Multiphoton ionization of azulene and naphthalene

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Abstract
Molecular beams of azulene and naphthalene are irradiated by the output of a tunable pulsed laser and the charged species produced by multiphoton ionization (MPI) are analyzed using a quadrupole mass spectrometer. A study of the ion fragmentation pattern as a function of laser power and laser wavelength permits these two C10H8 isomers to be readily distinguished. Compared to electron impact (EI) ionization, the MPI process can produce more gentle ionization at low laser powers and more extensive fragmentation at high laser powers. The former permits the study of azulene and naphthalene dimers from which their heats of dissociation are estimated. In the case of azulene, two‐color excitation yields a mass spectrum that differs from the mass spectrum obtained by excitation with either color, alone. The ionization efficiency of MPI is very high, exceeding that of EI during the time the pulsed laser is on. A qualitative description involving autoionizing states is presented to account for this efficiency and the growth of the daughter ion yield with increasing laser power.