J Comer

An electron spectrometer using a new multidetector system based on a charge-coupled imaging device

A new electron spectrometer has been developed which employs a position sensitive multidetector and can accumulate signals from a wide range of electron energies simultaneously. It employs a charge-coupled imaging device and results are presented showing that it gives an improvement of more than a factor of 100 in sensitivity over the best existing spectrometers.

Electron energy-loss spectroscopy of forbidden transitions to valence and Rydberg states of formaldehyde

Abstract The electronic spectrum of formaldehyde has been studied by the technique of electron scattering covering the range of energy loss from 0 to 12 eV with

Displacements of electron ejection energies in near-threshold excitation of autoionizing levels of helium by electron impact

Abstract The four lowest autoionizing levels in helium, namely (2s 2 ) 1 S, (2s2p) 3 P, (2p 2 ) 1 D and (2s2p) 1 P, have been studied near their thresholds for excitation by electron impact. Features in the spectra of electrons ejected from these states were seen to shift to steadily higher energies as the impact energy was lowered to within a few electron volts above threshold. This phenomenon is compared with a similar one which has been observed in the scattering of low energy helium ions from helium atoms. A qualitative explanation of these observations can be provided by considering a “post-collision interaction” between the scattered particle and the ejected electron. The limitations of this model are discussed. Structure in the spectra suggest that the situation is complicated by an interference phenomenon which cannot be explained in a classical treatment of the problem. The threshold electron excitation spectrum of these autoionizing levels has also been obtained, using an apparatus designed to detect very low energy electrons. As well as confirming the observations described above, this revealed the presence of one of the two negative ion states known to exist in this region. In an effort to obtain a complete picture of the processes taking place in helium at these energies, the negative ion states were also studied in direct ionization and in elastic scattering at various angles. The latter measurements confirmed the classifications of these states as (2s 2 2p) 2 P and (2s2p 2 ) 2 D, and we have obtained values for their energies of 57.22 ± 0.04 eV and 58.30 ± 0.04 eV respectively.

Ejected electron spectroscopy of autoionizing states excited by low energy electron impact

The autoionizing states of helium that exist beneath the n=2 threshold of He+ have been extensively investigated using a low energy (10-200 eV) electron impact spectrometer. The instrument was purpose-built for studying the electron ejection spectra of autoionizing states and features a high intensity ( approximately 1 mu A) incident beam and a high resolution ( approximately 40 meV) electron energy analyser. The energies of a number of the autoionizing states of helium have been measured, and our values are compared with previous results of theory and experiment. In a few cases the resolution of the instrument has allowed us to estimate the widths of the very short-lived states. An outstanding feature of the spectra obtained at energies just above the thresholds for excitation of the autoionizing states is the phenomenon of displaced thresholds.

The optimization of electrostatic energy selection systems for low energy electrons

Abstract Design criteria are presented which allow electrostatic energy selection systems to be optimized to give the highest currents for electron beams having a low mean energy (≲ 500 eV) and a small energy spread (≲ 30 meV). Limitations in various parts of the system are discussed and a relationship between the current delivered by the system and the energy spread is presented. Different types and sizes for the energy selectors in these systems are considered and the best results are obtained with hemispherical deflectors. The advantage of using, within reasonable limits, a large selector with small image sizes is also demonstrated.

Rydberg states of C2H4 and C2D4: assignments using the technique of low-energy electron energy-loss spectroscopy

Energy-loss spectra of C2H4 and C2D4 are presented covering the energy-loss range 6-11 eV, using incident electron energies in the range 5-100 eV above threshold, and scattering angles up to 60 degrees . Members of several optically assigned Rydberg series were identified, and their intensities measured at a number of scattering angles. Each transition was then classified as electric dipole or quadrupole allowed, and this information was used to assign the Rydberg orbitals involved. Four new triplet Rydberg states have been observed, and their assignments are also discussed. For two of the triplet states, more than one vibrational level was observed, and where possible the frequencies of the symmetric C=C stretch and CH2 torsional modes were obtained. For the lowest triplet Rydberg state, the barrier to internal rotation of the CH2 groups was estimated to be 0.08 eV.

Electron impact studies of a resonant state of N2

Studies have been made of resonant scattering of electrons from N2 in the energy region 11 eV to 13 eV. By measuring cross sections for scattering leaving the target molecule in vibrational levels of the ground state the potential curve of a state of N2- has been determined. The symmetry of this state appears to be 2 Sigma g+ and an estimate has been obtained of its width. Additional structure has been observed in the elastic channel and this is discussed.

Potential curves and symmetries of some resonant states of H2

By studying cross sections for resonance series in exit channels which are high vibrational levels of the ground state, and by assuming the compound molecule model with constant decay width Gamma , it has been possible to estimate the potential curves of resonant Feshbach states. Symmetries have also been obtained by measuring angular distributions. Evidence has been found for two new resonant states of symmetry 2 Sigma g+ and 2 Pi u. Five resonant states which may exist in the region 11 eV to 13.5 eV are discussed.

The production of autoionizing states of atomic oxygen by the photodissociation of O2

A study has been made of photodissociation processes in O2 which produce autoionizing fragments. Tunable VUV radiation was used to obtain photoelectron spectra. These contain structure due to electron emission from states of atomic oxygen decaying to the ground and first excitated states of the ion. The investigation has concentrated on the region of the thresholds for the production of the atomic autoionization, and spectra have also been recorded up to 60 eV photon energy.

High resolution electron impact studies of electric dipole-forbidden states of benzene

Electron energy-loss spectra have been obtained, with a resolution of 0.030 eV for the electronic states of benzene below 11.5 eV. At high scattering angles prominent vibrational structure has been observed in the first three triplet bands (3B1u, 3E1u, and 3B2u). A progression of three levels starting at 6.10 eV has been observed for the first time, and assigned to the 3E2g valence state. Rydberg states converging to the first and second ionisation potentials have been studied and in both cases quadrupole transitions have been detected for the first time. A series of four such levels converge to the first ionisation potential with a quantum defect of 0.266, and the Rydberg orbital is identified as nd(a1g). Two quadrupole transitions converging to the second ionisation limit have a similar quantum defect of 0.226 and are believed to involve transitions from the 3e2g orbital to the same Rydberg orbital.