Electronic spectroscopy of carbon chains

Abstract

Gas phase electronic spectra allow direct comparisons between laboratory data and the wealth of astrophysical measurements that have been tabulated. Over the past decade a number of assorted carbon chain systems have been spectroscopically investigated, yielding an understanding of the transitions and trends apparent among their homologous series. This rigorous program begins with trapping unstable carbon chain ions and radicals in neon matrices, where origin bands and vibrational data are obtained. From these results one can then seek out the gas phase transitions in higher resolution in order to more accurately determine assignments and spectroscopic constants. To this purpose, laser spectroscopic methods are employed, including pulsed and cw cavity ringdown, resonant enhanced multiphoton ionisation, and more recently, a two-photon two-colour excitation/fragmentation approach to investigate trapped ions. From a comparison involving the electronic spectra of a number of carbon chain systems measured in the laboratory with diffuse interstellar band (DIB) absorptions, one concludes that species involving only a handful of carbon atoms can not be the carriers. However, larger carbon chains and ring species still remain viable. In this review, we consider which systems require further investigation, including these longer species, metal-capped chains and other cationic radicals.