  Accurate parameters of molecular hot-band transitions, i.e., those starting
from vibrationally excited levels, are needed to accurately model
high-temperature spectra in astrophysics and combustion, yet laboratory spectra
measured at high temperatures are often unresolved and difficult to assign.
Optical-optical double-resonance (OODR) spectroscopy allows the measurement and
assignment of individual hot-band transitions from selectively pumped energy
levels without the need to heat the sample. However, previous demonstrations
lacked either sufficient resolution, spectral coverage, absorption sensitivity,
or frequency accuracy. Here we demonstrate OODR spectroscopy using a
cavity-enhanced frequency comb probe that combines all these advantages. We
detect and assign sub-Doppler transitions in the spectral range of the
3${\nu}$${_3}$${\leftarrow}$${\nu}$${_3}$ resonance of methane with frequency
precision and sensitivity more than an order of magnitude better than before.
This technique will provide high-accuracy data about excited states of a wide
range of molecules that is urgently needed for theoretical modeling of
high-temperature data and cannot be obtained using other methods.
