Coherent light sources have been widely used in control schemes that exploit quantum interference effects to direct the outcome of photochemical processes. The adaptive shaping of laser pulses is a particularly powerful tool in this context: experimental output as feedback in an iterative learning loop refines the applied laser field to render it best suited to constraints set by the experimenter. This approach has been experimentally implemented to control a variety of processes, but the extent to which coherent excitation can also be used to direct the dynamics of complex molecular systems in a condensed-phase environment remains unclear. Here we report feedback-optimized coherent control over the energy-flow pathways in the light-harvesting antenna complex LH2 from Rhodopseudomonas acidophila, a photosynthetic purple bacterium. We show that phases imprinted by the light field mediate the branching ratio of energy transfer between intra- and intermolecular channels in the complex's donor acceptor system. This result illustrates that molecular complexity need not prevent coherent control, which can thus be extended to probe and affect biological functions.

Using two-color pump-probe femtosecond spectroscopy, the temperature dependence of the energy transfer rate within the peripheral light-harvesting antenna (LH2) of the photosynthetic bacterium Rhodobacter sphaeroides has been measured. The energy transfer time from B800 to B850 is determined to be 0.7, 1.2, and 1.5 ps at 300, 77, and 4.2 K, respectively. These data, combined with earlier results, have been analyzed with regard to the crystal structure and spectroscopic properties of the purple bacterial LH2 complex. We conclude that the transfer within B800 occurs mainly via the incoherent Förster hopping mechanism. For B800 to B850 transfer, estimates based on the Förster formula yield considerably slower transfer times than experimentally observed, suggesting that an additional mechanism may be involved in enhancing the transfer rate. We suggest two possibilities: transfer via the upper excitonic component of B850 band and/or transfer mediated by a carotenoid molecule.

The real-time dynamics of hydrogen-atom-transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular "proton" transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl-group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen-transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of -7200 cm-'. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O-H-O reaction coordinate. The vibrational periods (2?r/w) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low-frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrationalenergy redistribution dynamics at longer times are important to the hydrogen-bond dissociation and to the nonradiative decay of the hydrogen-transferred form.

Carotenoids are involved in a variety of biological functions, yet the underlying mechanisms are poorly understood, in part because of the long-standing difficulty in assigning the location of the first excited (S 1 ) state. Here, we present a method for determining the energy of the forbidden S 1 state, on the basis of ultrafast spectroscopy of the short lived level. Femtosecond transient absorption spectra and kinetics of the S 1 3 S 2 transition revealed the location of the intermediate level in two carotenoid species involved in the xanthophyll cycle, zeaxanthin and violaxanthin, and yielded surprising implications regarding the mechanism of photoregulation in photosynthesis.The diversity and abundance of carotenoids in nature reflect their importance in protecting and supporting the organisms in which they are found (1). In photosynthetic and physiological systems alike, carotenoids are antioxidants that prevent damage by quenching triplet states and scavenging singlet oxygen. Furthermore, they play an integral role in photoregulation in plants, assisting with light harvesting or dissipating excess energy as needed (2). The mechanisms that underlie their functions are poorly understood, in part because of the long standing difficulty in assigning the location of their first excited (S 1 ) states. In higher conjugated carotenoids, the S 1 state is spectroscopically ''dark,'' hence previous assignments of its energy have relied on extrapolation of data from shorter-chain carotenoids (3). Whereas application of the ''energy-gap law'' (4) to carotenoids of similar composition (i.e., with changes only in conjugation length) may yield reasonable estimates of S 1 energies (5), quantitative information for markedly different species is not reliable.The S 1 level of carotenoids is likely involved in numerous functions and may be particularly relevant to the xanthophyll cycle of higher plants. In this photoprotection function, carotenoids are involved in regulating the amount of light energy funneled into the reaction centers according to the given environmental conditions. The equilibrium within an interconversion reaction cycle involving three carotenoid species, zeaxanthin N antheraxanthin N violaxanthin, is shifted according to the amount of light available. In high light, the concentration of zeaxanthin increases, and correspondingly excess energy is dissipated out of the antenna complexes (as monitored by fluorescence quenching), whereas in low light violaxanthin is the predominant carotenoid and no quenching is observed. The S 1 states of both carotenoids have been implicated in the mechanism of this function, according to estimates of the energy levels via application of the energy gap law (3, 6). However, without precise knowledge of the S 1 energetics, this interpretation is highly speculative.Here we present a general method for measuring the S 1 -state energy of carotenoids directly, with specific application to zeaxanthin and violaxanthin. The key factor of this experiment lies in the ability to ...