Observations of 6-200 mu m emission of the Ophiuchus cloud LDN 1688

We examine two positions, ON1 and ON2, within the Ophiuchus cloud LDN 1688 using observations made with the ISO photo-polarimeter (ISOPHOT) instrument aboard the Infrared Space Observatory (ISO) satellite. The data include mid-infrared spectra (similar to 6-12 mu m) and several photometric bands up to 200 mu m. The data probe the emission from molecular polycyclic aromatic hydrocarbon (PAH)-type species, transiently heated very small grains and large classical dust grains. We compare the observations to earlier studies, especially those carried out towards an isolated translucent cloud in Chamaeleon (Paper I). The spectra towards the two LDN 1688 positions are very similar to each other in spite of position ON1 having a larger column density and probably being subjected to a stronger radiation field. The ratios of the mid-infrared features are similar to those found in other diffuse and translucent clouds. Compared to Paper I, the 7.7/11.3 mu m band ratios are lower, similar to 2.0, at both LDN 1688 positions. A continuum is detected in the similar to 10 mu m region. This is stronger towards the position ON1 but still lower than on any of the sightlines in Paper I. The far-infrared opacities are higher than in diffuse medium. The value of the position ON2, tau(200)/N(H) = 3.9 x 10(-25) cm(2)/H, is twice the value found for ON1. The radiation field of LDN 1688 is dominated by the two embedded B-type double stars, rho Oph AB and HD 147889, with an additional contribution from the Upper Sco OB association. The strong heating is reflected in the high colour temperature, similar to 24 K, of the large grain emission. Radiative transfer modelling confirms a high level of the radiation field and points to an increased abundance of PAH grains. However, when the hardening of the radiation field caused by the local B stars is taken into account, the observations can be fitted with almost no change to the standard dust models. However, all the examined models underestimate the level of the mid-infrared continuum.