The spectroscopy of jet-cooled porphyrins: an insight into the vibronic structure of the Q band

摘要

We will present resonant two-photon ionization spectra for meso-tetraphenylporphyrin, H2TPP, measured under isolated conditions. The polycrystalline compound was vaporized, in vacuo, using both thermal and laser desorption, and seeded into a supersonic expansion of an inert-carrier gas. The molecules remain largely intact in the gaseous phase. However, the two techniques for vaporizing H2TPP give different internal temperatures for the isolated substrate, with greater vibrational cooling achieved using laser desorption. A comparison of the peak positions and intensities in the resonant two-photon ionization spectra of thermal- and laser-desorbed molecules provides an insight into the vibrational structure of the Q band. In particular, the greater contribution made by electronic transitions originating from higher vibrational levels in the ground state of H2TPP is emphasized. We conclude that vibronic coupling in the ground electronic state plays an important role in a quantum-mechanical interpretation of the Q band.