  We describe in this Letter how inhomogeneous line broadening affects the
Autler-Townes (AT) splitting in a three level open molecular cascade system.
For moderate Rabi frequencies in the range of 300 to 500 MHz the fluorescence
line shape from the uppermost level |3> in this system depends strongly on the
frequency ratio of the two laser fields. However, the fluorescence spectrum of
the intermediate level |2> appears as expected. We provide a description of the
conditions for optimally resolved AT splitting in terms of the probe
laser/coupling field frequency ratio and laser propagation geometry based on
our theoretical analysis of the Doppler integral. This is important for
applications such as molecular angular momentum alignment as well as for the
measurement of the transition dipole moment matrix element.
