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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 lunar surface is covered with a thick layer of sub-micron/micron size dust grains formed by meteoritic impact over billions of years. The fine dust grains are levitated and transported on the lunar surface, as indicated by the transient dust clouds observed over the lunar horizon during the Apollo 17 mission. Theoretical models suggest that the dust grains on the lunar surface are charged by the solar ultraviolet (UV) radiation as well as the solar wind. Even without any physical activity, the dust grains are levitated by electrostatic fields and transported away from the surface in the near vacuum environment of the Moon. The current dust charging and levitation models, however, do not fully explain the observed phenomena. Since the abundance of dust on the Moon's surface with its observed adhesive characteristics has the potential of severe impact on human habitat and operations and lifetime of a variety of equipment, it is necessary to investigate the charging properties and the lunar dust phenomena in order to develop appropriate mitigating strategies. Photoelectric emission induced by the solar UV radiation with photon energies higher than the work function (WF) of the grain materials is recognized to be the dominant process for charging of the lunar dust, and requires measurements of the photoelectric yields to determine the charging and equilibrium potentials of individual dust grains. In this paper, we present the first laboratory measurements of the photoelectric efficiencies and yields of individual sub-micron/micron size dust grains selected from sample returns of Apollo 17 and Luna-24 missions as well as similar size dust grains from the JSC-1 simulants. The measurements were made on a laboratory facility based on an electrodynamic balance that permits a variety of experiments to be conducted on individual sub-micron/micron size dust grains in simulated space environments. The photoelectric emission measurements indicate grain size dependence with the yield increasing by an order of magnitude for grains of sub-micron to several micron size radii, at which it reaches asymptotic values. The yield for large size grains is found to be more than an order of magnitude higher than the bulk measurements on lunar fines reported in the literature.

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.

Stark broadening widths for 138 Ti II and 210 Ti III spectral lines are presented as a function of temperature for an electron density of 10 23 m −3 . For Ti II, the Stark line shifts are given as well. The calculations have been performed using a modified semiempirical approach.

Stark broadening parameters for 53 Ra II spectral lines have been calculated. Calculations have been performed within the frame of the modified semiempirical approach within the temperature range 5000–50000 K. Stark broadening parameters for 43 Ra II spectral lines have been calculated for an electron density of 1020 m-3, while Stark broadening parameters for 10 Ra II spectral lines, belonging 5f2Fo–7d2D, 6d2D–7p2Po, 7d2D–8p2Po and 8d2D–9p2Po Ra II multiplets in the far infrared, have been presented for an electron density of 1020 m-3. Results have been used for a discussion of Stark broadening regularities within multiplets and systematic trends along spectral series. Moreover, the results obtained have been compared with simple estimates based on regularities and systematic trends.

The knowledge on regularities and systematic trends for Stark line shifts within a given spectrum as e.g. similarities among Stark line shifts within a multiplet, supermultiplet or transitions array and systematic trends within a spectral series, is useful for the interpolation of new data and the critical evaluation of experimental data. In this paper exceptions to the established regularities have been analyzed on the basis of critically selected experimental data as well as of results of semi-classical calculations. It has been demonstrated that the principal reason for the irregularities discussed is irregularities of energy level structure. An additional reason is the influence of the intercombination and forbidden perturbing transitions.

The Stark broadening parameters for six Au I lines and eight Au II transitions have been calculated. Two methods for calculation have been used: the semiclassical method (for Au I lines) and the modified semiempirical approach (for Au II transitions). In the case of Au II, the jj coupling approximation has been used for the matrix-element calculations. The importance of the electron-impact effect in the case of Au II nm line for several stellar atmosphere models has been tested.

Using the semiclassical perturbation formalism, we have calculated electron-, proton-, and ionized helium-impact line widths and shifts for six Au I lines as a function of temperature and perturber density. Electron-impact broadening parameters for eight Au II transitions have been calculated within the modified semiempirical approach.

In hot star atmospheres, the Stark broadening mechanism is the main pressure broadening mechanism. An interesting application where such a mechanism is of interest is the modeling and investigation of hot star spectra, stellar atmospheres and subphotospheric layers. Consequently, for the investigation and modeling of the Hg-Mn star and other type of hot star atmospheres, the Stark broadening parameters for Co III spectral lines may be of interest. Here we present Stark broadening data for 20 Co III spectral lines (from a 6 D- z 6 D o and a 6 D- z 6 F o Co III multiplets), as a function of temperature, calculated by using the modified semi-empirical approach. The importance of the electron-impact eect in the case of the Co III 194.98 nm line for several stellar atmosphere models has been tested.

The processes and mechanisms involved in the rotation and alignment of interstellar dust grains have been of great interest in astrophysics ever since the surprising discovery of the polarization of starlight more than half a century ago. Numerous theories, detailed mathematical models and numerical studies of grain rotation and alignment with respect to the Galactic magnetic field have been presented in the literature. In particular, the subject of grain rotation and alignment by radiative torques has been shown to be of particular interest in recent years. However, despite many investigations, a satisfactory theoretical understanding of the processes involved in grain rotation and alignment has not been achieved. As there appears to be no experimental data available on this subject, we have carried out some unique experiments to illuminate the processes involved in rotation of dust grains in the interstellar medium. In this paper we present the results of some preliminary laboratory experiments on the rotation of individual micron/submicron size nonspherical dust grains levitated in an electrodynamic balance evacuated to pressures of approximately 10(exp -3) to 10(exp -5) torr. The particles are illuminated by laser light at 5320 Angstroms, and the grain rotation rates are obtained by analyzing the low frequency (approximately 0-100 kHz) signal of the scattered light detected by a photodiode detector. The rotation rates are compared with simple theoretical models to retrieve some basic rotational parameters. The results are examined in the light of the current theories of alignment.

Measurements of electromagnetic radiation pressure have been made on individual silica (SiO2) particles levitated in an electrodynamic balance. These measurements were made by inserting single charged particles of known diameter in the 0.2 micron to 6.82 micron range and irradiating them from above with laser radiation focused to beam-widths of approx. 175-400 micron, at ambient pressures approx. 10(exp -3) to 10(exp -4) torr. The downward displacement of the particle due to the radiation force is balanced by the electrostatic force indicated by the compensating dc potential applied to the balance electrodes, providing a direct measure of the radiation force on the levitated particle. Theoretical calculations of the radiation pressure with a least-squares fit to the measured data yield the radiation pressure efficiencies of the particles, and comparisons with Mie scattering theory calculations provide the imaginary part of the refractive index of silica and the corresponding extinction and scattering efficiencies.

Investigations of the Stark line broadening parameters are important for a number of problems in laboratory and astrophysical plasma research. We use a modified semiempirical approach in order to complete a set of the Stark broadening data. Here we present the Stark broadening data for 6 Ag II spectral lines calculated within the modified semiempirical approach.

Dust grains in various astrophysical environments are generally charged electrostatically by photoelectric emissions with UV/X-ray radiation, as well as by electron/ion impact. Knowledge of physical and optical properties of individual dust grains is required for understanding of the physical and dynamical processes in space environments and the role of dust in formation of stellar and planetary systems. In this paper we focus on charging of individual micron/submicron dust grains by processes that include: (a) UV photoelectric emissions involving incident photon energies higher than the work function of the material and b) electron impact, where low energy electrons are scattered or stick to the dust grains, thereby charging the dust grains negatively, and at sufficiently high energies the incident electrons penetrate the grain leading to excitation and emission of electrons referred to as secondary electron emission (SEE). It is well accepted that the charging properties of individual micron/submicron size dust grains are expected to be substantially different from the bulk materials. However, no viable models for calculation of the charging properties of individual micron size dust grains are available at the present time. Therefore, the photoelectric yields, and secondary electron emission yields of micron-size dust grains have to be obtained by experimental methods. Currently, very limited experimental data are available for charging of individual micron-size dust grains. Our experimental results, obtained on individual, micron-size dust grains levitated in an electrodynamic balance facility (at NASA-MSFC), show that: (1) The measured photoelectric yields are substantially higher than the bulk values given in the literature and indicate a particle size dependence with larger particles having order-of-magnitude higher values than for submicron-size grains; (2) dust charging by low energy electron impact is a complex process. Also, our measurements indicate that the electron impact may lead to charging or discharging of dust grains depending upon the grain size, surface potential, electron energy, electron flux, grain composition, and configuration (e.g. Abbas et al, 2010). Laboratory measurements on charging of analogs of the interstellar dust as well as Apollo 11 dust grains conducted at the NASA-MSFC Dusty Plasma Lab. are presented here

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.

Photoemission experiments with UV radiation have been performed to investigate the microphysics and charge characteristics of individual isolated dust grains of various compositions and sizes by using the electrodynamic balance facility at NASA Marshall Space Flight Center. Dust particles of 1 - 100 micrometer diameter are levitated in a vacuum chamber at pressures approx. 10(exp -5) torr and exposed to a collimated beam of UV radiation in the 120-300 nanometers spectral range from a deuterium lamp source with a MgF2 window. A monochromator is used to select the UV radiation wavelength with a spectral resolution of 8 nanometers. The electrodynamic facility permits measurements of the charge and diameters of particles of known composition, and monitoring of photoemission rates with the incident UV radiation. Experiments have been conducted on Al2O3 and silicate particles, and in particular on JSC-1 Mars regolith simulants, to determine the photoelectron yields and surface equilibrium potentials of dust particles when exposed to UV radiation in the 120-250 micrometers spectral range. A brief discussion of the experimental procedure, the results of photoemission experiments, and comparisons with theoretical models will be presented.