A. Garscadden

Overview of growth and behaviour of clusters and particles in plasmas

A review is presented of the phenomena associated with particles in low pressure plasmas. Dust particles which are typically micrometers in diameter have been observed by laser light scattering in various low-pressure, radiofrequency-excited plasmas. Experiments have been designed so that the origin of the dust material is unambiguous and, to some extent, quantitative. The processes involved in the appearance of the mesoscopic dust particles are outlined and compared with our experimental observations. The source material and its required generation rate, nucleation, charging, growth mechanisms, growth rates, and saturation mechanisms are discussed. The mutual influences of dust and plasma, particularly the role of geometric and circuit boundary conditions in laboratory plasmas, are described.

On form and flow in dusty plasmas

Microscopic cauliflowers have been observed in a surprisingly diverse range of dusty plasmas. Their microstructure, as analysed by electron microscopy, is consistent with growth by ballistic deposition rather than diffusion limited aggregation. The morphology of the grains supports the inference from dust growth kinetics that they form by accretion of positive ions rather than neutral radicals. The dense, amorphous structure is capped by a fractal surface whose texture is concisely described by a recursion based on the modified midpoint method. The surface texture may be reconstructed by ion bombardment, providing a quantitative link between growth kinetics and roughness through a Mullins-Sekerka stability analysis of Laplacian growth.

Electron impact ionization and ion reactions in n-butane

Absolute cross sections for electron impact ionization of n-butane (n-C4H10) as functions of electron energy from 10 to 200 eV have been measured by Fourier transform mass spectrometry. The major ions including the parent ion and eight fragment ions , and are observed, with the total cross section reaching a maximum of 1.18 × 10−15 cm2 at ~80 eV. Studies of reactions of the major ionization product ions with n-C4H10 have been carried out, showing that , and are unreactive while the remaining ions react readily forming as the common product ion by hydride transfer. The observed reactivities of and combined with the calculated reaction enthalpies for their hydride transfer suggest that these two ions have the structures of cyclo- and cyclo- , respectively.

A mass spectrometry study of n-octane: Electron impact ionization and ion-molecule reactions

Electron impact ionization of n-octane over an energy range of 10–70 eV and the subsequent ion-molecule reactions with the parent molecule have been studied using Fourier-transform mass spectrometry. Molecular ion and fragment ions C1+–C6+ are produced from the electron impact with a total ionization cross section of 1.4±0.2×10−15 cm2 between 60 and 70 eV. C3H7+ is the most abundant ion at most of the ionizing energies with the exception for E⩽16 eV where C6H13+ and C6H12+ are the most abundant. Among the fragment ions only C4H7+ and smaller ions react readily with the parent molecule, primarily producing C5H11+ and C4H9+, with rate coefficients of 0.32–2.4×10−9 cm3 s−1. Essentially all of the ions, including the molecular ion and the large fragment ions, undergo decomposition upon collision with neutral molecules after they are kinetically excited to an energy range of 1–5 eV, forming a variety of small hydrocarbon ions. Many of the decomposition product ions in turn are capable of further reacting with ...

Comparisons of electron impact ionization and ion chemistries of CF3Br and CF3I

Abstract Comparisons of the electron impact ionization and of the ion-molecule reactions for CF 3 Br and CF 3 I are made from a study of the two compounds using Fourier-transform mass spectrometry. The ionization of the compounds over the energy range from threshold to 70 eV produces primarily the molecular ion and 6 fragment ions, with the dominant ion from CF 3 Br being CF 3 + and, from CF 3 I, CF 3 I + . The total cross sections at 70 eV are 8.3 ± 0.8 and 9.0 ± 0.9 × 10 −16 cm 2 for CF 3 Br and CF 3 I, respectively. These results appear to be the first for the molecules. The ion-molecule reactions in the two compounds are similar, with CF + and X + (X = Br or I) being the most reactive ions (k ∼ (8–13) × 10 −10 cm 3 s −1 ). Ar + reactions with the two compounds are also studied. Results of our study on the ion kinetics are compared with those from previous studies by other groups.

Influence of dielectric barrier discharges on low Mach number shock waves at low to medium pressures

For shock wave propagation in nonequilibrium plasmas, it has been shown that when the electron Debye length exceeds the shock wave discontinuity dimension, strong double layers are generated, propagating with the shock wave. Strong double layer formation leads to the enhancement of the local excitation, ionization, and local neutral gas heating which increases the shock wave velocity. It is shown that dielectric barrier discharges (DBD) in pure N2 also increase the shock wave velocity and broaden the shock wave. The DBD is considerably more energy efficient in producing these effects compared to a dc glow discharge and can operate over a wide pressure range. It is shown that these effects are also operative in the pure N2 discharge afterglow, allowing a wide range of pulse repetition frequencies.

Ionization of 2,5-dimethylfuran by electron impact and resulting ion-parent molecule reactions

2,5-dimethylfuran (C6H8O) is an important fuel additive and a possible renewable liquid fuel for the future. This paper presents a recent Fourier transfer mass spectrometry study on the formation of ions from C6H8O by electron impact ionization and by ion-molecule reactions. Cross sections of the partial electron impact ionization have been measured and the pathways of major fragmentation channels of the parent ion have been examined. The kinetics of the reactions of C6H8O with selected product ions from electron impact and Ar+ have been studied.

Gas-phase ion chemistries in perfluoromethylcyclohexane

Electron impact ionization and ion–molecule reactions of perfluoromethylcyclohexane (C7F14) using Fourier-transfer mass spectrometry are reported. The electron impact ionization produces dominant ions and throughout most of the energy range of 10–200 eV, with the total ionization cross section peaking at ~100 eV with a value of 1.8 × 10−15 cm2. Numerous ions are observed at energies within a few electron volts of the lowest ionization threshold: , , , , , and . The lighter ions CF+, and are found to react with C7F14 forming and other product ions that we believe to result from further fragmentation of the intermediate ion . The charge transfer reaction between Ar+ and C4F14 yields similar product ions.

Mass spectrometry study of decomposition of exo-tetrahydrodicyclopentadiene by low-power, low-pressure rf plasma

The plasma cracking of exo-tetrahydrodicyclopentadiene (JP-10) (C10H16) is investigated using a quadrupole mass spectrometer. The relative densities of the JP-10 molecule and its principal decomposition products, including H2, are determined for varying rf powers in the range of 3–30 W, using the measured ion intensities combined with ionization cross section data from the literature. The extent of the cracking of JP-10 and the formation of H2 as functions of the rf power are discussed.

Ionization of octafluorocyclopentene, c-C5F8

The ion chemistry in octafluorocyclopentene (c-C5F8) is examined by Fourier-transform mass spectrometry under single-collision conditions in the 10−7 Torr pressure range. Absolute total and partial cross-sections of electron impact ionization on c-C5F8 are measured as a function of the electron energy in the range 10–200 eV. The molecular ion and 11 fragment ions are observed with a total cross-section of 1.0 × 10−15 cm2 at ~70 eV. The ion population at low energies (< 40 eV) is dominated by , , , and . Ar+ charge transfer reaction with c-C5F8 produces the same important ions except the parent ion . All ions from c-C5F8 are non-reactive with the parent gas molecule, except CF+ and , which react to form .