In  the  past ten years many long carbon chains have been detected in
the  ISM and the CSM. Cyanopolyynes as long as HC7N, HC9N and HC11N are
abundant  molecules  in sources like TMC1 and IRC+10216 and long carbon
chain  radicals  have  been  detected in the same sources through their
pure  rotational  spectrum  at  radio  wavelengths. The relatively slow
decrease  in  the  abundance  of  the  higher  order chains in TMC1 and
IRC+10216  suggests  that  they are produced very efficiently. However,
while  the  abundance  of  the  long  and polar carbon chains have been
derived  in  the ISM and the CSM, many light molecules necessary to the
formation  and  synthesis  of  such  chains have not yet been detected,
mainly  due  to their lack of permanent dipole moment and hence of pure
rotational transitions.

  C3 and C4H2 are important molecular species for the synthesis and the
growth  of  long carbon chains and probably they also play an important
role    in   the  formation  of  the  first  cyclic  carbon  molecules.
Consequenly, a determination of their abundance in the interstellar and
circumstellar  medium is of a great importance for our understanding of
the  interstellar  and circumstellar chemistry. Both molecules have not
permanent  dipole  moment  and  hence  they dont have a pure rotational
spectrum.  They  can  be  only  detected  in  the  infrared through the
emission or absorption of their infrared active bands.

  We  propose to observe the v2 bending mode of C3 (at 63 cm-1), the v8
and v9 bending modes of C4H2, and the v11 and v13 bending modes of C6H2
towards  several  galactic  molecular  clouds  and  a  few prototypical
circumstellar  envelopes.  The  select  clouds  cover  a large range of
physical  conditions  allowing  to detect C3 in emission in the shocked
regions  of  recent  low-mass  star formation or in absorption/emission
towards strong continuum sources at ~150 um.