  The nucleus of M82 has been mapped in several 3mm and 1mm lines of CN, HCN,
C2H, c-C3H2, CH3C2H, HC3N and HOC+ using the IRAM 30m telescope. These species
have been purposely selected as good tracers of photon-dominated chemistry. We
have measured [CN]/[HCN] ~ 5 in the inner 650 pc galaxy disk. Furthermore, we
have detected the HOC+ 1--0 line with an intensity similar to that of the
H13CO+ 1--0 line. This implies a [HCO+]/[HOC+] ratio of ~40. These results
corroborate the existence of a giant photo-dissociation region (PDR) in the
nucleus of M82. In fact, the low [HCO+]/[HOC+] ratio can only be explained if
the nucleus of M82 is formed by small (r<0.02-0.2 pc) and dense (n ~ a few
10^4--10^5 cm^{-3}) clouds immersed in an intense UV field (G_0 ~ 10^4 in units
of the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2H in
the nucleus of M82 suggests that a complex carbon chemistry is developing in
this giant PDR.
