The formation of dust particles in argon diluted C2H2 plasmas was studied by means of Fourier transform infrared absorption spectroscopy and mass spectroscopy. The detection limit for infrared absorption was significantly improved by the use of a multipass technique. Measuring the intensity of the Rayleigh/Mie scattering of the infrared signal we found a periodicity of dust formation/vanishing (period of about 35 min in our experimental conditions). The fast disappearance of the dust from the plasma region at the end of every period is the evidence of a narrow particle size distribution, as confirmed by secondary electron micrographs of the collected powder. Characteristic infrared absorption features have their origin in absorption within the dust particles. Besides the strong presence of aliphatic hydrocarbons characteristic for amorphous hydrocarbon films, a significant amount of aromatic structures was detected. Heavy positive ions measured by ion-mass spectroscopy originate from polyacetilenic (C2nH2) and aromatic compounds. Time resolved mass spectra gave insight into the plasma response to the dust formation.

Different diagnostic techniques are used to monitor the dynamics of electrons and Ar * (1s 5 ) metastable atoms in the active plasma phase and in the afterglow of a capacitively coupled radiofrequency (RF) discharge operated in different gas mixtures and at different input powers. Diode laser absorption at 772.38 nm is used to measure the time resolved density of Ar * (1s 5 ) atoms in either continuous-wave mode or pulsed RF discharges with 100 Hz pulsing frequency. Simultaneously, microwave interferometry recorded the time dependence of the electron density. Different plasma conditions, namely: (1) pure argon, (2) argon +5.9% acetylene before nanoparticle formation, (3) argon +5.9% acetylene after dust particles have been formed and (4) argon with dust particles remaining in the plasma volume but without acetylene are studied. The measured steady-state Ar * (1s 5 ) density in the middle of the reactor is several times larger in the dusty argon plasma than in the pure argon discharge for the same discharge powers. At the same time, the electron density is several times less in the dusty plasma. These changes are caused by dust formation: the electric field in the bulk plasma is enhanced and thus consequently the electron temperature increases. Laser induced fluorescence (LIF) is used to measure the time and space resolved Ar * (1s 5 ) axial distribution. In the pure argon discharge, the axial Ar * (1s 5 ) metastable distribution has a characteristic saddle-like shape with maxima in the region of the sheaths. With dust particles inside, the axial distribution changes dramatically with the maximum at the discharge mid-plane, revealing an α-γ′ transition. The spatial distribution and absolute density of metastable atoms are influenced by the formation of a void in the cloud of nanoparticles. Depending on the size of the void, the Ar * (1s 5 ) density reduction inside the void is between 30% and 50%. The high Ar * (1s 5 ) metastable density in the dusty plasma afterglow strongly influences the time variation of the electron density in the afterglow. The observed increase of the electron density in the afterglow of the Ar/acetylene/dust plasmas is explained by the Penning ionization of acetylene by Ar * (1s 5 ) metastable atoms. The time evolution of the electron density in the Ar/dust plasmas reveals that the nowadays assumed rate coefficient for generation of electrons by pooling reaction of Ar * metastable atoms is highly overestimated.

Reactive plasmas are nowadays widely used for technological applications. The spontaneous formation and growth of dust is a phenomenon frequently observed in such plasmas. The formation of dust particles has been observed in a great variety of different discharge types and in different kind of gases or gas mixtures. Due to the large variety of different phenomena that can be observed in reactive complex plasmas this article will address some selected (general) problems and examples that are specific for the physics and chemistry of such systems. These examples concern the formation and growth of dust particles in reactive plasmas, the mechanisms responsible for that growth processes, the spatial distribution of the dust particles within the discharge, the response of the plasma to the formation and growth of dust particles and some technological aspects.

Zero-dimensional, space-averaged global models of argon dust-free and dusty afterglow plasmas are developed, which describe the time behaviour of electron n e(t) and Ar* metastable n m(t) densities. The theoretical description is based on the assumption that the free electron density is smaller than the dust charge density. In pure argon, fairly good agreement with the experimentally measured densities and their decay times in the afterglow is obtained when the electron energy loss term to the chamber walls is included in the electron energy balance equation. In dusty plasma afterglow, the agreement between theory and experiment is less satisfactory. The calculated metastable density is 3 times smaller than the measured one and the electron decay is much faster in the late afterglows. The difference should probably arise from the assumption that the electron energy distribution function is Maxwellian. Different sources of secondary electrons in the dusty plasma afterglow are analysed. Comparison of the model with experimental results of argon dusty plasma suggests that the metastable pooling could be the source of the experimentally observed electron density increase in the early afterglow but electron generation from metastable–dust interactions cannot be fully discarded.

Carbonaceous compounds are a significant component of interstellar dust, and the composition and structure of such materials is therefore of key importance. We present 1.5-15 m spectra of a plasma-polymerized carbonaceous material produced in radio-frequency discharge under low pressure, using C 2 H 2 as a precursor component. The infrared spectra of the resulting spheroidal carbonaceous nanoparticles reveal a strong aliphatic band (3.4 m feature), weak OH and carbonyl bands, and traces of aromatic compounds, all characteristics identified with dust in the diffuse interstellar medium of our Galaxy. The plasma polymerization process described here provides a convenient way to make carbonaceous interstellar dust analogs under controlled conditions and to compare their characteristics with astronomical observations. Here we focus on a comparison with the IR spectra of interstellar dust. The IR spectrum of carbonaceous dust in the diffuse interstellar medium is characterized by a strong 3.4 m CÀH stretching band and weak 6.8 and 7.2 m CÀH bending bands, with little evidence for the presence of oxygen in the form of carbonyl (C = O) or hydroxide (OH) groups. The plasma polymerization products produced under oxygen-poor conditions compare well with the peak position and profiles of the observed IR spectrum of diffuse dust. In addition, we find that addition of nitrogen to the plasma results in bands at 6.15 m (C = N band) and at 3 m (NH band). We note that, with the addition of nitrogen, the 3.4 m hydrocarbon band diminishes greatly in strength as the NH band grows. This may have implications for the puzzling absence of the 3.4 m hydrocarbon bands in the IR spectra of dust in dense molecular clouds, given that the presence of nitrogen-related bands has been established in dense-cloud dust.