  We have made one-dimensional raster-scan observations of the rho Oph and
sigma Sco star-forming regions with two spectrometers (SWS and LWS) on board
the ISO. In the rho Oph region, [SiII] 35um, [OI] 63um, 146um, [CII] 158um, and
the H2 pure rotational transition lines S(0) to S(3) are detected, and the PDR
properties are derived as the radiation field scaled by the solar neighborhood
value G_0~30-500, the gas density n~250--2500 /cc, and the surface temperature
T~100-400 K. The ratio of [SiII] 35um to [OI] 146um indicates that silicon of
10--20% of the solar abundance must be in the gaseous form in the
photodissociation region (PDR), suggesting that efficient dust destruction is
undergoing even in the PDR and that part of silicon atoms may be contained in
volatile forms in dust grains. The [OI] 63um and [CII] 158um emissions are too
weak relative to [OI] 146um to be accounted for by standard PDR models. We
propose a simple model, in which overlapping PDR clouds along the line of sight
absorb the [OI] 63um and [CII] 158um emissions, and show that the proposed
model reproduces the observed line intensities fairly well. In the sigma Sco
region, we have detected 3 fine-structure lines, [OI] 63um, [NII] 122um, and
[CII] 158um, and derived that 30-80% of the [CII] emission comes from the
ionized gas. The upper limit of the [SiII] 35um is compatible with the solar
abundance relative to nitrogen and no useful constraint on the gaseous Si is
obtained for the sigma Sco region.
