Multipole effects in the excitation of high-l doubly excited states in Sr

We report the observation of unexpected line strengths in the resonant multiphoton laser excitation of high-angular-momentum ${7d}_{5/2}{n}^{\prime }{l}^{\prime }$ and ${8p}_{3/2}{n}^{″}{l}^{″}{(n}^{″}\ne 16)$ states in Sr. The results differ from expected isolated-core excitation features usually explained on the basis of the multichannel-quantum-defect theory. In the present case, the observed line strengths and, in particular, the disappearance of single resonances can be explained by a configuration-interaction calculation, giving explicitly the multipole character of the correlation between the electrons. The spectra offer a sensitive way to test calculations involving electronic correlation.