The current peak that appears on a linear-scan voltammogram for the reductive desorption of alkanethiol
self-assembled monolayers (SAMs) from a gold surface in an aqueous alkaline solution exhibits intriguing
features: the narrow full width at half-maximum (fwhm) of the peak, e.g., 20 mV for dodecanethiol SAMs,
the saturation of fwhm in the SAM composed of long-chain alkanethiols, an asymmetric shape, the shift
of the peak potential with increasing the alkyl chain length, and the peak area that is greater than what
is expected from the (√3 × √3)R30° structure of adsorbed alkanethiols on Au(111). A Padé approximant
expression for the adsorption isotherm proposed by Blum and Huckaby based on the two-dimensional Ising
model, in combination with the semi-infinite linear diffusion of desorbed species, well explains these
salient features of the reductive desorption behavior. The double-layer charging current can amount to
one-third of the charge calculated from the area of the peak of the reductive desorption, explaining the
discrepancy between the adsorbed amount of an alkanethiol calculated from the peak area and that expected
from the (√3 × √3)R30° structure.
[1] We study formation process of electrostatic solitary waves (ESW) observed by recent spacecraft via one-and two-dimensional electrostatic particle simulations with open boundaries. The previous simulations have demonstrated that ESW correspond to Bernstein-Greene-Kruskal electron holes formed by electron beam instabilities. However, since the previous simulations were performed in uniform periodic systems, wave-particle interaction of an electron beam instability was taking place uniformly in the systems. In the present study, we inject a weak electron beam from an open boundary into the background plasma to study spatial and temporal development of a bump-on-tail instability from a localized source. In the open system, spatial structures of electron holes vary depending on the distance from the source of the electron beam. In an early phase of the simulation run, electron holes that are initially uniform in the direction perpendicular to the magnetic field become twisted through modulation by oblique electron beam modes. As the electron holes propagate along the magnetic field, they are aligned in the perpendicular direction through coalescence. Spatial structures of electron holes in a distant region from the source become one-dimensional. In a long-time evolution of the instability, ion dynamics becomes important in determining spatial structures of electron holes. A lower hybrid mode is excited locally in the region close to the source of the electron beam through coupling with electron holes at the same parallel phase velocity. The lower hybrid mode modulates electron holes excited in later phases, resulting in formation of modulated one-dimensional potentials. Since the perpendicular electric fields of electron holes are carried by the electron holes at the drift velocity of the electron holes, they can be observed even at a distant place from the source.
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A novel particle simulation code, the electromagnetic spacecraft environment simulator (EMSES), has been developed for the self-consistent analysis of spacecraft-plasma interactions on the full electromagnetic (EM) basis. EMSES includes several boundary treatments carefully coded for both longitudinal and transverse electric fields to satisfy perfect conductive surface conditions. For the longitudinal component, the following are considered: (1) the surface charge accumulation caused by impinging or emitted particles and (2) the surface charge redistribution, such that the surface becomes an equipotential. For item (1), a special treatment has been adopted for the current density calculated around the spacecraft surface, so that the charge accumulation occurs exactly on the surface. As a result, (1) is realized automatically in the updates of the charge density and the electric field through the current density. Item (2) is achieved by applying the capacity matrix method. Meanwhile, the transverse electric field is simply set to zero for components defined inside and tangential to the spacecraft surfaces. This paper also presents the validation of EMSES by performing test simulations for spacecraft charging and peculiar EM wave modes in a plasma sheath.
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