Measurements of Collisional-Excitation Rate Coefficients for Berylliumlike Ions

Electron collision-excitation rate coefficients have been determined experimentally for excited singlet and triplet ($(n=2,3, \mathrm{and} 4$) levels of the berylliumlike ions N IV, O V, Ne VII, and Si IX using various plasma cases produced in a $\theta$-pinch device. Line intensities of these ions were measured absolutely and converted into rate coefficients by knowledge of the pertinent plasma parameters. Electron density and temperature were obtained as function of radius and time from the spectrum of scattered laser light. The intercombination transition $2s2p{}_{}{}^3{}_{}{}^{}P\to 2s^2{}_{}{}^1{}_{}{}^{}S$ was observed, and its absolute intensity allowed determining the metastable triplet level population. The probable error of any individual rate coefficient is estimated to be a factor of 2.