The energy on the sample was less than 0.5 nJ/pulse for one-photon and 80 nJ/pulse for two-photon measurements, respectively. For the measurement of the one-photon fluorescence spectrum, the sample was excited at 330 nm this wavelength was obtained by frequency doubling the optical parametric generator/amplifier output with a β-barium borate crystal. Also in this case, the resolution of both fluorescence spectra and excitation profiles was set to 1 nm. The setup for this measurement is described in detail in Ref. Two-photon excitation profiles and one- and two-photon excited fluorescence spectra were measured on the picene nano-crystalline and on the micro-crystalline samples using a pulsed tunable source. Absorption and fluorescence spectra and excitation profiles were all measured with a resolution of 1 nm. Fluorescence spectra and excitation profiles on the solutions and on the suspension were measured with a Perkin-Elmer LS 55 fluorescence spectrometer using several excitation and detection wavelengths. One-photon absorption spectra were measured with a Perkin-Elmer Lambda 950 spectrophotometer. 13,16–18 All the works are in very good agreement showing that picene is a typical molecular crystal, where the band structure is dominated by the molecular states, but electronic correlations and anisotropy must be taken into account for an accurate description. 2 The electronic structure of pristine and doped picene, in the solid phase, has been studied theoretically, 3–11 and experimentally through XPS, 12,13 UPS, 13–15 and EELS. Picene has gained increasing attention in the last few years, since the discovery of superconductivity in K 3-picene. 1 The simplicity of their chemical composition is in contrast with the complexity of their highly anisotropic crystal structures and of their electronic properties, and investigation of their optical spectra in the solid phase is of great interest both from a fundamental and an applicative point of view. Molecular solids of hydrocarbons containing conjugated π-electron systems, such as fullerenes, graphite, and polycyclic aromatic hydrocarbons, have been extensively studied because of their potential use as superconductive materials upon doping with alkali metals.
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