ISOSWS-LWS observations of the prototypical reflection nebula NGC 7023

We present SWS and LWS ISO observations towards a strip across the photodis sociation region (PDR) in the reflection nebula NGC 7023. SWS02 and LWS01 s pectra have been taken towards the star and the brightest infrared filament s located NW and SW from the star (hereafter NW and SW PDRs). In addition, SWS02 spectra have been taken towards two intermediate positions (NW1 and S W1). This has provided important information about the extent and spatial d istribution of the warm H-2 and of the atomic species in this prototypical reflection nebula. Strong emission of the [SiII] 34.8 mu m line is detected towards the star. While all the PDR tracers (the [CII] 157.7 mu m, [OI] 63.2 and 145.6 mu m, [HI] 21cm and the H-2 rotational lines) present a ring-like morphology with the peaks toward the NW and SW PDRs and a minimum around the star, the SiI I emission is filling the hole of this ring with the peak towards the star. This morphology can only be explained if the Sill emission arise in the lo west extinction layers of the PDR (A(upsilon) < 2 mag) and the HII region. At least 20% - 30% of the Si must be in gas phase in these layers. For A(up silon) greater than or equal to 2 mag, the Si is mainly in solid form (delt a Si = -1.3). The NW and SW PDRs have Very similar excitation conditions, high density fi laments (n similar to 10(6) cm(-3)) immersed in a more diffuse interfilamen t medium (n similar to 10(4) cm(-3)). In both, the NW and SW PDRs, the inte nsities of the Hz rotational lines can only be fitted by assuming an ortho- to-para-H-2 ratio lower than 3 in gas with rotation temperatures from 400 t o 700 K. Therefore, there is a non-equilibrium OTP ratio in the region. Fur thermore, the comparison between the OTP ratio derived from Hz vibrational lines and the pure Hz rotational lines, shows that the OTP ratio increases from similar to 1.5 to 3 across the photodissociation region with larger va lues in the less shielded gas (A(upsilon) < 0.7 mag). This behavior is inte rpreted as a consequence of an advancing photodissociation front. We have not detected the OH, CH and CH2 lines towards the observed position s. This is consistent with the weakness of these lines in other sources and can be explained as a consequence of the small beam filling factor of the dense gas in the LWS aperture. The CO J=17-->16 line has been tentatively d etected towards the star.