Excitation of fluorescence in naphthalene and azulene by electron impact

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Abstract
Electron impact excitation of fluorescence in naphthalene and azulene is studied under low pressure conditions (?3×10−5 Torr). The data are obtained as (a) fluorescence spectra at a series of electron energies, and (b) optical excitation functions at several wavelengths. The naphthalene fluorescence spectrum is diffuse and exhibits an onset at the S2 spectroscopic energy, despite the fact that all emission appears to be S1 → S0. These observations are interpreted in terms of predominant S0 → S2 excitation, followed by rapid S2 → S1 internal conversion and then S1 → S0 fluorescence. For azulene S2 → S0 emission is observed which closely resembles solution results. The naphthalene and azulene fluorescence spectra exhibit no substantial variation for electron energies greater than 1 V above threshold. However, fragment emission from H, C, CH, and C2 becomes prominent above 30 eV. In addition, an unidentified continuum (or possibly many, closely spaced lines) is present throughout the 250−550 nm region for high electron energies. The excitation functions for both molecules exhibit several peaks at low electron energy (below 20 eV) and a broad peak at ∼150 eV. The low−energy peaks are assigned as due to previously unobserved resonances and to nonresonant excitation. The ∼150 eV peak is caused by continuum emission. The resonant peaks are observed to be strongest for wavelengths of maximum emission intensity. This behavior is explained in terms of preferred decay routes for the resonant states.