S Cvejanovic

A new technique for threshold excitation spectroscopy

A new technique has been developed for obtaining high resolution threshold excitation spectra in electron-atom and electron-molecule collisions. A very weak field penetrating into the target chamber produces a saddle point in the potential distribution which has the effect of focusing and enhancing the extraction efficiency of very low energy inelastically scattered electrons. Also the electron energy analyser is used in a special mode which sharpens the pass band for low energy electrons. Examples of threshold excitation spectra of He, Ar and N2 are given, and the onsets of the 23S and 21S states of helium have been used to determine accurately the energy (19.361+or-0.009 eV) of the 22S resonance in helium.

Studies of the threshold electron impact ionization of helium

Studies have been made of the two outgoing electrons in ionization events for which the energy difference E between the total energy and the ionization energy is in the range from 0.2 to 3.0 eV. A coincidence time-of-flight technique has been used to measure their energy distribution and angular correlation functions, and a non-coincidence technique has been used to measure the partial cross section for the production of very slow electrons. The energy distribution function has been found to be uniform in the range of E from 0.2 to 0.8 eV and the width of the angular correlation function has been found to increase with the energy E in a way which is consistent with a E1/4 dependence. The yield of very low energy electrons is consistent, in the range of E from 0.2 to 1.7 eV, with a total ionization cross section having the energy dependence En, where n=1.131+or-0.019. These results are all consistent with the Wannier-Peterkop-Rau theory.

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.

High resolution measurements of the 23S cusp and the 22S resonance in elastic electron-helium scattering

The cusp in the cross section for the elastic scattering of electrons by helium atoms at the energy of the 23S threshold has been measured at scattering angles from 25 degrees to 100 degrees . High resolution measurements at 90 degrees have shown that the s wave phase shift is 107 degrees +or-11 degrees at this energy. The energy of the 22S resonance has been accurately calibrated from the position of the cusp and it has been found to be at 19.367+or-0.009 eV, with a natural width of 9+or-1 meV. The s wave phase shift at the resonance energy has been found to be 106 degrees +or-3 degrees . The 21S cusp has also been studied.

Threshold electron impact excitation of hydrogen chloride

A high-resolution threshold-electron spectrometer was used to study excitation of HCl molecules O-10 meV above the thresholds for electronically excited states. The spectrum is obtained in an energy region from 4.2-14 eV corresponding to the excitation of lower valence states and Rydberg states below and just above the 2 Pi 3/2, 1/2, ionisation threshold. This first study of threshold excitation of HCl molecules reveals a considerable difference in relative intensities of some of the states compared with the case of higher-energy collisions. The transition into the lowest 3 Pi valence state is reported for the first time. The threshold ionisation region is discussed in terms of the superposition of the correlation-dominated direct ionisation process and the excitation function of an autoionising level decaying by releasing a zero-energy electron.

Energy partitioning in near-threshold excitation and ionisation of helium by electron impact

The processes of near-threshold ionisation and excitation of helium by electron impact have been studied experimentally by two different techniques to establish how the available excess energy is partitioned between the resulting free or bound electrons. The partitioning is found to be non-uniform and to be in agreement with that obtained from classical trajectory calculations.

Triplet states of ammonia excited by low-energy electrons

A low-energy crossed-beam electron spectrometer is used to investigate excitation of ammonia. Spectra obtained at low residual energies and large scattering angles in both constant residual energy mode and energy loss mode show excitation of two triplet states of ammonia, labelled a and b. A triplet b state is observed for the first time in the present work, while for the a state several lower, previously unobserved vibrational levels are detected. The threshold electron spectrum obtained by detecting zero residual energy electrons reveals a strong influence of threshold resonance in the excitation of he lowest Rydberg a state. Also a very large threshold peak in the excitation of the v=1 level of the symmetric stretch mode (v1) of the ground electronic state suggests a resonant process at very low energies.

Phenomena associated with near-threshold excitation of autoionizing levels of helium by electron impact

It is shown that the post-collision interaction model, previously proposed to explain the energy shifts of electrons ejected from autoionizing states of He excited by low-energy charged particles, can be extended to include cases where very low-energy scattered electrons are recaptured to form singly excited bound states. Electron excitation functions of three band states of He are presented over the range 57 to 60 eV. The photon yield from He over the incident energy range 56 to 61 eV is interpreted in the light of the above findings.

Threshold electron impact spectrum of N2O

Using a high-resolution electron impact monochromator combined with a penetrating-field analyser for efficient detection of electrons of nearly zero energy, the first high-resolution electron impact spectrum of N2O was obtained. The threshold spectrum is presented over a wide energy range for 1.5 to 17.5 eV. Decay of a low-energy shape resonance into the vibrationally excited ground state was observed. Several new optically forbidden and previously unobserved Rydberg states were detected.

Energy partitioning in near-threshold excitation and ionisation of atoms by photon or electron impact

Near-threshold processes are considered in which the reaction products consist of an ion and two low-energy electrons, one of which may be weakly bound to the ion. Using a classical model the partitioning of the available energy between the two electrons has been evaluated.