A technique for absolute measurements in near threshold excitation of positive ions: application to excitation of

Abstract

An electron-ion crossed beams experiment has been developed in which the energy loss of scattered electrons is used to determine cross sections for excitation in positive ions. Scattered electrons are confined by an axial magnetic field and are channelled into a detector using reflecting potentials and an (trochoidal) deflection. Complete collection of these electrons along with monitoring of the overlap and current of the beams enables absolute total cross sections to be determined. This technique has been used to study near threshold excitation of the D resonance transition in . , CII, trochoidal spectrometer

Effective collision strengths for electron impact excitation of C II

“…5 we display the collision strength for the transition between two LS levels 2s 2 2p 2 P o -2s2p 2 2 D and compare with experimental data. Data from two experiments have been noted: the crossed beams experiment of Williams et al (1998) and the merged beams experiment of Smith et al (1996). Both these experiments measure the excitation cross section, and so to make comparison with the current data we have converted the cross sections to collision strengths by applying Eq.…”

“…The experiment of Williams et al (1998) studies the near threshold excitation of the 2s 2 2p 2 P o -2s2p 2 2 D transition and so comparison can only be made over a small incident electron energy range. The data of Smith et al (1996) were taken over a larger range of energies and so here we have more values with which to compare.…”

“…However, it is useful to compare with previous work, and for this purpose we will consider the effective collision strengths of Blum & Pradhan (1992), who performed a 10-state R-matrix calculation, resulting in data at a number of temperatures for 153 fine-structure transitions. We also compare with the crossed beams experimental results of Williams et al (1998) and the merged beams results of Smith et al (1996) for the 2s 2 2p 2 P o -2s2p 2 2 D excitation.…”

Microcontroller based double beam modulation system for atomic scattering experiments

“…Molyneux et al [5] have described an electronic unit for providing the necessary beam modulation and scaler gating pulses. More recent implementations of the double beam modulation scheme have been carried out by Dunn's group at JILA [6], Chutjian's group at JPL [7] and in our own laboratory in Belfast [8]. These schemes place the beam switching sequence under computer control and allow counts from each of the four channels to be recorded separately.…”

“…The measured signal rate is also critically dependent on pulse deadtimes [8], but this effect will be ignored here. The modulation frequency is determined by the vacuum time constant of the experimental chamber.…”