In order to clarify the pump process in class II methanol masers
we intend to perform FP-observations of 4 torsional transitions
of methanol at 35,39,52,and 63 micron.The lines at 35 and 39 micron
are expected to be strong absorption lines,the measurement of which
will allow to estimate the column density of the methanol molecules
in the upper and lower level of the 12 GHz maser line.We intend to
combine the FP-observations of these lines with grating observations
of the stretching mode lines at 10 micron. The latter will give
information about the total column density of the methanol molecules
in the ground vibrational state and about the relative
excitation whithin the ground state.The lines at 52 and 63
micron are predicted to be in emission and may even turn out to be
iraser lines.If they are observed in emission against a bright
background continuum,this would be strong evidence for optical
pumping.- To clarify the pump process in some detail is also a
contribution to the determination of the physical parameters within the
maser region.

We have selected three targets -two for each launch date- which are
bright enough for FP-observations.Both belong to the strongest 12 GHz
masers.
The targets are W3(OH), NGC 6334 F (autumn only), and CepA (spring
only).