Electronic Spectroscopy of Carbon Chains

Abstract

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.

Theoretical Characterization of C6N, C6N−, and C6N+

“…Our previous studies of astrophysical carbon chains (Massó et al 2007;Inostroza et al 2008;Massó & Senent 2009;Inostroza & Senent 2010;Senent & Hochlaf 2010Hammoutene et al 2012;Al-Mogren et al 2013), as well as other previous works (van Order & Saykally 1998;Jochnowitz & Maier 2008), show that unsaturated carbon clusters can display a large density of low-energy electronic states and can adopt different stable geometrical structures with peculiar electronic distribution and spin multiplicities. In the case of the relatively large C 6 N chain, which involves a unique nitrogen atom, the number of configurations is extremely large.…”

Electronic spectra of C6H+ and C6H3+ in the gas phase

“…In all these cases, the comparisons with the DIB data were negative, leading to upper limits of column densities of around 10 12 cm Ϫ3 [3]. Only in the case of C 3 could very weak interstellar absorptions be detected and the species unambiguously identified in the diffuse medium [5].…”

“…Following the suggestion by Douglas [2] that bare carbon chains comprising a handful or two of carbon atoms are good candidates as carriers of these absorptions-the diffuse interstellar bands (DIBs)-it took around 20 years to obtain a number of gas-phase electronic spectra in the laboratory [3]. Thus, it became possible for the first time to make a direct comparison with the astronomical data under comparable conditions: collision-free environment and low, 10 -50 K, temperatures.…”

“…Thus, it became possible for the first time to make a direct comparison with the astronomical data under comparable conditions: collision-free environment and low, 10 -50 K, temperatures. This was achieved in the laboratory by producing the species in supersonic expansions through which a discharge runs, coupled with sensitive laser methods such as cavity ringdown absorption, frequency modulation, and resonant multi-photon ionization [4]. Important were the observations of the neutral carbon chains C 4 , C 5 , and of their hydrogen derivatives, C n H, the latter being well known constituents of dark interstellar clouds, identified by mm-wave astronomy.…”

Electronic spectra of carbon chains and rings: Astrophysical relevance?

“…Ions are relevant because it is often argued that the degree of ionization can be high in the diffuse region (Le Page et al 2003). Furthermore, while the neutrals do not possess electronic transitions in the visible (these lie below 300 nm), their ions do (Jochnowitz & Maier 2008). Thus the hypothesis concerning smaller carbon chains (Douglas 1977) has been tested and can now be rejected (Maier et al 2004).…”