The simulation of electron beam induced welding of crossed carbon nanotubes is considered with classical molecular dynamics simulations. Covalent junctions are predicted to form between various types of carbon nanotubes that contain many defects and are likely to be representative of experimentally welded nanotubes under highly nonequilibrium synthesis conditions. The effect of the junction structure and hydrogen termination of dangling bonds on the mechanical responses of the junctions is also considered.

New experimental and theoretical results are presented that address the movement of O+ ions through argon gas. On the experimental front, improved ion mobility results are presented. These results confirm the presence of the oft-cited mobility minimum as a function of electrostatic field strength at room temperature. On the theoretical side, high-level ab initio potential energy curves are calculated for the Ar-O+ system and, from these, transport properties are calculated and compared to experiment. A crossing between the lowest 2Pi curve and the ground state 4Sigma(-) curve near the minimum of each potential becomes an avoided crossing on the inclusion of spin-orbit coupling. It is shown that the more appropriate potential for the description of the motion of O+(4S degree) through Ar at the energies of interest is the diabatic potential, neglecting fine structure. By using an improved 4Sigma(-) potential, agreement with the mobility measurements is obtained for low and intermediate electrostatic field strengths, although small discrepancies remain for high field strengths. The appropriate choice of diabatic or adiabatic potentials is also considered for related systems of interest: He-O+, Ne-O+, and Rg-O(-) (Rg=He,Ne,Ar).

Recent moment theories of ion motion in devices where the external fields vary with position and time are applied to non-ideal ion traps. In first approximation, the theories give differential equations with collision frequencies that vary with the effective temperature characterizing the relative kinetic energy of the ion–neutral collisions. Solutions of the set of coupled differential equations provide the ion number density, average velocities, average energies and average temperatures as functions of time and of position in the apparatus. Solutions of the coupled equations are discussed for the Maxwell model, rigid spheres and general ion–neutral interactions. A surprising finding is that ac fields on the endcaps lead to changes in the stability region for ion motion in traps, changes that can cause ion ejection both for small and large values of the ac field applied to the rings.

Fundamental aspects are presented of a two-temperature moment theory for quadrupole ion traps developed via transformation of the Boltzmann equation. Solutions of the moment equations correspond to changes in the ensemble average for any function of ion velocity, because the Boltzmann equation reflects changes to an ion distribution as a whole. The function of primary interest in this paper is the ion effective temperature and its behavior during ion storage and resonance excitation. Calculations suggest that increases in ion effective temperature during resonance excitation are due primarily to power absorption from the main RF trapping field rather than from the dipolar excitation signal. The dipolar excitation signal apparently serves mainly to move ions into regions of the ion trap where the RF electric field, and thus ion RF heating, is greater than near the trap center. Both ideal and non-ideal ion trap configurations are accounted for in the moment equations by incorporating parameterized variables ã and q, which are modified versions of the commonly used forms for the DC and AC ring voltages, and b and d, which are new forms that account for the voltages applied to the endcaps. Besides extending the applicability of the moment equations to non-ideal quadrupole ion traps, the modified versions of the parameterized variables can have additional utility. T oday, a large number of analytical mass spectrometers depend exclusively upon electric fields for ion transport, manipulation, and mass analysis. The fields employed may be static (e.g., time-offlight) [1], dynamic (e.g., RF quadrupole [2][3][4][5]), or a combination of the two [6 -8]. Improvements in performance of RF devices have been realized by changes in the physical configuration and applied potentials [9 -12], which produce electric fields of increased complexity (e.g., hexapole, octapole, etc.). Furthermore, ion motion and physicochemical phenomena in such RF devices are influenced by the introduction of a buffer gas at substantial pressure. The high number of ionneutral collisions resulting from extended ion residence times and elevated buffer gas number density can prove beneficial for ion cooling and focusing [13,14]. In contrast, the average kinetic energy of the ions also can be increased significantly above the thermal energy of the neutrals via acceleration in an electric field. The most widely used approach for ion acceleration in electrodynamic ion traps, termed resonance excitation, uses a relatively low-amplitude AC signal at the fundamental frequency of ion axial oscillations applied to the endcaps [15]. An important application of that process in RF multipole devices is collisional activation (CA) [15], in which a portion of the ion-neutral relative (i.e., center-of-mass) kinetic energy is transferred into internal energy of the ions. In such instances, although the kinetic energy associated with any individual collision is generally small, the cumulative effect of multiple collisions enables the collision-induced dissociation (CID) o...

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