Paper 308

Abstract Title:

X-RAY ABSORPTION AND PHOTOELECTRON SPECTROSCOPY OF CAROTENOIDS

Authors:

H. Stiel, H. Lokstein, M. Beck, U. Vogt, I. Will and D. Leupold

Institutions:

Max-Born-Institut, D-12489 BERLIN, GermanyInstitut f|r Biologie HU-Berlin, D-10099 BERLIN, Germany

Key Words:

Abstract:

Carotenoids (and their oxygen-containing derivatives, the xanthophylls) are essential components of pigment-protein complexes in photosynthetic organisms. Much attention has been devoted to elucidate the symmetry forbidden ''dark'' S₁(2¹A_g) level of the carotenoids as the basis of its participation in the energy transfer process xanthophyll Chl as well as in a photoprotective transfer Chl xanthophyll. Because the S₁(2¹A_g) level can not be assessed by optical spectroscopy, we propose a new experimental approach: For molecules with p-electron systems the transition 1s2p(p*) from inner atomic to the lowest unoccupied molecular orbital (LUMO) appears in an X-ray absorption spectrum (XAS) as an intense, sharp peak few eV below the carbon K-edge. Whereas the peak position reflects the energy of the first excited singlet state in relation to the ionization potential of the molecule, intensity and width of the transition depend on hybridization and bonding partners of the selected atom. An additional peak related to the 1s2p(p) transition appears in the spectrum if an intense laser pulse tuned to the S₀S₂ energy creates a vacancy in the highest occupied molecular orbital (HOMO). Complementary information can be obtained from ultraviolet photoelectron spectroscopy (UPS): At low-binding-energy site of the spectrum peaks related to HOMO appear. We have measured the XAS and UPS of b-carotene and luteine. Based on these measurements and quantum chemical calculations the HOMO and LUMO-energies of both molecules were derived. The relevance of the data for the determination of the energetic position of the S₁(2¹A_g) level will be discussed. Support by DFG (SFB 429) is gratefully acknowledged.