  We aim to investigate the chemistry of internal photon-dominated regions
surrounding deeply embedded hypercompact and ultracompact HII regions. We
search for specific tracers of this evolutionary stage of massive star
formation that can be detected with current astronomical facilities. We modeled
hot cores with embedded HC/UCHII regions, by coupling the astrochemical code
Saptarsy to a radiative transfer framework obtaining the spatio-temporal
evolution of abundances as well as time-dependent synthetic spectra. In these
models where we focused on the internal PDR surrounding the HI region, the gas
temperature is set to the dust temperature and we do not include dynamics thus
the density structure is fixed. We compared this to hot molecular core models
and studied the effect on the chemistry of the radiation field which is
included in the HII region models only during the computation of abundances. In
addition, we investigated the chemical evolution of the gas surrounding HII
regions with models of different densities at the ionization front, different
sizes of the ionized cavity and different initial abundances. We obtain the
time evolution of synthetic spectra for a dozen of selected species as well as
ratios of their integrated intensities. We find that some molecules such as C,
N2H+, CN, and HCO do not trace the inner core and so are not good tracers to
distinguish the HII/PDR regions to the HMCs phase. On the contrary, C+ and O
trace the internal PDRs, in the two models starting with different initial
abundances, but are unfortunately currently unobservable with the current
achievable spatial resolution because of the very thin internal PDR (r < 100
AU). In addition, we find that the abundance profiles are highly affected by
the choice of the initial abundances, hence the importance to properly define
them.
