The Gas Phase Spectrum of Cyclic C18 and the Diffuse Interstellar Bands

The gas phase spectrum of the cyclic C18 molecule recorded in the laboratory at a temperature typical of diffuse interstellar clouds is compared with absorption features toward Oph and HD 204827 in the 5730–5934 8 region. For the origin band at 5928.5 8 an upper limit to the column density of 1:8 ; 10 cm 2 is inferred. The origin band pattern in the laboratory spectrum changes on lowering the internal temperature in the 100–20 K range and bears a striking resemblance to the observed structure of a number of DIBs at other wavelengths. This suggests that platelike molecules or ions, comprising a couple of dozen to hundred carbon atoms, could be responsible for some of the latter absorptions. Subject headinggs: ISM: general — ISM: lines and bands — ISM: molecules — stars: individual ( Oph, HD 204827)