  The structure and photoexcitation dynamics of high lying doubly excited
states of the strontium atom with high angular momenta are studied in the
vicinity of the Sr$^+(N=5)$ threshold. The spectra recorded using resonant
multiphoton isolated core excitation are analyzed with calculations based on
configuration interaction with exterior complex scaling, which treats the
correlated motion of the two valence electrons of Sr from first principles. The
results are rationalized with a model based on multichannel quantum-defect
theory and transition dipole moments calculated with a perturbative treatment
of electron correlations. Together, both approaches reveal that most of the
lines observed in the spectra arise from the interaction of a single optically
active state, coupled to the initial state by an electric-dipole transition,
with entire doubly-excited Rydberg series. The long-range electron correlations
responsible for this interaction unexpectedly vanish for identical values of
the initial and final principal quantum numbers, a fact related to the quasi
hydrogenic nature of the high-$l$ Rydberg electron. This special situation, and
in particular the vanishing interaction, leads to the surprising observation of
an electric \emph{quadrupole} isolated-core excitation with a similar intensity
as the neighboring electric dipole transitions.
