Electron-impact excitation of the(…6s5d1D2)to(…6s6p1P1)transition in barium

Abstract

We have measured and calculated scattering parameters (differential cross sections and four electron-impact coherence parameters) for electron-impact excitation of the $(\dots{}{6s5d}^{1}{D}_{2})$ to $(\dots{}{6s6p}^{1}{P}_{1})$ transition in barium in the low to intermediate impact energy regime (10, 20, and 40 eV). Measurements were carried out by the superelastic scattering technique in which collisional deexcitation from the laser-excited $(\dots{}{6s6p}^{1}{P}_{1})$ state to the metastable $(\dots{}{6s5d}^{1}{D}_{2})$ level was detected by electron-energy-loss spectroscopy in a crossed-beam apparatus. Supporting calculations were carried out in the convergent close-coupling scheme. Measured differential cross sections were significantly lower than those measured for $(\dots{}{6s6p}^{1}{P}_{1})$ excitation out of the ground $(\dots{}{6s}^{21}{S}_{0})$ state and good agreement with theory was found. Measured and calculated coherence parameters show strikingly different behavior from that associated with ${}^{1}{P}_{1}$ excitations out of the ground state. Agreement between measured and calculated coherence parameters was found to be less satisfactory than in the case of the differential cross sections but was, nevertheless, encouraging considering the weak nature of the transition.