The nucleus of M82 has been mapped in several 3 and 1 mm lines of CN, HCN, C_2H, c-C_3H_2, CH_3C_2H, HC_3N, and HOC~+ using the IRAM 30 m 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 H~(13)CO~+ 1 → 0 line. This implies an [HCO~+]/[HOC~+] ratio of ～40. These results corroborate the existence of a giant photodissociation 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 times 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-C_3H_2 and CH_3C_2H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.
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