P J Hicks

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.

High resolution studies of dipole-forbidden states of N2 using low-energy electron energy-loss spectroscopy

Energy-loss spectra for molecular nitrogen have been measured with impact energies between 0.5 and 100 eV above the threshold for excitation and at scattering angles between 2 and 60 degrees . The energy-loss range between 5 and 25 eV was studied, with a typical resolution of approximately 0.025 eV. At energy losses below 11 eV the observed spectra can be fully assigned, with the exception of one progression of very weak peaks, using dipole-forbidden states previously observed in emission studies. The angular distribution of one of these states, w1 Delta u was measured. Between 14.30 and 15.90 eV, six energy-loss peaks were found to have angular distributions typical of dipole-forbidden transitions which cannot be related to previously observed states of N2. Above the ionisation limit, three new Rydberg series of states converging to the nu =0 level of the B2 Sigma u+ state of N2+ were detected at high scattering angles and low impact energies.

Autoionization of N2 studied using an electron time-of-flight coincidence spectrometer

Autoionizing states of N2 have been studied using a new electron-electron coincidence technique, by simultaneous measurement of their excitation energies and ejection energies in decay to states of N2+. This method gives a much clearer understanding of molecular autoionization than methods which measure either energy separately. Low energy electron impact is employed using hemispherical analysers to select the incident beam and to detect scattered electrons which have excited a selected autoionizing level. In coincidence with these scattered electrons, ejected electrons are analysed by measurement of their flight times along a field-free region. Transitions between the selected autoionizing level and levels of N2+ appear as peaks in the time spectra. The design, construction and operation of the spectrometer are discussed and factors limiting the time and energy resolutions are considered. Results are presented showing time spectra for electrons ejected from five autoionizing states of N2 and good agreement is observed between the predicted and measured resolutions of the spectrometer.

An electron impact energy-loss study of triplet states of acetylene

Electron impact energy-loss spectra have been obtained for acetylene in the range 4 to 8 eV at various impact energies and scattering angles. At low impact energies structure attributed to two triplet states is observed as long vibrational progressions. Analysis of the structure indicated that in the ground vibrational level of each triplet state, the molecule has a trans-bent geometry, and the configurations 13Au and 13Bu are suggested. The lowest vibrational level observed for the 13Au state has an excitation energy of 4.36+or-70.007 eV.

Electron energy-loss spectroscopy of carbon monoxide using a new position-sensitive multidetector spectrometer. I. The energy region 6-10.9 eV

An electron impact spectrometer incorporating a novel position-sensitive multidetector has been used to study the excited electronic states of carbon monoxide. This paper presents results for the energy region from 6 to 10.9 eV. Many new lines are observed, some of which correspond to high vibrational levels of known electronic states and some of which have been provisionally assigned to new valence states of CO. The resolution with which the measurements are made is sufficient for the effects of rotational excitation to be observed.

Autoionizing transitions in neon studied by low-energy electron impact

Measurements have been made of the energies of electrons ejected from autoionizing states of neon following excitation by low-energy electron impact. Singly and doubly excited states with energies in the range 43-46 eV were studied using a resolution and signal level which enabled previously unobserved structure to be detected. Ejected-electron spectra measured down to about 1.5 eV above the excitation threshold show a shift in energy of some of the structure as the excess energy above the excitation threshold is reduced. This is interpreted as a post-collision interaction between the ejected electron and the low-energy scattered electron.

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.

Autoionizing transitions in N2 and H2 produced by electron impact

Studies have been made of autoionizing transitions in N2 with excitation by low energy electron impact. Electron ejection is observed from states of the neutral molecule in the continuum region 16-19 eV giving levels of the x 2 Sigma g+ state of N2+. Measurements are made of autoionizing states which had previously been detected using photon and high energy electron excitation. In addition new levels are observed which appear to be favoured by low energy electron excitation.

Electron impact studies of resonances and autoionizing states of neon

Measurements have been made of the energy spectrum of electrons ejected from autoionizing states of neon with excitation energies in the region 41-46 eV; in particular, the lowest doubly excited autoionizing state of neon, the 2s22p43s2(3P) state, has been observed positively. These studies have given detailed information on post-collision interaction effects. In addition, it has been shown that resonant excitation of autoionizing states via negative-ion states can be an important process and measurements have been made on a number of excited negative-ion states in the region 41-48 eV.