  We present a method to measure rotational transitions of molecular anions in
the terahertz domain by sequential two-photon absorption. Ion excitation by
bound-bound terahertz absorption is probed by absorption in the visible on a
bound-free transition. The visible frequency is tuned to a state-selective
photodetachment transition of the excited anions. This provides a terahertz
action spectrum for just few hundred molecular ions. To demonstrate this we
measure the two lowest rotational transitions, J=1<-0 and J =2<-1 of OD- anions
in a cryogenic 22-pole trap. We obtain rotational transition frequencies of
598596.08(19) MHz for J=1<-0 and 1196791.57(27) MHz for J=2<-1 of OD-, in good
agreement with their only previous measurement. This two-photon scheme opens up
terahertz rovibrational spectroscopy for a range of molecular anions, in
particular for polyatomic and cluster anions.
