G. Hinojosa

Test of Blanc's law for negative ion mobility in mixtures of SF6 with N2, O2 and air

We have measured the mobility of negative ion species drifting in mixtures of SF6 with N2, O2 and air. The pulsed Townsend experiment was used for this purpose. The conditions of the experiment, high pressures and low values of the reduced electric field, E/N, ensured that the majority species drifting in the gap was , to which the present mobilities are ascribed. The extrapolated, zero field mobilities for several mixture compositions were used to test them successfully with Blancs law. Moreover, the measured zero field mobilities in air could also be explained in terms of the measured mobilities for this ionic species in N2 and O2.

Polar dissociation of vibrationally de-excited H3+ and D3+ in collision with He

A discharge ion source under high pressure was used to produce de-excited H3+ and D3+ beams. The energy spectra of the positive and negative ion fragments were studied in coincidence over the energy range 5.5-9.8 keV, when the molecular ion beams, in their ground state, are excited, they undergo polar dissociation in collisions with an He target. Accurate energy distributions of the ionic fragments were obtained by transforming the measured laboratory spectra to the projectile centre-of-mass (CM) frame of reference. From the transformation to the CM frame, the energy shared by the positive (H+,D+) and negative (H-,D-) fragments was evaluated; it was found that the energy taken by the positive fragments was about 25 times larger than that of the corresponding negative fragments, and it is shared equally by the positive fragments. The energy threshold for the positive fragments resulting from the polar dissociation channel is in agreement with that derived from the potential energy curves for H3+ with D3h symmetry for transitions from v = 0 in the Franck-Condon region.

Electron detachment cross sections of colliding with O2 and N2 below 10 keV energies

Negative methane is an interesting molecular anion from a fundamental point of view and also because it can be a source of thermal electrons in low temperature hydrocarbon plasma. Measurements of collisional electron detachment from methane anions colliding with O2 and N2 in the energy range of 0.3 to 10.0 keV are presented. Single electron detachment was measured with the signal grow-rate method and total electron detachment was measured with the beam attenuation technique. Both techniques were employed in order to study the process of electron detachment over the present energy range. The validity of the geometrical model is discussed.

Relaxation effects in ionic mobility and cluster formation: negative ions in SF6 at high pressures

The relaxation effects of the ionic mobility and the formation of negative-ion clusters in SF6 are studied in this work. For this purpose, we have measured the mobility of negative ions in SF6 over the pressure range 100–800u2009Torr at a fixed value of density-normalized electric field, E/N, of 20u2009Td (1u2009Townsend = 10−17u2009Vu2009cm2). The data obtained show a clear dependence of the negative-ion drift velocity on drift distance. It is observed that the drift velocity (mobility) reaches a steady-state value only for drift distances above 2u2009cm, over the studied pressure range. In addition to this, we have observed that the ionic mobility depends strongly on the gas pressure. An explanation of this dependence of the ionic mobility on gas pressure is given in terms of a negative-ion clustering formation process. It was found that the assumption of a linear dependence of the cluster ion mass on pressure provides a satisfactory explanation for the observed mobilities.

Generation of an H3+(v = 0) beam and its diagnosis from the observation of its polar dissociation in collision with He

Abstract Ground state H 3 + ions in their lowest vibrational level were produced, and the initial vibrational state of the H 3 + ions was qualitatively observed as a function of the source pressure. Quantitatively, the polar dissociation channel was studied in order to verify the initial vibrational state of the ions. The energy distributions of H + and H − , generated from the collisional dissociation of H 3 + in He at 9 keV of projectile energy, resulting from the polar dissociating channel, were measured in coincidence. The good agreement between the center of mass energy threshold of the proton distributions from the polar dissociation process and the theoretical potential curves for the adiabatic Born-Oppenheimer energies of the H 3 + ions confirms the vibrational cooling of the H 3 + beam by operating the ion source at high pressure. We conclude that the process is effectively originated from H 3 + ( v =0).

Time-resolved study on negative ion motion in SF/sub 6/ at high pressures

We have used a pulsed Townsend technique to measure the drift velocity of two negative ion species in SF/sub 6/ at low values of the density-normalized electric field intensity E/N, from 2.3 to 100 Td (1 Townsend = 10/sup -17/ V cm/sup 2/), and very high pressures in the range 50-720 torr. Within experimental uncertainties of between 2-3%, we could group the observed mass-unidentified ionic reduced mobilities near the value of 0.48 cm/sup 2/ V/sup -1/ s/sup -1/ and 0.42 cm/sup 2/ V/sup -1/ s/sup -1/. This finding is consistent with the mobility ascribed to the clusters SF/sub 6//sup -/SF/sub 6/ and SF/sub 6//sup -/(SF/sub 6/)/sub 2/, respectively. These ions are assumed to be formed by ion-molecule reactions between SF/sub 6//sup -/ and the gas neutrals. Our findings may be of relevance for the modelling of high pressure SF/sub 6/ discharges, like those taking place in circuit breakers.

H3+ and D3+ ground state beams production with a filament-type ion source

H3+ and D3+ molecular ions in their ground state have been produced by using a filament-type ion source. This was achieved by increasing the ion source pressure up to 0.82 Torr. These molecular ion beams were used in collision with He to study the polar dissociation of H3+ and D3+. The polar dissociating channel for H3+ with He was used to qualitatively verify the internal vibrational energy of the initial H3+ ions by observing the protons in coincidence with H− fragments. Variation of the source pressure influenced the vibrational population of the initial H3+ ions, and this was reflected in the distribution of the coincident H+ and H− fragments resulting from H3+ ions in their ground state.