  Electromagnetically induced transparency (EIT) is a well-known phenomenon due
in part to its applicability to quantum devices such as quantum memories and
quantum gates. EIT is commonly modeled with a three-level lambda system due to
the simplicity of the calculations. However, this simplified model does not
capture all the physics of EIT experiments with real atoms. We present a
theoretical study of the effect of two closely-spaced excited states on EIT and
off-resonance Raman transitions. We find that the coherent interaction of the
fields with two excited states whose separation is smaller than their Doppler
broadened linewidth can enhance the EIT transmission and broaden the width of
the EIT peak. However, a shift of the two-photon resonance frequency for
systems with transitions of unequal dipole strengths leads to a reduction of
the maximum transparency that can be achieved when Doppler broadening is taken
into account even under ideal conditions of no decoherence. As a result,
complete transparency cannot be achieved in a vapor cell. Only when the
separation between the two excited states is of the order of the Doppler width
or larger can complete transparency be recovered. In addition, we show that
off-resonance Raman absorption is enhanced and its resonance frequency is
shifted. Finally, we present experimental EIT measurements on the D1 line of
$^{85}$Rb that agree with the theoretical predictions when the interaction of
the fields with the four levels is taken into account.
