The Herschel Space Observatory opens the sky for observations in the far infrared at high spectral and spatial resolution .
A particular class of molecules will be directly observable ; light diatomic hydrides and their ions ( CH , OH , SH , NH , CH ^ { + } , OH ^ { + } , SH ^ { + } , NH ^ { + } ) .
These simple constituents are important both for the chemical evolution of the region and as tracers of high-energy radiation .
If outflows of a forming star erode cavities in the envelope , protostellar far UV ( FUV ; 6 < E _ { \gamma } < 13.6 eV ) radiation may escape through such low-density regions .
Depending on the shape of the cavity , the FUV radiation then irradiates the quiescent envelope in the walls along the outflow .
The chemical composition in these outflow walls is altered by photoreactions and heating via FUV photons in a manner similar to photo dominated regions ( PDRs ) .

In this work , we study the effect of cavity shapes , outflow density , and of a disk with the two-dimensional chemical model of a high-mass young stellar object introduced in the second paper in this series .
The model has been extended with a self-consistent calculation of the dust temperature and a multi-zone escape probability method for the calculation of the molecular excitation and the prediction of line fluxes .

We find that the shape of the cavity is particularly important in the innermost part of the envelope , where the dust temperatures are high enough ( \gtrsim 100 K ) for water ice to evaporate .
If the cavity shape allows FUV radiation to penetrate this hot-core region , the abundance of FUV destroyed species ( e.g .
water ) is decreased .
On larger scales , the shape of the cavity is less important for the chemistry in the outflow wall .
In particular , diatomic hydrides and their ions CH ^ { + } , OH ^ { + } and NH ^ { + } are enhanced by many orders of magnitude in the outflow walls due to the combination of high gas temperatures and rapid photodissociation of more saturated species .
The enhancement of these diatomic hydrides is sufficient for a detection using the HIFI and PACS instruments onboard Herschel .
The effect of X-ray ionization on the chemistry is found to be small , due to the much larger luminosity in FUV bands compared to X-rays .