Energy-resolved Electrons Research Articles

Electron-ion coincidence study of ionizing thermal energy collisions between He*(21S) and Ar atoms

Ionization in thermal energy collisions between excited He*(21S) and ground state Ar atoms leads to final states involving a free electron and either dissociating He–Ar+(2P3/2,1/2) (PI) or bound HeAr+(2P3/2,1/2) molecular ion states (AI). The higher-lying initially bound HeAr+(2P1/2) levels may be subject to predissociation to unbound He−Ar+(2P3/2) states with release of kinetic energy. By means of coincidence studies of the mass-selected ions and the energy-resolved electrons in conjunction with quantum mechanical calculations of the PI and AI processes we show that essentially all of the initially bound HeAr+(2P1/2) levels are predissociated over the ion detection time of 4 µs.

Interaction-induced charge transfer in a mesoscopic electron spectrometer

A novel mesoscopic electron spectrometer allows for the probing of relaxation processes in quantum Hall edge channels. The device is composed of an emitter quantum dot that injects energy-resolved electrons into the channel closest to the sample edge, to be subsequently probed downstream by a detector quantum dot of the same type. In addition to inelastic processes in the sample that stem from interactions inside the region between the quantum dot energy filters (inner region), anomalous signals are measured when the detector energy exceeds the emitter energy. Considering finite range Coulomb interactions in the sample, we find that energy exchange between electrons in the current inducing source channel and the inner region, similar to Auger recombination processes, is responsible for such anomalous currents. In addition, our perturbative treatment of interactions shows that electrons emitted from the source, which dissipate energy to the inner region before entering the detector, contribute to the current most strongly when emitter-detector energies are comparable. Charge transfer in which the emitted electron is exchanged for a charge carrier from the Fermi sea, on the other hand, preferentially occurs close to the Fermi level.

A double toroidal analyzer for scanning probe electron energy spectrometer

An ultra-high vacuum (UHV) compatible electron spectrometer employing a double toroidal analyzer has been developed. It is designed to be combined with a custom-made scanning tunneling microscope (STM) to study the spatially localized electron energy spectrum on a surface. A tip—sample system composed of a piezo-driven field-emission tungsten tip and a sample of highly ordered pyrolytic graphite (HOPG) is employed to test the performance of the spectrometer. Two-dimensional images of the energy-resolved and angle-dispersed electrons backscattered from the surface of HOPG are obtained, the performance is optimized and the spectrometer is calibrated. A complete electron energy loss spectrum covering the elastic peak to the secondary electron peaks for the HOPG surface, acquired at a tip voltage of −140 V and a sample current of 0.5 pA, is presented, demonstrating the viability of the spectrometer.

Study of the dissociation of nitrous oxide following resonant excitation of the nitrogen and oxygen K-shells

A photochemistry study on nitrous oxide making use of site-selective excitation of terminal nitrogen, central nitrogen, and oxygen 1s-->3pi excitations is presented. The resonant Auger decay which takes place following excitation can lead to dissociation of the N2O+ ion. To elucidate the nuclear dynamics, energy-resolved Auger electrons were detected in coincidence with the ionic dissociation products, and a strong dependence of the fragmentation pathways on the core-hole site was observed in the binding energy region of the first satellite states. A description based on the molecular orbitals as well as the correlation between the thermodynamical thresholds of ion formation and the first electronic states of N2O+ has been used to qualitatively explain the observed fragmentation patterns.

Site-selective ion pair production via normal Auger decay of free CH 3F molecules studied by electron–ion–ion coincidence spectroscopy

Site-specific fragmentation caused by C KVV and F KVV Auger decay of free CH 3F molecules has been studied. Energy-resolved Auger electrons and pairs of mass-resolved ions were detected in coincidence. Strong enhancements for the production of CH 3 + – F + and H +–CF + ion pairs were found for the F KVV and C KVV Auger decays, respectively.

Site-specific fragmentation following F 1s photoionization of free CF 3SF 5 molecules studied by electron–ion coincidence spectroscopy

We have studied site-specific fragmentation caused by F 1s photoionization of free CF 3SF 5 molecules. Energy-resolved electrons and mass-resolved ions were detected in coincidence. We found an enhancement of the CF 3 + ion production in coincidence with photoelectrons emitted from F atoms in the SF 5 group due to a reaction path leading to CF 3 + – SF + pair production. We found an enhancement of the CF + and C + ions due to F 1s electron emission from the CF 3 group. Site-selectivity was also observed for the CF +–SF +, CF + – SF 2 + , CF + – SF 3 + , C +–F +, C +–S + and C +–SF + ion pair production.

Site-selective ion production of the core-excitedCH3Fmolecule probed by Auger-electron–ion coincidence measurements

We have carried out a coincidence experiment between energy-resolved resonant Auger electrons and mass-resolved ions on ${\mathrm{CH}}_{3}\mathrm{F}$ molecules following F $1s$ and C $1s$ excitation to the lowest unoccupied C--F antibonding molecular orbital ${\ensuremath{\sigma}}_{\mathrm{CF}}^{*}$. We found a strong enhancement of the ${\mathrm{F}}^{+}$ or ${\mathrm{CH}}^{+}$ ion production in coincidence with the F $KVV$ and C $KVV$ spectator Auger electrons, respectively, in the wide binding energy range of 28--36 eV. This site-selective ion production is interpreted as a consequence of the resonant Auger emission taking place in the transient region where the C--F elongation caused by the core excitation transforms the molecular valence orbitals gradually into nonoverlapping valence orbitals of each fragment.

3D-ion-momentum/high-resolution-electron coincidence measurements

We have set up a coincidence apparatus consisting of a high-resolution electron spectrometer and a k-resolving ion spectrometer. As an example for measurements performed with this apparatus, we present ion mass spectra that are coincident with energy-resolved resonant Auger electrons emitted from F 1s excited SF 6 molecules and resonant Auger electron spectra that are coincident with mass-selected fragment ions, as a function of the detuning from the F(1s −1)a 1g ∗ resonance in SF 6. Significant changes of these spectra due to detuning suggest that a substantial fraction of the excess photon energy goes into fragmentation rather than into electron kinetic energy.

SELECTIVE FRAGMENTATION OF VALENCE- AND CORE-ELECTRON-EXCITED CD4 and SF6 MOLECULES

Electron–ion coincidence measurements with energy-resolved electrons are a powerful tool in studies of molecular fragmentation processes, since fragmentation from a specific doorway state can be monitored while the remaining reactions are discriminated. Presently, we have applied this technique in measurements of coincidence spectra of the CD 4 and SF 6 molecules after valence and core electron excitation. The newly constructed energy-resolved electron–ion coincidence station developed for this project has been used. Our results on CD 4 are the first demonstration of a correlation between vibrations and molecular dissociation observed in this molecule. The SF 6 spectra reveal strong selectivity in dissociation following distinct electronic states of the molecule. This selectivity reflects the bonding properties of the potential surfaces involved in the studied processes. The obtained results exemplify the potential of the new multicoincidence station used in this work.

The double ionization of and by energy-resolved electrons

Absolute cross sections for the double ionization of and by energy-resolved electrons have been measured from their threshold energies to 130 and 180 eV, respectively. The double ionization cross section of displays a broad rise beginning at about 80 eV which reaches a peak at around 125 eV. This peak can be attributed to a giant resonance as a result of reactions involving the excitation or ionization of a 4d electron followed by autoionization. Many steps and resonant peaks are observed, some of which can be exclusively assigned to resonant recombination followed by triple autoionization (RETA). There is no evidence of a giant resonance in the double ionization of , but a wealth of structure, due to autoionizing processes, is displayed in the cross section. The cross section of is concave at threshold.

Measurements of cross sections for the ionization of Al+ and Cd+ by energy-resolved electrons

Absolute cross sections for the single ionization of Al+ and Cd+ by energy-resolved electrons have been measured from their thresholds to 115 eV and 95 eV respectively. The Al+ cross section was similar in magnitude to those reported by Montague and Harrison (1983) and Belie et al (1987) but differed in detail. Contributions from excitation-autoionization and the states that may give rise to it are discussed. The Cd+ cross section was similar to that of Belie et al except that considerable step-like structure due to indirect ionization processes was resolved for electron energies above 20 eV.

The double ionization of Ba+ by energy-resolved electrons

Absolute cross sections for double ionization of Ba+ by energy-resolved electrons have been measured from below threshold to 120 eV. The results are similar to those of Hirayama et al. (1987) except that considerable step-like structure was resolved in the ionization function for electron energies between 80 and 120 eV. The origin of this structure is discussed and it probably arises from excitation of a 4d electron followed, either by double autoionization, or by auto-double ionization.

Direct and indirect ionization of Mg+ by energy-resolved electrons

Absolute cross sections for electron impact ionization of Mg+ have been measured between threshold and 70 eV. The electrons had a near-Gaussian energy profile with a width (FWHM) of 170 meV and their energy was defined to within 40 meV. Considerable structure due to indirect ionization was revealed, and from comparisons with calculations by Tayal and others, it can be identified as excitation-autoionization and resonant-excitation-double-autoionization. The measurements confirm the existence of several of the resonances seen by Muller et al. (1988-90) but there are some differences between the two experimental results.

The ionization of Zn+ by energy-resolved electrons

Cross sections for the ionization of Zn+ by energy-resolved electrons have been measured from below threshold (17.96 eV) to 39 eV. The results are similar to others that have been reported except that much more structure, due to indirect ionization processes, was resolved. The origin of much of the autoionization was determined by comparing the measurements with theoretical predictions. Comparisons were also made with measured photoionization cross sections of Zn+.

The ionisation of Ga+ by energy-resolved electrons

Cross sections have been measured for the ionisation of Ga+ by energy-resolved electrons ( Delta E=0.1 eV) with energies between threshold at 20.51 eV and 41 eV. There was good agreement with the measurements of Rogers et al (1982) who reported very strong contributions from indirect ionisation, but the enhanced energy resolution of the present measurements enabled much more structure in the ionisation function to be resolved. This structure is compared with the photoionisation cross sections of Ga+ of Peart et al (1989) and with the calculations of autoionisation by Pindzola et al (1982). The calculations reproduced some autoionisation features quite accurately but optically permitted transitions were found to contribute less to ionisation than was predicted.

The ionisation of Sr+ by energy-resolved electrons

Cross sections have been measured for the ionisation of Sr+ by energy-resolved electrons with energies between threshold (11.030 eV) and 50 eV. Considerable structure due to autoionisation was resolved. The results were compared with energies of autoionising levels deduced from measurements, by others, of the ejected electron spectra and optical spectra of neutral strontium. Further comparisons were made between the present results and those of the authors' earlier experiments on electron impact ionisation of Ca+ and Ba+ and on the photoionisation of Sr+.

The ionisation of Ca2+ by energy-resolved electrons

Cross sections for the ionisation of Ca2+ by energy-resolved electrons have been measured for electron energies between 21 and 50 eV. Considerable structure was resolved in the ionisation function. At energies where autoionisation is dominant, the cross sections differ substantially from the results of recent calculations by Burke et al. (1983), Griffin et al. (1984) and Pindzola et al. (1987). The experiment indicates that many more transitions are significant than have so far been included in theory. Comparisons are also made with energies of autoionising levels deduced from theory, from ejected-electron spectra of neutral calcium and from the photoionisation of Ca2+.

Autoionisation and threshold ionisation of Ba+ by energy-resolved electrons

Detailed measurements have been made of the ionisation of Ba+ by energy-resolved electrons, drawn from a double 127' electron energy selector, for energies between threshold and 30 eV. Several autoionisation thresholds have been resolved and their energies are compared with values obtained by others from optical and ejected-electron spectra of neutral barium. Comparisons are also made with results of Hartree-Fock calculations by Hansen (1974-1975). These comparisons enabled the probable configurations of some principal autoionising transitions to be suggested. Detailed measurements have also been made of the near-threshold ionisation of Ba+.

Angular distributions of photoelectrons from excited valence1,3P1ostates of Ca, Sr, and Ba

Angular distributions are reported for energy-resolved electrons obtained by resonance-enhanced multiphoton ionization of alkaline-earth atoms through excited intermediate valence states. The angular distributions are measured as functions of the independently variable polarizations of the exciting and ionizing laser beams. This report treats the following states, resonantly formed and photoionized: Ca(4s4p) $^{1}P_{1}^{o}$, Sr(5s5p) $^{1}P_{1}^{o}$, Sr(5s5p) $^{3}P_{1}^{o}$, Ba(6s6p) $^{1}P_{1}^{o}$, and Ba(6s6p) $^{3}P_{1}^{o}$. Systematic behavior in the resulting photoelectron angular distributions is explored. The effects of the hyperfine interaction are apparent.

Measurements of cross sections for the ionization of Ba+by energy resolved electrons

The crossed beam method for observing the ionisation of ions by electrons has been refined by the inclusion of a 127 degrees electrostatic electron energy selector. This enhances the resolution of the measurements, by an order of magnitude, to about 0.15 eV. With this apparatus, autoionization thresholds have been resolved in the ionization function for, Ba++e to Ba2++2e, at energies of 15.8, 16.8, 19.2 and 21.9 eV. Observations have also been made of the near-threshold ionization of K+.