  We present SWS and LWS ISO observations towards a strip across the
photodissociation region (PDR) in the reflection nebula NGC 7023. SWS02 and
LWS01 spectra have been taken towards the star and the brightest infrared
filaments located NW and SW from the star (hereafter NW and SW PDRs). In
addition, SWS02 spectra have been taken towards two intermediate positions.
  While all the PDR tracers (the [CII] 157.7$\mic$, [OI] 63.2 and 145.6$\mic$,
[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 SiII
emission is filling the hole of this ring with the peak towards the star. This
morphology can only be explained if the SiII emission arise in the lowest
extinction layers of the PDR (A$_v$ $<$ 2 mag) and the HII region. At least 20%
-- 30% of the Si must be in gas phase in these layers. For A$_v$ $\geq$ 2 mag,
the Si is mainly in solid form ($\delta$ Si = -1.3).
  In both, the NW and SW PDRs, the intensities of the H$_2$ 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 to 700 K. Therefore, there is a
non-equilibrium OTP ratio in the region. The comparison between the OTP ratio
derived from H$_2$ vibrational lines and the pure H$_2$ rotational lines, shows
that the OTP ratio increases from $\sim$ 1.5 to 3 across the photodissociation
region with larger values in the less shielded gas (A$_v$$<$ 0.7 mag). This
behavior is interpreted as a consequence of an advancing photodissociation
front.
