Search

Dust grains in various astrophysical environments are generally charged electrostatically by photoelectric emissions with radiation from nearby sources, or by electron/ion collisions by sticking or secondary electron emissions (SEES). The high vacuum environment on the lunar surface leads to some unusual physical and dynamical phenomena involving dust grains with high adhesive characteristics, and levitation and transportation over long distances. Knowledge of the dust grain charges and equilibrium potentials is important for understanding a variety of physical and dynamical processes in the interstellar medium, and heliospheric, interplanetary/ planetary, and lunar environments. It has been well recognized that the charging properties of individual micron-/submicron-size dust grains are expected to be substantially different from the corresponding values for bulk materials. In this paper, we present experimental results on the charging of individual 0.2-13 m size dust grains selected from Apollo 11 and 17 dust samples, and spherical silica particles by exposing them to mono-energetic electron beams in the 10-200 eV energy range. The dust charging process by electron impact involving the SEES discussed is found to be a complex charging phenomenon with strong particle size dependence. The measurements indicate substantial differences between the polarity and magnitude of the dust charging rates of individual small-size dust grains, and the measurements and model properties of corresponding bulk materials. A more comprehensive plan of measurements of the charging properties of individual dust grains for developing a database for realistic models of dust charging in astrophysical and lunar environments is in progress.

The photoelectric emission process is considered to be the dominant mechanism for charging of cosmic dust grains in many astrophysical environments. The grain charge and equilibrium potentials play an important role in the dynamical and physical processes that include heating of the neutral gas in the interstellar medium, coagulation processes in the dust clouds, and levitation and dynamical processes in the interplanetary medium and planetary surfaces and rings. An accurate evaluation of photoelectric emission processes requires knowledge of the photoelectric yields of individual dust grains of astrophysical composition as opposed to the values obtained from measurements on flat surfaces of bulk materials, as it is generally assumed on theoretical considerations that the yields for the small grains are much different from the bulk values. We present laboratory measurements of the photoelectric yields of individual dust grains of silica, olivine, and graphite of approx. 0.09-5 micrometer radii levitated in an electrodynamic balance and illuminated with ultraviolet radiation at 120-160 nm wavelengths. The measured yields are found to be substantially higher than the bulk values given in the literature and indicate a size dependence with larger particles having order-of-magnitude higher values than for submicron-size grains.

The dominant charging processes in various astrophysical environments are considered to be by photoelectric emissions with radiation from nearby stars and secondary electron emissions (SEE) by impact of electrons in a medium with sufficiently high energies. The charging of bulk materials with planar surfaces by SEE with electron impact appears to be well understood with theoretical expressions as well as by experimental techniques. However, the charging of submicron/micron-size dust grains by SEE with sufficiently high-energy electrons is a complex process, and is a function of electron energies, the electron current, and the grain size, and the charge or the surface potential. Development of viable theoretical models and acquisition of experimental data for charging properties of micron-size dust grains are still in the early stages. This paper focuses on SEE charging properties of individual micron-size dust grains by low-energy electron impact, obtained from laboratory measurements on an experimental facility based on an electrodynamic balance. The measurements of SEE yields of positively charged dust grains indicate the yields increase with decreasing grain size and the equilibrium surface potentials showing generally linear size dependence. These experimental results are generally in agreement with several independent experimental and analytical model studies in the literature, with the exception of a recently published paper which is in fundamental conflict with our studies as well as with several other experimental and theoretical studies. The sources and causes of this conflict are critically examined and discussed in this paper.