Electron Density Oscillations in CC-RF Oxygen Plasma Investigated by Gaussian Beam Microwave Interferometry
Gaussian beam microwave interferometry (160 GHz, beam waist of 5 mm) is applied to study the electron density in capacitively coupled radio-frequency (RF) oxygen plasma. The microwave interferometry provides immediately the line-integrated electron density without model assumptions. In the considered range of RF power and total pressure, the investigations have shown that the oxygen plasma is dominated by electron density oscillations due to the attachment-induced ionization instability. The frequency of the mostly nonharmonic oscillations ranges between 0.3 and 3 kHz with peak-to-peak line-integrated electron density variations up to 3.5 × 1015 m-2.
Attachment-induced ionization instability in electronegative capacitive RF discharges
Attachment-induced ionization instability has been experimentally observed in O2 and CF4 capacitive RF discharges using time-resolved voltage probe, Langmuir probe, optical emission and mass spectrometry measurements. This instability occurs under specific conditions of power and pressure, and produces synchronized oscillations in the kilohertz range on potentials, emission intensity and positive ion fluxes. In contrast, the SF6 plasma was observed to remain stable under all experimental conditions. This can be understood by considering attachment and ionization cross sections of these gases and applying the theoretical criterion of instability. Contrary to O2 and CF4, the attachment rate coefficient of SF6 is very high at low energy and has a negative dependence on the electronic temperature. The application of the criterion shows clearly that O2 and CF4 plasmas are unstable at low electronic temperature, and that the SF6 plasma is stable due to its particular low-energy attachment cross section.
ThAT is The qUesTion : CoMPAring The AdvAnTAges And disAdvAnTAges oF TesTiFying ACross siTUATions
Criminal defendants are under no obligation to testify on their own behalf and doing so can lead to deleterious outcomes. Specifically, if the defendant has a criminal record, the prosecution can introduce this into evidence in an effort to impeach the defendant’s testimony. Yet, failure to take the stand and proclaim one’s innocence arouses suspicion among jurors that may influence verdicts. In this study, we manipulated whether the defendant testified or not. In those instances where he did testify, we varied whether he had (1) no criminal record, (2) a criminal record for a similar offense, or (3) a criminal record for a dissimilar offense. Results indicated that mock jurors who adjudicated the defendant guilty were influenced by the defendant’s failure to take the stand. Furthermore, mock jurors were more likely to render guilty verdicts when they perceived the defendant as less trustworthy, more aggressive, and less credible (as a witness). However, in no instance were these decisions and perceptions related to the manipulations. This suggests that mock jurors were suspicious of the defendant regardless of whether or not he testified, or whether he had a criminal record.