  We investigate electromagnetically induced transparency (EIT) and
Autler-Townes (AT) splitting in Rydberg rubidium atoms for a six-level
excitation scheme. In this six-level system, one radio-frequency field
simultaneously couples to two high-laying Rydberg states and results in
interesting atomic spectra observed in the EIT lines. We present experimental
results for several excitation parameters. We also present two theoretical
models for this atomic system, where these two models capture different aspects
of the observed spectra. One is a six-level model used to predict dominant
spectral features and the other a more complex eight-level model used to
predict the full characteristics of this system. Both models shows very good
agreement with the experimental data.
