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

The electronic transition spectra of the indene molecular cation (In + , C 9 H 8 + ) 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 2 A ′ ′ - X 2 A ′ ′ electronic transition system of In + . A set of indicative spectroscopic parameters have been obtained from a band contour analysis of the 0 0 0 band in combination with supporting ab-initio calculations. The inferred Lorentz line width of 4.0 ± 0.5 cm − 1 indicates an upper state lifetime of 1.2 ± 0.1 ps . 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. [ABSTRACT FROM AUTHOR]