The Electronic Spectra of Carbon Chains, Rings, and Ions of Astrophysical Interest.

On the basis of a comparison of the electronic spectra of a number of carbon chains measured in the laboratory with diffuse interstellar band (DIB) absorptions, it is concluded that carbon chains and related systems comprising up to a handful of carbon atoms can not be the carriers, as originally suggested by Douglas. However, the detection of the weak absorptions in diffuse clouds due to C3 enables arguments to be brought forward why certain larger carbon chains remain viable candidates. Specifically the odd-numbered carbon chains, C17, C19, ... have special spectroscopic properties: their lowest electronic transitions lie in the 400–900 nm DIB range, as is known from observation in neon matrices, and manifest very large oscillator strengths. An attempt to measure these in the gas phase was unsuccessful, but the electronic spectrum of a bare C18 ring could be observed. Particularly striking is the similarity of its origin band profile at temperatures in the 20–100 K range with some DIB measurements at high resolution, though at other wavelengths. The peak structure of the DIBs and the laboratory band is almost superimposable, suggesting that other plate-like species, with electronic transitions in the visible with high oscillator strengths, could be some of the DIB carriers. In order to measure the electronic spectra of larger cations under conditions relevant to diffuse interstellar clouds, an ion trap instrument has been set-up. In this ions are held in a 22-pole trap, where the vibrational and rotational degrees of freedom are equilibrated to temperatures down to 20 K by collisions with cryogenically cooled helium. The analysis of the vibrational pattern and of the rotational K-structure in the electronic spectrum of 2,4-hexadiyne cation, detected by a one photon predissociation process, shows that the relaxation to such low temperatures has taken place. Measurement of the electronic spectrum of 1,4-dichlorobenzene cation illustrates a scheme to detect the electronic spectra for bound excited states using two photon absorption. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]