Optical signatures of the charge of a dielectric particle in a plasma

Abstract

With an eye on dust particles immersed into an ionized gas, we study the effect of a negative charge on the scattering of light by a dielectric particle with a strong transverse optical phonon resonance in the dielectric constant. Surplus electrons alter the scattering behavior of the particle by their phonon-limited conductivity in the surface layer (negative electron affinity) or in the bulk of the particle (positive electron affinity). We identify a charge-dependent increase of the extinction efficiency for low frequencies, a shift of the extinction resonance above the transverse optical phonon frequency, and a rapid variation of the polarization angles over this resonance. These effects could be used for noninvasive optical measurements of the charge of the particle.

Electron kinetics at the plasma interface

“…We found light scattering by particles made out of dielectric materials featuring transverse optical phonons, for instance, MgO, CaO, Al 2 O 3 to be very charge sensitive in the vicinity of anomalous optical resonances [43,44], defined by ε (ω) < 0 and ε (ω) 1, where ε(ω) = ε (ω)+iε (ω) is the complex dielectric function of the material [77,78]. The resonances are in the infrared and shift with increasing charge towards higher wave numbers.…”

“…In an effort to establish a method for measuring the charge of dust grains in a plasma optically we investigated Mie scattering by charged dielectric particles. Initially we considered homogeneous particles with negative and positive electron affinity [43,44] but later we also investigated coreshell particles, where an electro-negative core is coated by an electro-positive film [42]. The idea was based on the observation that the particle charge modifies either the boundary condition for the electromagnetic field (particles with negative electron affinity) or the bulk dielectric function (particles with positive electron affinity).…”

Optical signatures of electrically charged particles: Fundamental problems and solutions

“…Basically, the RHS is a very complicated function of the coordinates φ and ϑ (see Eqs. (11), (13)- (14) and Eqs. (15)- (16)).…”

“…At present, the optical effects due to excess charges on a small particle or on a collection of small particles are largely unknown; however, they are of high importance for new technologies [8][9][10], for correct understanding of many physical processes [11], and for the revelation of new physical phenomena [12,13]. Specifically, the interstellar extinction features that are notoriously associated with material composition also may be affected by electric charge that acts as a modulator of the optical properties of small particles.…”

“…The surface conductivity can involve various physical processes, such as the movement of unbounded charge on metallic or semi-metallic surfaces, passing the excess electrons into the conductive band of an isolant [11] or flow of bounded electrons in the polarizable states on the particle surface.…”