  We report the first detection of PDR molecular tracers, namely HOC+, and CO+,
and confirm the detection of the also PDR tracer HCO towards the starburst
galaxy NGC 253, claimed to be mainly dominated by shock heating and in an
earlier stage of evolution than M 82, the prototypical extragalactic PDR. Our
CO+ detection suffers from significant blending to a group of transitions of
13CH3OH, tentatively detected for the first time in the extragalactic
interstellar medium. These species are efficiently formed in the highly UV
irradiated outer layers of molecular clouds, as observed in the late stage
nuclear starburst in M 82. The molecular abundance ratios we derive for these
molecules are very similar to those found in M 82. This strongly supports the
idea that these molecules are tracing the PDR component associated with the
starburst in the nuclear region of NGC 253. A comparison with the predictions
of chemical models for PDRs shows that the observed molecular ratios are
tracing the outer layers of UV illuminated clouds up to two magnitudes of
visual extinction. Chemical models, which include grain formation and
photodissociation of HNCO, support the scenario of a photo-dominated chemistry
as an explanation to the abundances of the observed species. From this
comparison we conclude that the molecular clouds in NGC 253 are more massive
and with larger column densities than those in M 82, as expected from the
evolutionary stage of the starbursts in both galaxies.
