Absolute partial and total cross‐section functions for the electron impact ionization of C60 and C70

Electron Ionization of Size-Selected Positively and Negatively Charged Helium Droplets

A beam of size-selected charged helium droplets was crossed with an electron beam, and the ion efficiency curves for the product droplets in all different charge states were recorded. We estimate that the selected helium droplets on their passage through the electron beam are hit by several hundred electrons which can interact with the individual He atoms of the droplets. Reaction channels corresponding to the removal or capture of up to eight electrons were identified, and in all cases, inelastic scattering and the formation of metastable helium played a significant role.

Collective mode properties in a paired fullerene-ion plasma

A pair-ion plasma without electrons consisting of C60+ and C60- is generated through the processes of electron-impact ionization, electron attachment, and magnetic filtering. Properties of electrostatic modes propagating along magnetic-field lines are experimentally investigated by externally exciting them with two types of electrodes. It is found that four kinds of wave modes exist and a frequency spectrum of phase lag between the density fluctuations of C60+ and C60- is unique in comparison with ordinary electron-ion plasmas. One of the modes is an ion acoustic wave which is divided into two branches at around the ion cyclotron frequency in the presence of a backwardlike mode joining them. The phase lag of the ion acoustic wave strongly depends on the frequency, while those for the other ion plasma and intermediate-frequency waves are constant at pi independent of the frequency.

Double photoionization of C(60) and C(70) in the valence region

Photoion yields from gaseous fullerenes, C(60) and C(70), for production of singly and doubly charged ions are measured by mass spectrometry combined with tunable synchrotron radiation at hnu=25-150 eV. Since the signal of triply or highly charged ions is very weak, the total photoionization yield curve can be estimated from the sum of the yields of the singly and doubly charged ions. A distinct feature appears in the resultant curve of C(60) which is absent in the calculated total photoabsorption cross section previously reported. This difference is attributed to C(60) (2+) ions chiefly produced by spectator Auger ionization of the shape resonance states followed by tunneling of the trapped electron or by cascade Auger ionization. Ratios between the yields of doubly and singly charged ions for C(60) and C(70) are larger than unity at hnu>50 eV. These ratios are quite different from those reported in the experiments using electron impact ionization.

Electrostatic waves in a paired fullerene-ion plasma

Three kinds of electrostatic modes are experimentally observed to propagate along magnetic-field lines for the first time in the pair-ion plasma consisting of only positive and negative fullerene ions with an equal mass. It is found that the phase lag between the density fluctuations of positive and negative ions varies from 0 to pi depending on the frequency for ion acoustic wave and is fixed at pi for an ion plasma wave. In addition, a new mode with the phase lag about pi appears in an intermediate-frequency band between the frequency ranges of the acoustic and plasma waves.

Energetic ion bombardment of Ag surfaces by C60+ and Ga+ projectiles

The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C60+ cluster projectiles compared with Ga+ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C60+ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga+ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. The experimental findings are in reasonable agreement with recent molecular dynamics simulations.

Numerical investigation of the discharge characteristics of the pulsed discharge nozzle

The characteristics of the plasma generated by a pulsed discharge slit nozzle (PDN) are investigated. The PDN source is designed to produce and cool molecular ions creating an astrophysically relevant environment in the laboratory. A discharge model is applied to this system to provide a qualitative as well as a quantitative picture of the plasma. We find that the plasma's properties and behavior are characteristic of those of a glow discharge. We model the electron density and energy, as well as the argon ion and metastable atom number density. The results reveal a high abundance of metastable argon atoms in the expansion region, which is more than one order of magnitude higher than the abundance of electrons and ions. These findings confirm experimental observations, which concluded that large molecular ions are dominantly formed through Penning ionization of the neutral molecular precursors seeded in the supersonic expansion of argon gas. The simulations presented here will help optimize the yield of formation of molecular ions and radicals in the PDN source; they will also provide key physical insight into the characteristics of interstellar molecules and ions analogs in laboratory experiments.

Pair-ion plasma generation using fullerenes

We have developed a novel method for generating pure pair plasma which consists of positive- and negative-charged particles with an equal mass. The pair-ion plasma without electrons is generated using fullerene as an ion source through the processes of hollow-electron-beam impact ionization, electron attachment, preferential radial diffusion of ions, and resultant electron separation in an axial magnetic field. Basic characteristics of this plasma are discussed in terms of the differences from ordinary electron-ion plasmas, such as a phenomenon in the absence of sheath and potential structure formation.

A C60 primary ion beam system for time of flight secondary ion mass spectrometry: its development and secondary ion yield characteristics

A buckminsterfullerene (C60)-based primary ion beam system has been developed for routine application in TOF-SIMS analysis of organic materials. The ion beam system is described, and its performance is characterized. Nanoamp beam currents of C60+ are obtainable in continuous current mode. C60(2+) can be obtained in pulsed mode. At 10 keV, the beam can be focused to less than 3 microm with 0.1 nA currents. TOF-SIMS studies of a series of molecular solids and a number of polymer systems in monolayer and thick film forms are reported. Very significant enhancement of secondary ion yields, particularly at higher mass, were observed using 10-keV C60+ for all samples other than PTFE, as compared to those observed from 10 keV Ga+ primary ions. Three materials (PS2000, Irganox 1010, PET) were studied in detail to investigate primary ion-induced disappearance (damage) cross sections to determine the increase in secondary ion formation efficiency. The C60 disappearance cross sections observed from monolayer film PS2000 and self-supporting PET film are close to those observed from Ga+. The resulting C60 efficiencies are 30-100 times those observed from gallium. The cross sections observed from C60 bombardment of multilayer molecular solids are approximately 100 times less, such that essentially zero damage sputtering is possible. The resulting efficiencies are > 10(3) greater than from gallium. It is also shown that C60 primary ions do not generate any more low-mass fragments than any other ion beam system does. C60 is shown to be a very favorable ion beam system for TOF-SIMS, delivering high yield, close to 10% total yield, favoring high-mass ions, and on thick samples, offering the possibility of analysis well beyond the static limit.

Manifestation of charge-density fluctuations in metal clusters: suppression of the ionization channel

The fluctuations in the electronic charge density of metallic clusters in response to an approaching electron suppress the single-ionization channel. This conclusion is made on the basis of numerical calculations for the total ionization cross sections using the random-phase approximation with exchange to describe the particle-hole (de)excitations. The general trends can be understood by means of the Thomas-Fermi model. The present theory explains, for the first time, the behavior of the measured total ionization cross section for C(60). The interplay between finite size and non-local screening effects is studied by tracing the changes in the ionization cross sections for Li clusters with an increasing cluster radius.

Electron-impact induced fragmentation of fullerene ions

The C2 fragmentation of fullerene ions C(q+)(60) (q = 1,2,3) induced by electron impact was studied for the first time. The cross sections for the loss of a C2 fragment indicate the presence of two different processes. At low electron energies the projectile electron leads to the direct excitation of the giant plasmon resonance. At electron energies larger than 100 eV the fragmentation of the fullerene ions can be described as an unsuccessful ionization. Only this second part of the cross section shows a dependence on the charge state q of the precursor ion.

Multiple ionization and fragmentation of negatively charged fullerene ions by electron impact

Cross sections for the electron-impact multiple ionization and fragmentation of negatively charged fullerene ions C(-)(n) ( n = 60, 70) to C(q+)(n-m) ( q = 1,2,3 and m = 0,2,4) have been measured for electron energies up to 1 keV. In the case of pure ionization all threshold energies are about 10 eV higher than the values expected. This shift, however, is not observed for the fragment ions. The experimental data indicate that there is no strong electron-electron interaction between the incident electron and the attached electron. A novel ionization mechanism is proposed which can be expected to be valid for all negatively charged molecular or cluster ions which are able to shield the attached electron from the incident electron.