H G M Heideman

Relative optical excitation functions of helium (excitation by electrons)

Synopsis When helium atoms are excited by electrons, the resulting light shows a line spectrum. The intensity of such a line depends on the energy of the electrons; this dependence is given by the so-called optical excitation function of the line. In onr case the excitation was achieved by a beam of electrons of adjustable velocity in helium. The beam current was low (10 – 50 μA) and the helium pressure was also low (about 5 × 10−3 Torr), so as to avoid errors due to space charge. Since under these circumstances, the light-intensities were very small a special photon-counting method was developed for the measurement. The excitation curves, obtained by us, have roughly the same shape as those, which were measured earlier. However, in the region immediately after the excitation energy many of our excitation curves show a detailed structure. This structure was not measured by most of the earlier investigators, the energy spread in their electron beam being to large.

Cross sections and coherence terms for associative ionization of two differently excited Na(3p) atoms

The associative ionization process in thermal Na(3p)–Na(3p) encounters has been studied in a series of crossed‐beam experiments where the light polarization of the two laser beams preparing the excited atoms before collision was varied independently. It is shown how in this way maximum possible information for our geometry is extracted about the dependence of the ion formation process on the shape and spatial orientation of the electron clouds of the two approaching atoms, including all coherence terms. The experimental findings are discussed in the light of recent theoretical results for the states of the Na2 molecule. It is concluded that just a few of the possible geometrical approaches are favorable for molecular–ion formation.

Coherences between autoionising states of different excitation energies

The authors have observed coherences between the electron impact excitation of autoionising states of helium that are separated in energy up to more than 2 eV. The method is based on the occurrences of interferences between scattered and ejected electrons resulting from the electron impact excitation and subsequent autoionisation of the states concerned. No energy shifting or broadening mechanisms such as those caused by post-collision interactions play a role in the experiment. The authors also present a theoretical description of the observed coherence effects. From this theory a parametrisation for scattered- and ejected-electron profiles is deduced which is fitted to experimental data. This parametrisation appears to describe the observed state-state interference very well.

Polarization of the radiation induced by electron impact on He and Hg

Abstract The polarization of a number of He and Hg spectral lines, excited by a beam of electrons, has been measured as a function of the electron energy. The transitions studied are: for helium 4 1 D → 2 1 P, 5 1 D → 2 1 P, 4 3 S → 2 3 P, 3 3 D → 2 3 P 4 3 D → 2 3 P, for mercury 6 1 D 2 → 6 1 P 1 , 6 3 D 2 → 6 1 P 1 , 7 3 S 1 → 6 3 P 1 , and 7 1 D 1 → 6 1 P 1 . The electron energy was varied from the threshold energy to a few eV above it. The results indicate that in most cases the polarization measured near the threshold is much lower than the theoretically predicted threshold polarization. For some transitions a detailed structure near threshold is observed in the polarization function. The observed behaviour of the polarization near threshold is discussed and a qualitative interpretation is presented.

Generalized Oscillator Strengths and Absolute‐Zero‐Angle Cross Sections for Inelastic Electron—Helium Collisions

Absolute‐zero‐angle cross sections for excitation of helium to the 21P level have been calculated with the Bethe theory, using Lassettres generalized oscillator strengths (for 11S→21P) normalized to the optical oscillator strength of Schiff and Pekeris. Absolute‐zero‐angle cross sections (in the 25–1000 eV energy region) for transitions to several other helium states were obtained, using (a) the calculated cross section for excitation to the 21P level and (b) already measured cross‐section ratios (obtained from energy‐loss spectra) or known optical oscillator strengths. These cross sections, which are not very accurate for small incident electron energies due to application of the Bethe theory, have been used to determine generalized oscillator strengths for the transitions 11S→31P, 41P, 51P, 61P, 21S, and 31S. It is pointed out that in the calculation of generalized oscillator strengths possible errors in the zero‐angle cross sections used may cancel out, which is illustrated for the transition 11S→31P....

The effect of long-range electron correlations on the polarisation of atomic line radiation, excited by electron impact

The polarisation of atomic spectral lines, excited by electron impact, shows in some cases an anomalous behaviour near threshold. It is suggested that this anomaly may be caused by the influence of electron correlation effects similar to those which give rise to specific threshold laws for electron impact ionisation. Both model calculations and experiments have been performed which tend to support the proposed model.

The use of weak magnetic fields in collisions of polarized atoms

We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determin...

An angle-resolving cylindrical mirror analyser for measurements of photo- and Auger electrons☆

Abstract An apparatus is described that allows angular distribution measurements of photo- and Auger electrons with high energy resolution. This apparatus is well suited to the investigation of inner-shell processes requiring photon energies from 100 to 1000 eV. Its proper functioning is tested by measuring the resonant Auger spectrum of argon, which is known for its unusually large anisotropy. Two preliminary results are presented for the asymmetry parameter of Ar3s photoelectrons at 244.4 eV photon energy (1.95±0.15) and of Ar2p photo-electrons at 450 eV photon energy (1.57 ± 0.15). Both values agree well with those from the literature.

Threshold effects in Auger spectra of photoionized argon

The Auger spectra of photoionized argon have been studied around the threshold for ejection of a 2p electron. Above threshold the normal Auger line shape is distorted due to post-collision interaction. The line shape is well described using existing theories. Below threshold two high-resolution spectra have been recorded of resonantly excited inner-shell states. In these spectra the multiplet structure of the final state is partly resolved. Shake up of the excited electron appears to be an important exit channel. The authors find evidence that orbital and spin angular momentum of the excited electron are conserved during the shake up and shake down processes under the influence of the Auger decay.

Shape resonances and the excitation of helium autoionising states by electrons in the 57-66 eV region

The authors present optical excitation functions of singly excited helium states, measured by detecting the yield of emitted photons as a function of the incident electron energy from 56 to 66 eV. Many structures are observed, which are caused by negative-ion resonances and by the decay of autoionising states followed by post-collision interaction. Some of the structures are interpreted as being caused by hitherto unknown shape resonances lying very close to the thresholds of a particular class of autoionising states. As these shape resonances almost exclusively decay to their respective parent (autoionising) states, thereby considerably enhancing the threshold excitation cross sections of these states, they can only be observed via the PCI effect on the excitation functions of (higher lying) singly excited states. Using the recently introduced supermultiplet classification for doubly excited states a selection rule for the near-threshold excitation of doubly excited states by electron impact is deduced from the measurements.