Isomer effects in the double-to-single photoionization ratio of aromatic hydrocarbons

We have measured the ratios of doubly to singly charged molecular parent ions of azulene and phenanthrene, which are isomers of naphthalene and anthracene, respectively. The photon-energy dependences of the ratio curves have been analyzed and compared to each other from threshold to the carbon K edge. Qualitatively, the ratio curves for azulene and naphthalene as well as phenanthrene and anthracene are very similar, but we have found a distinct quantitative difference in both cases for the two isomers.