Absolute-rate coefficient for C3+(2s→2p) electron-impact excitation

摘要

We have measured the absolute-rate coefficient for electron-impact excitation (EIE) of C3+(2s 2S1/2→2p 2P1/2,3/2) for energies near threshold. A delayed coincidence technique with inclined beams was used. Radiation from the excited ions was detected using an optical system that subtends slightly over π steradians. At an ion-rest-frame energy of 10.10 eV the measured rate coefficient was (7.79±2.10)×10-8 cm3 s-1 and the measured cross section was (4.15±1.12)×10-16 cm2. The uncertainties quoted here represent the total experimental uncertainty, statistical and systematic, at a confidence level considered to be equivalent to a statistical 90% confidence level. Good agreement is found with other measurements. Agreement is not good with Coulomb-Born with exchange and two-state close-coupling calculations, both of which fall outside our 90% confidence limits. Nine-state close-coupling calculations are in better agreement. However, the calculations lie at the extreme edge of our uncertainty limits. Taking into account previous measurements of EIE in C3+ and also a measurement of EIE in Li-like Be+ that falls significantly below theory at threshold, there is a suggestion that the C3+(2s→2p) EIE rate coefficient near threshold may fall slightly below presently accepted values.