Electronic Spectrum of C7H3+ in the Gas Phase at 10 K

The analysis of the λ5797.1 diffuse interstellar band (DIB) by Huang & Oka concludes that the carrier is a chain-like molecule with five to seven heavy atoms with a large oscillator strength, f ≈ 1, for the electronic transition. The spectra of carbon chains of this size with transitions in the visible have been obtained in the gas phase, but the f-values are too small. We have now found that certain carbon-chain cations with transitions in the DIB range have large f-values. An example is the origin band at 4387.7 Å of the 1 1AA1 electronic transition of the H2C7H+ chain with f ≈ 0.3. This could be measured in the gas phase at 10 K in an ion trap. Astrophysical relevance of such cations is discussed.