No abstract

We report results of direct measurements of the reduced Stokes parameters P 1 ,P 2 andP 3 for electron impact excitation of H(2p) at 54.4 eV over the scattering range 10-40 •. These three parameters have been measured simultaneously for the first time using a VUV double-rotation polarization analyser consisting of a MgF 2 retarder followed by a SiO 2 reflection linear polarizer. As expected, our measurements forP 1 andP 2 are in good agreement with theoretical calculations and previous experimental data. Our data forP 3 differ significantly from previous experimental measurements and theoretical calculations. Consequently, we find that the coherence parameter P + deviates significantly from unity at 30 •. If correct, this signifies that spin-exchange scattering may be more important than has previously been thought.

Hypomagnesaemic tetany and composition ofbody fluids. Use of vitreous for the postmortem diagnosis of hypomagnesaemia. Annales Universitatis Mariae Curie Sklodowska. Sectio DD, Medicina Veterinaria 51, 179-198 TODD, J. R. (1965) The influences of soil type on the effectiveness of single dressings of magnesia in raising pasture magnesium content and in controlling hypomagnesaemia.

First direct measurements are reported of the linear reduced Stokes parameters P 1 , P 2 for H(2p) excited by electron impact at the benchmark energy of 54.4 eV. The results differ significantly from previous values deduced from angular correlation measurements which are in serious conflict with all sophisticated theoretical approaches. Our results support the trend of theoretical predictions for P 2 and confirm that its value is negative at electron scattering angles above 100 ± , as predicted by theory. [S0031-9007(98)05388-5]

Abstract-A continuous wave near-infrared spectroscopy (NIRS) instrument for brain-computer interface (BCI) applications is presented. In the literature, experiments have been carried out on subjects with such motor degenerative diseases as amyotrophic lateral sclerosis, which have demonstrated the suitability of NIRS to access intentional functional activity, which could be used in a BCI as a communication aid. Specifically, a real-time, multiple channel NIRS tool is needed to realise access to even a few different mental states, for reasonable baud rates. The 12-channel instrument described here has a spatial resolution of 30mm, employing a flexible software demodulation scheme. Temporal resolution of ~100ms is maintained since typical topographic imaging is not needed, since we are only interested in exploiting the vascular response for BCI control. A simple experiment demonstrates the ability of the system to report on haemodynamics during single trial mental arithmetic tasks. Multiple trial averaging is not required.

The yields and time-of-flight spectra for ions produced in three-photon ionization of atomic deuterium have been investigated as a function of laser intensity from 0.4 to 200 MW/cm 2 . A pulsed laser beam of 243 nm is used, so that the photoionization is resonant with the metastable 2s state of deuterium. The production of resonantly excited 2s atoms is measured by detecting the quenched Lyman-␣ radiation at time intervals both during and after the laser pulse. The near degeneracy of the 2s-2p state manifold provides a particularly sensitive way to monitor the presence of any electric field in the interaction volume. As the laser intensity is increased, we observe the effects of a collective field arising from the electron-ion plasma produced by the laser pulse. We present the results of a theoretical model where the collective motion of the ions and the electrons is analyzed in terms of the fluid approximation. The essential features of the experimental observations including ion yields, ion time-of-flight spectra, and metastable production versus time, are well described by the theory. ͓S1050-2947͑98͒06508-1͔

Measurements of absolute panid cross sections, integrated over the angular m g e 9Oo-18O0, are reported for the elastic scattering of electrons from Art at impact energies 3.5-6.5 eV. These constitute the firs1 measurements of backward scattering and for energies below lhe inelastic lhreshold in the elastic scamring of free electrons from positive ions. Comparison with the predictions of the Rutherford formula, and pariial wave analysis using semiempirical s!alic polentials, suggesls ha1 low-energy, large-angle scanering from singly charged positive ions provides an excellent regime for the lesling of dynamic effects in electron-ion collisions.

An electron-ion crossed beams experiment has been developed in which the energy loss of scattered electrons is used to determine cross sections for excitation in positive ions. Scattered electrons are confined by an axial magnetic field and are channelled into a detector using reflecting potentials and an (trochoidal) deflection. Complete collection of these electrons along with monitoring of the overlap and current of the beams enables absolute total cross sections to be determined. This technique has been used to study near threshold excitation of the D resonance transition in . , CII, trochoidal spectrometer