Advantages of the Photon Detection Method for Beams Measurements of Electron Impact Excitation Rates

Beams experiments are well suited to measurements of electron impact excitation (EIE) cross sections for specific transitions of positive ions. Excitation events can be determined directly by detecting either the emitted photon following excitation and radiative decay or by detecting the inelastically scattered electron. The photon method has been used at Harvard-Smithsonian to measure absolute cross sections involved in the production of C II 133.5 nm, C III 117.6 nm, C IV 155.0 nm, and Si III 120.6 nm. The experiments allow the cross sections to be determined completely from measured quantities such as the distribution of electrons and ions in the beams, the beam energy, the photon emission rate and the ground state and metastable state population fractions (when required). Typical standard uncertainties are +/- 9 percent. Population fractions are determined by two methods: 1) the beams attenuation method, which relies on differences between the ground state and metastable state charge transfer cross sections with a selected gas target, and 2) measurement of the radiative decay intensity from metastable ions in the incident beam. Collision energies from threshold to well above threshold are accessable allowing the determination of empirical electron collision rates for both maxwellian and non-maxwellian electron velocity distributions. Recent results, comparison to theoretical values, astrophysical applications, the experimental method and the apparatus, including a new 5 GHz Electron Cyclotron Resonance (ECR) ion source, will be described. This work was supported by NASA Grant NAG5-12863 to the Smithsonian Astrophysical Observatory.