Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: experiment and theory

The C K-edge photoabsorption and 1s core-level photoemission spectra of pentacene (C22H14) molecules in the gas phase are experimentally determined, and calculated by Self Consistent Field (SCF) and Static-Exchange Approximation (STEX) ab-initio methods.The contribution of the six non-equivalent C atoms present in the molecule to the C 1s photoemission spectrum, is clearly brought to light. We observe and interpret the complex near-edge structures of the carbon K-edge absorption spectrum, with two main groups of discrete transitions between 283eV and 288eV photon energy, as due to absorption to pi* virtual orbitals, while the broader structures at higher energy involve sigma* virtual orbitals. The sharp absorption structures to the pi* empty orbitals lay well below the threshold for the C 1s ionization, demonstrating strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of pi* symmetry, from the six chemically shifted core orbitals. These results constitute an important basis and prerequisite