Time-resolved two-photon spectroscopy of photosystem I determines hidden carotenoid dark-state dynamics

We present time-resolved fs two-photon pump-probe data measured with photosystem I (PS I) of Thermosynechococcus elongatus. Two-photon excitation (λexc/2) 575 nm) in the spectral region of the optically forbidden first excited singlet state of the carotenoids, Car S1, gives rise to a 800 fs and a 9 ps decay component of the Car S1f Sn excited-state absorption with an amplitude of about 47 ( 16 % and 53 ( 10%, respectively. By measuring a solution of pure "-carotene under exactly the same conditions, only a 9 ps decay component can be observed. Exciting PS I at exactly the same spectral region via one-photon excitation (λexc) 575 nm) also does not show any sub-ps component. We ascribe the observed constant of 800 fs to a portion of about 47 ( 16 % "-carotene states that can potentially transfer their energy efficiently to chlorophyll pigments via the optically dark Car S1 state. We compared these data with conventional one-photon pump-probe data, exciting the optically allowed second excited state, Car S2. This comparison demonstrates that the fast dynamics of the optically forbidden state can hardly be unravelled via conventional one-photon excitation only because the corresponding Car S1 populations are too small after Car S2 f Car S1 internal conversion. A direct comparison of the amplitudes of the Car S1 f Sn excited-state absorption of PS I and "-carotene observed after Car S2 excitation allows determination of a quantum yield for the Car S1 formation in PS I of 44 ( 5%. In conclusion, an overall Car S2f Chl energy-transfer efficiency of∼69 ( 5 % is observed at room temperature with 56 ( 5 % being transferred via Car S2 and probably very hot Car S1 states and 13 ( 5 % being transferred via hot and “cold ” Car S1 states

Local impurity effects in superconducting graphene

We study the effect of impurities in superconducting graphene and discuss their influence on the local electronic properties. In particular, we consider the case of magnetic and non-magnetic impurities being either strongly localized or acting as a potential averaged over one unit cell. The spin dependent local density of states is calculated and possibilities for visualizing impurities by means of scanning tunneling experiments is pointed out. A possibility of identifying magnetic scatters even by non spin-polarized scanning tunneling spectroscopy is explained.Comment: 4 pages, 4 figure

Bandwidth renormalization due to the intersite Coulomb interaction

The theory of correlated electrons is currently moving beyond the paradigmatic Hubbard $U$, towards the investigation of intersite Coulomb interactions. Recent investigations have revealed that these interactions are relevant for the quantitative description of realistic materials. Physically, intersite interactions are responsible for two rather different effects: screening and bandwidth renormalization. We use a variational principle to disentangle the roles of these two processes and study how appropriate the recently proposed Fock treatment of intersite interactions is in correlated systems. The magnitude of this effect in graphene is calculated based on cRPA values of the intersite interaction. We also observe that the most interesting charge fluctuation phenomena actually occur at elevated temperatures, substantially higher than studied in previous investigations.Comment: New appendix on benzen