Collision Strength Measurement for
Electron-Impact Excitation of Neonlike Barium

Abstract

Relative collision strengths have been measured for electron-impact excitation to (2p_1/2^-13s)_J=1 and (2p_3/2^-13d_5/2)_J=1 in neonlike barium, whose wave functions are strongly mixed with each other. An experimental method has been developed for the collision strength measurement from X-ray observations by using the Tokyo electron beam ion trap. X-ray transitions from these excited states were observed with a flat crystal spectrometer. The electron energy was adjusted to 5.02 keV which is just above the threshold, and the electron energy width was narrowed to 16±8 eV by reducing the electron beam current. This allowed us to exclude indirect excitation followed by radiative cascades. The experimental collision strength ratio was found to be larger than the distorted-wave calculation by [Zhang and Sampson: At. Data Nucl. Data Tables 43 (1989) 1]. This discrepancy is probably attributed to the contribution of resonant excitation.