Picosecond UV pump (248.5 nm)/IR probe spectroscopy has been applied to the study of the decomposition of several aromatic diacyl peroxides, peroxycarbonates, and of a peroxyester dissolved in CH 2 Cl 2 . Measuring the IR transient absorbance in the 2100 to 2450 cm −1 range allows to monitor the formation and vibrational relaxation of the photoproduct CO 2 via the asymmetric stretching mode (ν 3 ) with a time resolution of 1.8 ps. With each of the six peroxides a CO 2 molecule is released at delay times below 10 ps. The energy relaxation of the initially formed vibrationally hot CO 2 is followed over the time range up to 500 ps. Analysis of the transient IR spectra, via an anharmonic oscillator model proposed by Hamm et al. [1], shows a monoexponential decay of internal energy. Irrespective of the type of peroxide a single relaxation time of 67 ± 5 ps is found to adequately represent the cooling behavior of CO 2 in liquid CH 2 Cl 2 . The "initial" temperatures of vibrationally hot CO 2 at a delay time of 20 ps after applying the pump pulse differ considerably, between 1400 and 2700 K for the decompositions of tert-butyl benzoyl peroxide and tert-butyl benzoyl carbonate, respectively. These remarkably high temperatures are assumed to originate from energy release associated with structural relaxation of the bent OCO moiety to form the linear CO 2 molecule.

The ultrafast internal conversion (IC) dynamics of seven C(40) carotenoids have been investigated at room temperature in a variety of solvents using two-color transient lens (TL) pump-probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and-for the first time-echinenone, as well as new data for lycopene, beta-carotene, (3R,3'R)-zeaxanthin and (3R,3'R,6'R)-lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S2 band at 400 nm. For the S2 lifetime solvent-independent upper limits on the order of 100-200 fs are estimated for all carotenoids studied. The S1 lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthin/echinenone/beta-carotene (13/12/11 double bonds) one finds tau1 approximately 5, 7.7 and 9 ps for the S1-->S0 IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on tau1, as observed for canthaxanthin/astaxanthin (tau1 approximately 5 ps in both cases). For all carotenoids studied, tau1 is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e.g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C40 carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured tau1.

We present the theoretical details and experimental demonstration of fifth-order three-dimensional (3D) electronic spectroscopy using a pump-probe beam geometry. This is achieved using a pulse shaper and appropriate phase cycling schemes. We show how 8-step and 27-step phase cycling schemes can measure purely absorptive 3D spectra as well as 3D spectra for the individual fifth-order processes that contribute to the purely absorptive spectrum. 3D spectra as a function of two separate controllable waiting time periods can be obtained. The peak shapes and positions of the peaks in the experimental measurement correspond well to theory.

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The structure and dynamics of a biological model bilayer are reported with atomic-scale resolution by using ultrafast electron crystallography. The bilayer was deposited as a Langmuir-Blodgett structure of arachidic (eicosanoic) fatty acids with the two chains containing 40 carbon atoms (Ϸ50 Å), on a hydrophobic substrate, the hydrogen terminated silicon(111) surface. We determined the structure of the 2D assembly, establishing the orientation of the chains and the subunit cell of the CH 2 distances: a0 ‫؍‬ 4.7 Å, b0 ‫؍‬ 8.0 Å, and c 0 ‫؍‬ 2.54 Å. For structural dynamics, the diffraction frames were taken every 1 picosecond after a femtosecond temperature jump. The observed motions, with sub-Å resolution and monolayer sensitivity, clearly indicate the coherent anisotropic expansion of the bilayer solely along the aliphatic chains, followed by nonequilibrium contraction and restructuring at longer times. This motion is indicative of a nonlinear behavior among the anharmonically coupled bonds on the ultrashort time scale and energy redistribution and diffusion on the longer time scale. The ability to observe such atomic motions of complex structures and at interfaces is a significant leap forward for the determination of macromolecular dynamical structures by using ultrafast electron crystallography.dynamics ͉ ultrafast electron diffraction ͉ Langmuir-Blodgett films M embranes of bilayers are an essential part of biological structures and are model systems for behavior on surfaces and at interfaces (1-3). Even though there exists a rich literature about the structure and spectroscopy of membranes (4-6) and their function in biological environments, very little is known about their dynamical structures on the ultrashort time scale. Because the atomic motions on these time scales are critical to the subsequent global changes responsible for the biological function (7), it is essential to elucidate the primary dynamical behavior of the structures in the nonequilibrium regime and with atomic-scale resolution. The combined spatial and temporal resolutions of the recently developed ultrafast electron crystallography (UEC) (8-10) make the methodology ideal for determining dynamical structures of nanometer-scale (2D) bilayers, but the challenges are numerous.As a step in this direction, we studied a bilayer of fatty acids deposited on a hydrophobic surface substrate, invoking the well known Langmuir-Blodgett (LB) technique. It allows for a controlled layer-by-layer deposition of ordered molecular films and has been used to create model biological membranes for studies under controlled conditions (11,12). Although biomembranes are more complicated by the presence of intercalaters, the LB bilayers represent the building blocks of lipid bilayers, in our case with the molecular chains extending up to Ϸ50 Å. LB films themselves are of considerable interest in various areas of research involving self-assembly and self-organization and in technology developments such as molecular electronics and nonlinear optics.Previous static ...