  The chemistry along the mass loss of Hot Jupiters is generally considered to
be simple, consisting mainly of atoms, prevented from forming more complex
species by the intense radiation field from their host stars. In order to probe
the region where the temperature is low (T < 2000 K), we developed a 1D
chemical and photochemical reaction model of the atmospheric mass loss of HD
209458 b, involving 56 species, including carbon chain and oxygen bearing ones,
interacting through 566 reactions. The simulation results indicate that simple
molecules like OH+, H2O+ and H3O+ are formed inside the region, considering
that residual H2 survives in the exosphere, a possibility indicated by recent
observational work. The molecules are formed and destroyed within a radial
distance of less than 10^7 km, but the estimated integrated column density of
OH+, a potential tracer of H2, is high enough to allow detection, which, once
achieved, would indicate a revision of chemical models of the upper atmosphere
of Hot Jupiters. For low density Hot Jupiters receiving less intense XUV
radiation from their host stars than HD 209458 b, molecular species could
conceivably be formed with a higher total column density.
