Photoelectron spectroscopy of cluster anions of naphthalene and related aromatic hydrocarbons

The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene) n- (n=3-150), and its related aromatic cluster anions, (acenaphthene) n- (n=4-100) and (azulene) n- (n=1-100), are studied using anion photoelectron spectroscopy. For (naphthalene) n- clusters, two isomers coexist over a wide size range: isomers I and II-1 (28≤n≤60) or isomers I and II-2 (n∼60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene) n- and (azulene) n- clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7≤n≤100) in all the anionic clusters depend linearly on n-13 and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene) n- clusters with n∼30 remain constant at 0.84 and 0.99 eV, respectively, 0.4-0.6 eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene) n- and (azulene) n- and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric π -conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable.