Gas-phase optical absorption spectra of the indene cation (C9H8 +)

Abstract

The electronic transition spectra of the indene molecular cation (In, ) in the 550–580 nm region has been measured in the gas phase in direct absorption. The spectra were experimentally recorded using sensitive cavity ring-down spectroscopy in a supersonically expanding planar plasma. Two lifetime-broadened vibronic bands are assigned to the - electronic transition system of In. A set of indicative spectroscopic parameters have been obtained from a band contour analysis of the 0 band in combination with supporting ab-initio calculations. The inferred Lorentz line width of indicates an upper state lifetime of . The laboratory data are compared to the diffuse interstellar bands (DIBs), and no matches can be found. Upper limits for In column densities in the diffuse interstellar medium have also been estimated from six sightline spectra.