We performed ultrafast polarized light experiments in which we pumped the 800 nm band and probed several near- and mid-IR transitions of photosynthetic reaction centers from Rhodobacter sphaeroides. Absorption into the upper excitonic level of the special pair (PY+) is part of this band, but it is not known whether PY+ behaves as a localized state or if it mixes with accessory bacteriochlorophyll (BChl) states. A calculation of the anisotropy of pump−probe signals fails to reproduce the experimental results if the localized picture is used. In fact, the transition into PY+ has to be 4-fold intensified over a simple exciton model prediction in order to give rise to anisotropies which are consistent with the experiment. This substantial intensification is inconsistent with previous experimental results. Agreement between theory and experiment can be achieved if the PY+ state is mixed with excited states of accessory BChl. Stimulated emission from the lower excitonic level of the special pair (PY -), probed at 950 nm after pumping at 800 nm, does not appear instantaneously, but rises with a time constant of 110 fs. A novel excited state absorption of the accessory BChls at 1200 nm, assigned as a monomer transition by comparison with a pump−probe experiment on free BChl dissolved in acetone, also decays with a ∼100 fs time constant. Although Förster energy transfer from accessory Bchl states to PY - can account for the fast transients, under the delocalized state picture suggested in this paper they would rather correspond to an internal conversion process from the mixed states to the PY - state.

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Electronic transitions of the special pair excited state, P*, and of its positive ion, P+, have been identified by transient infrared spectroscopy in the 170&2000-~m-~ region. The P* transition is present immediately (ca. 300 fs) after light absorption and decays with time constant 3.4 ps. The P+ transition appears with time constant 3.4 ps. The transition dipoles of these transitions are both measured to have a squared projection of 0.63 onto the direction of the ground state to Qv-(870 nm) transition. This is interpreted to imply that both P* and P+ transitions have dipoles along the line joining the centroids of charge of the two bacteriochlorophylls (BChl) composing the dimer. The P* transition is assigned as an interexciton transition brought about by mixing of exciton and charge-separated states. The P+ transition is assigned as a transition between the symmetric and antisymmetric combination of the localized hole states of the dimer. The results are compared with theoretical calculations, static FTIR, and Stark effect measurements on the reaction center. While the results are in qualitative agreement with recent theoretical calculations, better agreement requires a larger admixture of charge resonance states in the QU-state than is found in most calculations.

Span-type surfactants (sorbitan fatty acid esters) and Tween-type surfactants (sorbitan polyoxyethylene fatty acid esters) are employed by us to generate stabilized microbubbles for use in diagnostic ultrasound. After sonication of an aqueous surfactant solution, only some mixtures of Span-type and Tween-type surfactants at certain conditions can form stable microbubbles. This work investigated the stability of the surfactant-stabilized microbubbles by using a Langmuir trough to measure the π-A isotherms of the surfactant monolayer. The experimental results, which agreed with a theoretical analysis of the microbubble stability, indicate that the surfactant-stabilized microbubbles have a solid-condensed monolayer "skin" which functions to reduce the surface tension, prevent coalescence between microbubbles, and increase their aqueous compatibility. The higher surface pressure obtained for the case of a microbubble preparation, compared with that of unsonicated mixtures, indicates that sonication enhances the structure of surfactant monolayer and makes the microbubbles extremely stable.