Two-color photoionization of naphthalene and benzene at threshold

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Abstract
We observe the resonant two‐photon ionization (R2PI) spectrum of naphthalene and benzene at and just above the first ionization potential in a supersonic molecular beam. For naphthalene, the spectrum consists of direct ionization step functions indicating the relative Franck–Condon factors for transitions into the ground state ion. The spacings observed between these steps measure the ν8̄ vibrational frequency in the naphthalene ion. The electric field dependence of these steps is found to be consistent with a field ionization red shift of the thresholds. In benzene we observe similar direct ionization. Superimposed on the threshold steps, however, are broad features (∼100 cm−1) due to vibrational autoionization of Rydberg states at this same energy. Implications of this data on the observation of high Rydbergs in such large organic molecules are discussed.

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