Multiplex Integrated Cavity Output Spectroscopy (MICOS) is a new cavity-enhanced absorption method that allows the use of broadband dye nanosecond pulsed laser sources and offers a sensitivity equivalent to CRDS. MICOS has been coupled to a pulsed discharge slit nozzle to measure the spectra of the cold naphthalene (C10H8+), acenaplithene (C12H10divided by) and pyrene (C16H10divided by) cations in the gas phase. A femtosecond relaxation timescale is measured for the D-5 <--D-0 (0-0) transition of C16H10divided by. Spectra recorded at high plasma energies also show evidence of fragmentation. The CH radical is observed and carbon nanoparticles are generated in the plasma. Published by Elsevier B.V.