  A new millimeter- to millimeter-wave double-modulation double-resonance
(MMW-MMW DM-DR) scheme has been applied to record spectra of two astronomically
relevant complex organic molecules (COMs), propanal (C2H5CHO) and ethyl cyanide
(C2H5CN), to demonstrate advantages of the DM-DR experimental technique. The DR
technique helps to identify target transitions in a forest of lines and the
implementation of a DM procedure (modulation of the pump and probe source)
allows for confusion- and baseline-free spectra containing only the line(s) of
interest. In particular the unambiguous assignment of weak and blended
transitions in very dense MMW spectra is highlighted. Details of the observed
Autler-Townes line splitting and possible future applications, such as
automated analyses and adaptions of DM-DR methods to other experimental setups,
are discussed.
