Ion beam experiments

Figure 1 shows three configurations for colliding-beam experiments that have been used for studies of dissociative recombination. These are the inclined-beam apparatus at the University of Newcastle-upon-Tyne,30 the merged-electron—ion-beam experiment (MEIBE) at the University of Western Ontario,31 and an electron-cooler apparatus at the Manne Siegbahn laboratory in Stockholm.32 In the inclined-beam method (Figure la), the ion beam is accelerated to an energy of 30... 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

It is seen from this table that, at the LD50 level, the nucleus has received about the same dose, irrespective of whether X-rays, 3H-dThd or 125I-concanavalin are used as the source of ionizing radiation, while the membrane has received an immense dose with 125I-concanavalin and very little with 3H-dThd. As expected, the cytoplasm lies in between these two extremes. Yet, irradiation of the cytoplasm (single-ion-beam experiments) is not without an effect. It may cause mutations (Wu et al. 1999) and the formation of products that induce apoptosis in nearby (unirradiated) cells (bystander effect Shao et al. 2004). 

In the microwave ion beam experiments described in this section, it is important to identify the microwave mode corresponding to the resonance line studied in a magnetic field. For a TM mode the microwave electric field along the central axis of the waveguide is parallel to the static magnetic field. We then put p = 0 in equation (10.161) so that the Zeeman components obey the selection rule AMj = 0. Alternatively in a TE mode the microwave electric field is perpendicular to the static magnetic field and the selection rule is A Mj = 1. This is the case for the Zeeman pattern shown in figure 10.73 each J = 3/2 level splits into four Mj components and the six allowed transitions should,... 

In a guided ion beam experiment, reactant ions are created in the source region, mass selected by a mass spectrometer (a magnetic sector in our instruments), decelerated to a desired kinetic energy, and injected into a radiofrequency (rf) octopole ion beam guide [10,11]. This device comprises eight rods cylindrically surrounding the ion beam path. Opposite phases of an rf potential... 

In ion beam experiments [30,31] a very pronounced Ts-dependence is observed the yield can vary by nearly two orders of magnitude between room temperature and 900 K. In tokamaks with flux densities several orders of... 

In ion beam experiments, carbon materials show an unexpected additional erosion mechanism which dominates the carbon erosion in the temperature range 1200 K 2200 K, called radiation enhanced sublimation (RES) [35,36]. There are doubts whether this effect also exists with very high particle flux densities as they are typical for tokamaks. Test limiter experiments in TEXTOR have demonstrated that RES is not important under such conditions. However, other devices reported a carbon influx by RES and recent measurements indicate also an enhanced erosion of metals at high temperatures under low energy particle impact [34]. Further R D is needed to clarify these issues. 

Since the reviews on chemical erosion of carbon in 1992 [35] considerable improvements have been reached in the fundamental understanding. The thorough investigations by Horn et al. [39] on the thermal chemical reactivity has been extended to the case of energetic ion bombardment by Roth and Garcfa-Rosales [41]. Simultaneously, new ion beam experiments at energies down to 10 eV [36-38] have improved the understanding of ion induced hydrocarbon emission. 

In the material modification due to non-recycling ions in a hydrogen plasma the mobility of the constituents will modify the surface concentrations and influence the erosion of the substrate. These effects can only be investigated in a controlled way using dual ion beam experiments. Such a system is being commissioned at IPP and results for the interaction of different plasma facing materials are expected in the near future. 

The M H dissociation energies see Bond Dissociation Energy) estimated from ion beam experiments are quite... 

Fig. 15. Sodium charge transfer cross sections with atmospheric molecular ions as measured using the high temperature guided-ion beam experiment (circles). Triangles are the flowing-afterglow measurements by Farragher et and squares are the...

The interactions of bare metal ions, M (M = Co, Fe) performed in an ion beam experiment, with cyclopropanes have also been studied in great detail. It has been demonstrated that reaction of Co or Fe" with c-CaH or c-CaHsMe results initially in the formation of metallacyclobutanes (236). The latter rearrange to olefin/carbene complexes (237) (Scheme 34). 

In the ion beam experiments, the beam-current density is measured and converted to a flux, ion/cm s. The fluence (in ions/cm experienced by the specimen is the flux multiplied by time. In many papers, the measured fluence is converted to an ion dose in units of displacements per atom (dpa). This is essentially a measure of the actual amount of damage (i.e. number of atomic displacements) that results from cascade formation, but it does not consider subsequent relaxation or recrystallization events. The fluence-to-dose... 

ENERGY RESOLVED CID VIA GUIDED ION BEAM EXPERIMENTS. Guided ion beam experiments have been used to measure a wealth of gas-phase thermochemistry [23]. Because these experiments measure thresholds for fragmentation, they can be probes of the activation energy [24]. 

He thanks Prof. Jacob Klein and other colleagues from the Weizmann Institute of Science for their friendly and helpful assistance which enabled him to complete the ion beam experiments illustrating this work. He is also very indebted to J. Klein, L.J. Fetters, U. Steiner, T. Hashimoto, R. Brenn, K. Binder, R.A. Jerry, J. Genzer, G. Reiter, S. Kumar, and M. Stamm for all the collaboration and discussions benefiting this work. 

In the simulations described here, an initially bare Si layer of 512 atoms, with a lateral area of about 500 and a depth of about 20 A, was used. Periodic lateral boundaries and a fixed bottom layer were used as before. The top Si layer was amorphized by repeated impacts of 200-eV Ar before exposure to GF). The goal was to simulate an ion beam experiment in which a beam of monoenergetic CFj is directed at a silicon surface, with no other species impacting the surface. The initial transient associated with the buildup of a mixed C-F-Si layer at the surface would be followed by a period of steady-state etching in which the composition and thickness of the top mixed layer would not change. Each ion impact was followed for 0.2 ps, and tests were made to be certain that the results did not depend on the length of the impact trajectory time. Clusters of material that were weakly... 

The reaction of B" with H2 gives rise to a variety of products which have been studied in ion beam experiments by several groupsfor collision energies up to about 13 eV. These are summarized as follows ... 

Guella, G., Ascenzi, D., Franceschi, P., Tosi, P., Gas phase Synthesis and Detection of the Benzenediazonium Ion, C6H5N2 +. A Joint Atmospheric Pressure Chemical Ionization and Guided Ion Beam Experiment, Rapid Commun. Mass Spectrom. 2005, 19, 1951 1955. 

This article discusses the importance of the weak interaction rates in different astrophysical processes. Though a lot of effort has gone in to calculate these rates accurately, more work is still needed, particularly for the highly neutron-rich nuclei. Radioactive ion beam experiments are also expected to give valuable information for these nuclei. So one hopes that these will help in a more accurate description of these processes in the near future. 

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