  We present a combined experimental and theoretical study of the effects of
Rydberg interactions on Autler-Townes spectra of ultracold gases of atomic
strontium. Realizing two-photon Rydberg excitation via a long-lived triplet
state allows us to probe the thus far unexplored regime where Rydberg state
decay presents the dominant decoherence mechanism. The effects of Rydberg
interactions are observed in shifts, asymmetries, and broadening of the
measured atom-loss spectra. The experiment is analyzed within a one-body
density matrix approach, accounting for interaction-induced level shifts and
dephasing through nonlinear terms that approximately incorporate correlations
due to the Rydberg blockade. This description yields good agreement with our
experimental observations for short excitation times. For longer excitation
times, the loss spectrum is altered qualitatively, suggesting additional
dephasing mechanisms beyond the standard blockade mechanism based on pure van
der Waals interactions.
