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

The electronic transition spectra of the indene molecular cation (In+,C9H8+) in the 550-580 nmregion has been measured in the gas phase in direct absorption. The spectra were experimen-tally recorded using sensitive cavity ring-down spectroscopy in a supersonically expanding planarplasma. Two lifetime-broadened vibronic bands are assigned to the 2A ''-X(2)A '' electronic transitionsystem of In+. A set of indicative spectroscopic parameters have been obtained from a band contouranalysis of the 000band in combination with supporting ab-initio calculations. The inferred Lorentzline width of 4.0 +/- 0.5 cm(-1)indicates an upper state lifetime of 1.2 +/- 0.1 ps. The laboratory data arecompared to the diffuse interstellar bands (DIBs), and no matches can be found. Upper limits forIn+column densities in the diffuse interstellar medium have also been estimated from six sightlinespectra