Femtosecond-picosecond dynamics

Figure 3. Selective vibrational transitions OH(l>, = 0) - OH(ty = 5) and OH(u, = 5)->-OH(iy = 10) induced by two individual IR femtosecond/picosecond laser pulses. The electric fields c(i) and the population dynamics Pv(t) are shown in panels (a) and (b), respectively. Sequential combination of the two individual laser pulses yields the overall transition OH(u = 0) - OH(u = 5) - OH(u/ = 10) cf. Fig. 1 and Table I. For the isolated system, the population of the target state Pv= fo(t) is constant after the series of IR femtosecond/picosecond laser pulses, i > 1 ps.

The MD approach is one of the most elaborated techniques for simulating the dynamic behaviour of molecules. In this approach, spatial coordinates and velocity components of each atom are considered. At each time step the whole set of equations of motion, corresponding to all the atoms, is solved in order to define the new positions and velocity components of the atoms. Time steps are in the range of femtoseconds, the dynamics is usually performed (for computer time reason) over a rather short time, typically a few hundred picoseconds, in such a way that a limited number of events are picked up along the considered trajectories. 

Femtosecond spectroscopy has an ideal temporal resolution for the study of ultrafast water motions from femtosecond to picosecond time scales [33-36]. Femtosecond solvation dynamics is sensitive to both time and length scales and can be a good probe for protein hydration dynamics [16, 37-50]. Recent femtosecond studies by an extrinsic labeling of a protein with a dye molecule showed certain ultrafast water motions [37-42]. This kind of labeling usually relies on hydrophobic interactions, and the probe is typically located in the hydrophobic crevice. The resulting dynamics mostly reflects bound water behavior. The recent success of incorporating a synthetic fluorescent amino acid into the protein showed another way to probe protein electrostatic interactions [43, 48]. 

Here, we will focus on fast (picosecond regime) and ultrafast (femtosecond regime) dynamics of proton (or H-atom) transfer and related events that may occur before or/and after the atomic rearrangement in selected systems trapped in cydodextrin cavities. The information is relevant for a better understanding of many systems where confinement is important for reactivity and function. To this end, we first give a short overview of the photochemistry and photophysics of CD complexes. 

The technique of fluorescence up-conversion (see Chapter 11), allowing observations at the time-scale of picoseconds and femtoseconds, prompted a number of fundamental investigations on solvation dynamics that turned out to be quite complex (Barbara and Jarzeba, 1990) (see Box 7.1). 

A fundamental goal of chemical research has always been to understand the reaction mechanisms leading to specific reaction products. Reaction mechanisms, in turn, are a consequence of the structural dynamics of molecules participating in the chemical process, with atomic motions occurring on the ultrafast timescale of femtoseconds (10 s) and picoseconds (10" s). Although kinetic studies often allow reaction mechanisms as well as the kind and properties of reaction intermediates to be determined, the obtained information is not sufficient to deduce the ultrafast molecular dynamics. Because these ultrafast motions are the essence of every chemical process, detailed knowledge about their nature is of fundamental importance. 

The time-resolved techniques that are usually used for FLIM are based on electronic-basis detection methods such as the time-correlated single photon counting or streak camera. Therefore, the time resolution of the FLIM system has been limited by several tens of picoseconds. However, fluorescence microscopy has the potential to provide much more information if we can observe the fluorescence dynamics in a microscopic region with higher time resolution. Given this background, we developed two types of ultrafast time-resolved fluorescence microscopes, i.e., the femtosecond fluorescence up-conversion microscope and the... 

This article highlights the recent development of ultrafast electron diffraction at Caltech. This development has made it possible to resolve transient structures both spatially (0.01 A) and temporally (picosecond and now femtosecond) in the gas phase and condensed media, surfaces and crystals, with wide ranging applications. We also present some advances made in the studies of mesoscopic ionic solvation and biological dynamics and function. 

In conclusion visible and IR experiments on the timescale of femtoseconds and picoseconds in combination with molecular dynamics simulations have given a detailed picture of the fast structural dynamics in light triggered azobenzene peptides. This reaction exhibits three phases ... 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

An intense femtosecond laser spectroscopy-based research focusing on the fast relaxation processes of excited electrons in nanoparticles has started in the past decade. The electron dynamics and non-linear optical properties of nanoparticles in colloidal solutions [1], thin films [2] and glasses [3] have been studied in the femto- and picosecond time scales. Most work has been done with noble metal nanoparticles Au, Ag and Cu, providing information about the electron-electron and electron-phonon coupling [4] or coherent phenomenon [5], A large surface-to-volume ratio of the particle gives a possibility to investigate the surface/interface processes. 

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