P J M van der Burgt

Photoemission Cross Sections for Atomic Transitions in the Extreme Ultraviolet due to Electron Collisions with Atoms and Molecules

This article reviews experimental photoemission cross sections in the extreme ultraviolet, for transitions in excited atoms and atomic ions formed in electron collisions with atoms and molecules. A survey of the available experimental data for each investigated target gas reveals severe inconsistencies between cross sections reported by different laboratories. As almost all reported cross sections are based on relative measurements, a detailed discussion is given of the methods used for normalization of the cross sections.

DETECTION OF NEUTRAL METASTABLE FRAGMENTS FROM ELECTRON-IMPACT ON ARGON CLUSTERS

We have studied the production of neutral metastable fragments in electron collisions with neutral argon clusters. The fragments are detected using a time‐of‐flight technique. The time‐of‐flight spectra show that the metastable fragments appear in two velocity ranges. Kinetic energy distributions are obtained, showing that the faster fragments are ejected with energies from 0.2 to 1.5 eV and that the slower fragments have energies less than 0.2 eV. It is argued that the fragmentation of the clusters involves the excitation and decay of excitons in the clusters.The faster fragments are produced by n=2 excitons, which localize on an excimer or an excited trimer within the cluster and upon dissociation cause the ejection of a metastable atom. The slower fragments are produced by n=1 excitons, which tend to localize on the periphery of the cluster, leading to the ejection of a metastable atom due to weak repulsive forces with neighboring atoms. Four different production mechanisms for neutral metastable fragm...

Metastable fragment production by electron-impact dissociation of CF4

Time-of-flight detection of neutral metastable fragments has been applied to the study of electron-impact dissociation of CF4 molecules. The time-of-flight distributions indicate that a number of distinct processes are involved in the production of metastable fragments. Excitation functions for these processes have been measured up to 400 eV. The results presented are compared with other studies of fragmentation of CF4 and indicate that fragmentation into an excited CF3+ ion and an excited F atom is a prominent process.

Metastable fragment production by electron-impact dissociation of SO2

Time-of-flight detection of neutral metastable fragments has been applied to the study of electron-impact dissociation of SO2 molecules. The time-of-flight distributions indicate that a variety of processes are involved in the production of metastable fragments with a wide range of energies. Rydberg S and O atoms and SO molecules have been identified. Data for these processes have been measured up to 300 eV. The results presented are compared with other studies of fragmentation of SO2.

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.

A new type of He- resonance states in the autoionisation region

The authors have studied the near-threshold excitation of autoionising states in electron-helium scattering. Only a particular set of autoionising states is observed in the measurements, which has led them to formulate a selection rule for threshold excitation of autoionising states. Evidence is presented that autoionising states belonging to this particular set are strongly excited near their thresholds by hitherto unknown shape resonances.

The effect of a finite interaction region on the measurement of coherence parameters in electron-photon coincidence experiments

Electron-photon coincidence experiments are usually carried out by crossing an electron beam with a beam of atoms emerging from a capillary tube or array. Recently it has been found that the finite dimensions of the interaction region formed by the crossing beams can affect the coherence parameters that are measured in these experiments. The authors have developed models with which these effects and those related to the finite acceptance angles of the detectors can be simulated numerically. This article presents a description of the models and presents results which illustrate the possible magnitude of these effects in electron-rare gas scattering experiments. Other depolarizing effects such as internal atomic interactions are also discussed.

Determination of density matrices of H(n = 3) atoms formed in charge transfer between protons and helium atoms

Abstract Fast hydrogen atoms are formed in charge transfer reactions between incident protons (60 keV) and helium atoms. The formed excited hydrogen atoms are allowed to decay in the presence of externally applied electric fields which dramatically change the intensity and polarization of the emitted radiation. Balmer-alpha radiation emitted from a 7 mm long section of the proton beam is measured as a function of applied electric field strength using a precision polarimeter. Detailed analysis of the experiment in which the time evolution of the hydrogen atoms in the presence of external fields is calculated allows the extraction of the full, axially symmetric density matrix of the H(n = 3) atoms at the time of their production.

A new He- resonance in the excitation of the (2s2p)3P autoionising state by electrons

Four He- resonances with all three electrons in the n=2 shell are accessible by electron impact from the He ground state but only two resonances have been identified unambiguously in electron-helium scattering experiments. Experimental evidence for the presence of a He- resonance at 59.90 eV in the excitation cross section of the He**(2s2p)3P autoionising state is presented and discussed.

Positive ion mass spectrometry for fragmentation of 5-fluorouracil by low energy electron impact

Fragmentation of 5-fluorouracil in the gas phase has been studied using low energy electron impact. Positive ions have been mass resolved and detected using a reflectron time-of-flight mass spectrometer. Mass spectra have been measured as a function of electron impact from 5 to 100 eV, and ion yield curves and appearance energies for most of the positively charged fragments have been extracted.