C.A. DeJoseph

Application of excitation cross sections to optical plasma diagnostics

Many optical-based plasma diagnostic techniques require electron-impact excitation cross sections. In recent years, a considerable number of new results have become available for excitation of rare-gas atoms from both the ground state and metastable states. Using relatively simple techniques these cross sections can be combined with plasma emission measurements to extract many useful plasma parameters such as the electron temperature. Many of the limitations of simple plasma emission models such as the corona model can be overcome by using cross section measurements to select what particular emission lines to use in the analysis.

Time-resolved current and voltage measurements on a pulsed rf inductively coupled plasma

Time resolved current and voltage measurements have been made on a pulsed radio frequency (rf) inductively coupled plasma (ICP) at 13.56 MHz in argon. Measurements were made on the rf coil using a high-voltage probe, a Rogowski current probe, and a high-performance digital oscilloscope. Relative phase information was also obtained so that time resolved rf power measurements could be made. Due to the inductive nature of the load, measurement of the phase had to be better than 0.6 mrad at 13.56 MHz in order that the power measurements were accurate to 10%. This accuracy in phase measurement was achieved by careful positioning of the probes and by establishing accurate phase calibration procedures. The power was calculated by three methods: discrete Fourier transform, integral of the current voltage product over N periods, and least-squares fits of a sine wave to the measured data. Time-resolved measurements of the system complex impedance, power loss in the ICP planar coil, and the actual amount of rf power delivered to the plasma were made. These measurements give details during plasma breakdown and show the transition from capacitive to inductive discharge. The results are compared with both time-resolved plasma emission and time-resolved Langmuir probe measurements.

Large-area surface treatment by ion beam technique

Abstract A large-area ion beam deposition system has been used extensively for depositing diamond-like carbon films by a direct ion-beam process. An ultra high vacuum ion beam system, consisting of a 20-cm diameter RF excited (13.56 MHz) ion gun and a four-axis substrate scanner, has been used to modify large surfaces (up to 1000 cm 2 ) of various materials, including 304 and 316 stainless steel, 440C and M50 steels, aluminum, aluminum alloys, Ti-6Al-4V, silicon carbide, silicon nitride, polycarbonates, infrared windows and polycrystalline diamond, by depositing varying chemical compositions of diamond-like carbon films. The influence of ion energy, RF power, and gas composition (H 2 /CH 4 , Ar/CH 4 , O 2 /H 2 /CH 4 and N 2 /H 2 /CH 4 ), on the diamond-like carbon characteristics has been investigated. Particular attention was focused on the adhesion, environmental effects, coefficient of friction and wear factors of the diamond-like carbon films on the various substrates under space-like environments. A quadrupole mass spectrometer and a total ion-current measuring device have been utilized to monitor the ion compositions of the gas mixtures of CH 4 /H 2 , CH 4 /Ar, CH 4 /H 2 /O 2 and CH 4 /H 2 /N 2 during the deposition process for quality control and process optimization.

Formation and electron-ion recombination of N4+ following photoionization in near-atmospheric pressure N2

The time dependent behavior of molecular nitrogen ions has been investigated following pulsed photoionization of near atmospheric pressure N(2) using multiphoton laser techniques and kinetic modeling. Multiple fluorescence bands, some unreported previously, with various temporal behaviors were observed after ultraviolet laser photoionization of N(2)(X (1)Sigma(g)). The initial N(2) ionization was generated via resonance-enhanced multiphoton ionization with focused radiation in the 275-290 nm range, where several resonant transitions are accessible. The observed optical fluorescence bands appeared to be unique to the near-atmospheric pressure N(2) condition and were shown by the evidence in this work to be the result of collisional formation and recombination of N(4)(+). Measured time dependent fluorescence spectra during and after pulsed laser photoionization of N(2), together with a coupled rate equation model, allowed for the determination of the absolute densities of N(2)(+) and N(4)(+) as these species evolved.

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.

Spectroscopic study of a pulsed argon rf ICP discharge: stepwise excitation in the afterglow and its application in optical spectroscopy

It is shown experimentally in a noble-gas, pulsed rf ICP at low pressure (~20 mTorr) that stepwise excitation may cause a maximum in the time dependence of the optical emission during the power-free stage following the rf pulse. During this stage, fast electrons are created by chemi-ionization of metastable atoms and collisions of second kind between metastables and slow electrons. These fast electrons have energies much greater than the overall average electron energy, which is typically ~0.1 eV for the power-free stage. If the flux of fast electrons exceeds the ambipolar flux of ions to the chamber walls, a dramatic increase in the near-wall potential drop occurs, from a few tenths of a volt to several volts, and a corresponding increase in the near-wall electric fields. In this case, a fraction of the fast electrons are reflected by the near-wall potential drop and accumulate in the plasma volume, leading to an increase in density as a function of time. Eventually, the fast electrons have sufficient density to produce significant excitation of the metastable states, increasing with time and creating a maximum in the optical emission. In the opposite case, when the flux of fast electrons is less than the flux of ions to the wall, stepwise excitation in the afterglow is usually negligible. In this case, a maximum can be caused by recombination, but generally, only at higher gas pressures. These effects should be taken into account in spectroscopy of post-discharge plasmas of various types and may be useful for measurements of relative cross-sections for stepwise excitation of atoms and molecules by monoenergetic electrons. The latter is possible because the collisional energy relaxation time of the fast electrons at these pressures is on the order of milliseconds, leading to the existence of a near monoenergetic energy spectrum for a significant time.

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.

Ionization kinetics and E?H mode transition in a noble gas, low-pressure pulsed ICP discharge

Under some conditions of power modulation, an rf ICP will exhibit a rapid transition between E and H modes following application of the rf power. It is shown that in noble gases this transition may be connected with the dynamics of the electron density and metastable density of the atoms, and competition between direct and stepwise ionization of the atoms by electron impact. A simple model allows us to demonstrate that after application of rf power the initial slow growth of electron density changes to a rapidly rising function. This rapid rise is consistent with observed E–H transitions that take place in these discharges. The model thus allows us to calculate the characteristic time for the transition beteween modes.

Laser REMPI Triggering of an Air Spark Gap with Nanosecond Jitter

A laser-triggering scheme for air spark gap switches was conceived and investigated for its potential to reduce shot-to-short time jitter. The scheme utilizes a pulsed ultraviolet laser of relatively low energy to generate resonant enhanced multi-photon ionization (REMPI) within the atmospheric air medium of the spark gap switch. With an applied voltage below the self-breakdown level, the laser-induced pre-ionization initiated avalanche breakdown within the gap and the subsequent triggering of the switch. This laser induced pre-ionization process relied solely on gas phase ionization and not surface effects, since the laser does not strike either electrode. This triggering scheme produced sub-nanosecond jitter with low enough laser power that it could be transmitted through fiber optics, which would be advantageous for multi-switch triggering of a high current pulse. The laser pre-ionization effects of space charge, electric field distribution, and active species within the gap were analyzed for their role in driving electron multiplication leading to avalanche breakdown below the self-breakdown voltage. Experimental results will be presented, including arc timing and statistical jitter measurements, as well as optical images and spectral analysis of the arc emission.