  Detecting the internal state of polar molecules is a substantial challenge
when standard techniques such as resonance-enhanced multi photon ionization
(REMPI) or laser-induced fluorescense (LIF) do not work. As this is the case
for most polyatomic molecule species, we here investigate an alternative based
on state selective removal of molecules from an electrically trapped ensemble.
Specifically, we deplete molecules by driving rotational and/or vibrational
transitions to untrapped states. Fully resolving the rotational state with this
method can be a considerable challenge as the frequency differences between
various transitions is easily substantially less than the Stark broadening in
an electric trap. However, making use of a unique trap design providing
homogeneous fields in a large fraction of the trap volume, we successfully
discriminate all rotational quantum numbers, including the rotational
M-substate.
