D. S. Newman

Electron scattering from vibrationally excited CO2

The total cross section for electron scattering from vibrationally excited (principally 010) CO2 molecules has been measured in the energy range 0.12-2.0 eV with a time of flight electron spectrometer. The vibrationally excited molecules were produced by thermal excitation of the gas contained in a scattering cell at a temperature of 300 degrees C. The cross section for scattering from these excited molecules was deduced by comparison with similar measurements performed at room temperature. At energies below 2 eV the excited-state cross section is considerably larger than that for scattering from ground-state CO2. The ground-state measurements are in excellent agreement with other recent experimental and theoretical work. No other experimental or theoretical data exist for the excited-state cross section.

A trap-based positron beamline for the study of materials

We present the design of a new positron beamline for lifetime and Doppler studies of materials. The design is based on the Surko buffer gas trap system and is capable of producing sub-nanosecond pulses with a spot size at the sample of 10 mm. The present timing resolution of ≈800 ps allows lifetimes as small as 500 ps to be resolved. Typically count rates are between 100 and 150 counts per second.

An experimental and theoretical study of transient negative ions in Mg, Zn, Cd and Hg

A range of experimental and theoretical techniques have been applied to the study of transient negative ions (resonances) formed in electron scattering from the Group II metals Mg, Zn, Cd, and Hg at incident electron energies below the first ionization potential. A wealth of resonance structures have been observed and from the experimental observations and theoretical information, classifications are proposed for some of these negative ion states.

A further study of highly excited resonances in He

High-resolution electron impact excitation studies of helium in the energy region just below the first ionisation threshold provide further evidence for the existence of a Rydberg series of doubly excited resonances. These measurements extended the work of Buckman et al (1983) to states in which both excited electrons have values of the principal quantum number n>or=8. The energies of these states show good agreement with the values predicted by a number of semi-empirical double-Rydberg formulae and with a recent quantum mechanical calculation.

Role of negative ion resonances in electron scattering from atoms and molecules

Transient negative ions (resonances) formed during the collision of an electron with an atom or molecule have been extensively studied for over thirty years. The continued interest in these states, both experimentally and theoretically, stems from the profound effects that they can have on electron scattering cross sections and the role that electron–electron correlations play in their formation and quasi-stability. A selective discussion of examples of such resonances, involving one, two and three excited electrons is given for a wide range of atomic and molecular systems.

Angle-resolving time-of-flight electron spectrometer for near-threshold precision measurements of differential cross sections of electron-impact excitation of atoms and molecules

This article presents a new type of low-energy crossed-beam electron spectrometer for measuring angular differential cross sections of electron-impact excitation of atomic and molecular targets. Designed for investigations at energies close to excitation thresholds, the spectrometer combines a pulsed electron beam with the time-of-flight technique to distinguish between scattering channels. A large-area, position-sensitive detector is used to offset the low average scattering rate resulting from the pulsing duty cycle, without sacrificing angular resolution. A total energy resolution better than 150 meV (full width at half maximum) at scattered energies of 0.5-3 eV is achieved by monochromating the electron beam prior to pulsing it. The results of a precision measurement of the differential cross section for electron-impact excitation of helium, at an energy of 22 eV, are used to assess the sensitivity and resolution of the spectrometer.

The `5.2 eV resonance' in electron scattering from Hg

A phaseshift analysis technique has been used to analyse both elastic backscattering and elastic spin-polarization measurements (Duweke et al, Albert et al) to provide compelling evidence for the existence of a negative ion resonance at about 5.2 eV in Hg.

Low Energy Electron Scattering From H2

Absolute differential and integrated total cross sections for elastic electron scattering and vibrational excitation of molecular hydrogen have been measured at an incident energy of 1·5 eV. The results are presented and discussed with particular reference to a long-standing impasse which has existed between and within experiment and theory for the near-threshold excitation of the first vibrational state of H2. The integral vibrational cross section is in good agreement (±10%) with previous beam experiments and theory but is some 60% higher than the cross section derived from a swarm analysis.