exhibits a sharp increase below 1.5 eV and a resonant maximum around 3.8 eV. It then rises again up to a broad maximum near 30 eV and then decreases slowly. A very broad feature was observed centred at an energy of about 300 eV and extending from about 200 to 600 eV. Comparison is made with other available measurements and calculations.

Absolute total cross section measurements for intermediate energy electron scattering: III. Ne and Ar

Absolute total cross section measurements are presented for e--Ne and e--Ar scattering. The covered energy range spans from 100 to 3000 eV. The overall systematic uncertainty was +or-2.4%, the random uncertainty was +or-1.5% and the angular resolution error was less than 0.5%. A comparison with existing experimental and theoretical data is given. This comparison suggests that the semi-empirical data of de Heer et al (1979) for Ne are possibly overestimated by 5% at 100 eV.

Local order in hydrogenated amorphous germanium thin films studied by extended x-ray absorption fine-structure spectroscopy

The effect of hydrogenation on the local order in amorphous germanium has been studied by EXAFS. Measurements have been carried out on sputtered a-Ge:H films with hydrogen concentrations of 0, 7, 10, and 15 at.%, as a function of temperature in the range 11 - 300 K. The first-shell EXAFS data were analysed by the ratio method based on cumulant expansion. The asymmetric distributions reconstructed from cumulants are in very good agreement with a parametrized distribution obtained by other researchers using calculated phase-shifts. For the unhydrogenated a-Ge (deposited at ), increases of the interatomic distance, at 11 K, static disorder, , and thermal disorder, , have been found with respect to those for c-Ge. Both the static and the thermal disorder are smaller than for an evaporated sample (deposited at ) previously studied. The insertion of hydrogen in a-Ge produces a sharp reduction of the interatomic distance, static disorder, and asymmetry of the distribution already at the lowest H concentration (7%); then these parameters decrease almost linearly when the hydrogen content increases. No appreciable influence of hydrogenation on the thermal disorder has been detected.

Chemical composition and local structure of plasma enhanced chemical vapor-deposited Si nanodots and their embedding silica matrix

X-ray absorption measurements in total electron yield mode have been carried out on Si nanodots embedded in amorphous silica produced by plasma enhanced chemical vapor deposition (PECVD). The amount of Si atoms composing the Si nanodots and the chemical composition of the amorphous host matrix has been determined thanks to the comparison with Rutherford backscattering spectrometry data. The influence of nitrogen, incorporated during the PECVD procedure, on the structure of the host silica matrix has been discussed.

Total absolute cross sections for electron scattering on H2O at intermediate energies

Absolute measurements of the total cross section for electron scattering on H2O molecules are presented. The energy range extends from 81 to 3000 eV. A comparison with previous experimental data is given.

Absolute total cross section measurements for intermediate-energy electron scattering. IV. Kr and Xe

For pt.III, see ibid., vol.20, no.19, p.5157-64 (1987). Absolute total cross section measurements with an overall experimental uncertainty below 5% were performed for Kr and Xe in the energy range from 80 eV to 4000 eV. The Xe measurements above 750 eV are the first available in the literature . In the overlapping energy range the present data are in good agreement with other experimental and semi-empirical results. Comparing the total cross sections for all noble gases it has been found that their energy dependencies do not follow the Born approximation even at the highest measured energies.

Fast timing with hamamatsu R2083Q photomultipliers

Timing results are presented obtained with Hamamatsu R2083Q photomultipliers and BaF2 scintillators. For 60Co radiation the best time resolution was 101(1) ps FWHM per detector pair. For measuring positron lifetimes the best resolution was 140(1) ps FWHM.

X-ray absorption spectroscopy on light emitting porous silicon by XEOL and TEY

This paper presents an experimental investigation of the local structure of porous silicon based on X-ray absorption fine structure by two different techniques: Total electron yield (TEY) and X-ray excited optical luminescence (XEOL). The influence of the main parameters of sample preparation on both the photoluminescence and the local structure around Si atoms has been investigated. The dependence of the optical emission energy on the current density shows a saturation limit. The analysis of both the optical emission and the short range order (up to the third coordination shell) confirms the presence of crystalline cores on a scale of a few nanometers in freshly prepared porous silicon samples. Although the local structural parameters obtained by TEY are similar to those reported in recent X-ray absorption studies, a direct connection between optical emission energy and short range order has not always been found. In particular, while the role of HF concentration in determining optical and microstructural properties is clear, a change of current density affects the energy shift of the X-ray excited optical band but not the short range order properties monitored by TEY.