Electronic spectroscopy of carbon chains.

Investigators have recorded the electronic spectra of assorted carbon-chain systems in the gas phase using a variety of methods, ranging from direct cavity ringdown absorption spectroscopy to photofragmentation techniques that utilize the cooling capabilities of an ion trap. We summarize the results from these studies and compare them with astronomical measurements of the diffuse interstellar band (DIB) absorptions. Although carbon chains comprising up to a handful of carbon atoms cannot be the carrier species, we explore which chains remain viable. In particular, the (1)Sigma(u)(+)-Chi(1)Sigma(g)(+) transitions of the odd-numbered carbon chains (n = 17,19,...) possess large oscillator strengths and lie in the 400-900-nm DIB range. The origin bands of larger bare carbon rings, such as C(18), have also been observed, with striking similarities to some DIB measurements at high resolution, although at other wavelengths. Finally, we consider recently obtained electronic spectra of metal-containing carbon chains.