Ultrafast conversion

On the ultrafast conversions from excited FC (Franck-Condon) state to FI (Fluorescence) state of chromophores in PNS of photoactive proteins PYP, Rh and FP... 

As a result of the impact with the surface, the leading edge atoms of the cluster reverse their direction of motion and collide with the atoms of the cluster which are still moving forward. Further collisions result in an ultrafast conversion of the initially directed energy to random motion and ultimately to the extensive or complete breakup of the cluster. In this section we show that the finite cluster reaches thermal equilibrium in a very short time that is available before all collisions cease due to the rapid expansion of the cluster. Then we will present a simple mechanism which suggests an interpretation for what causes this fast relaxation. 

Photoisomerizations. Photoinduced isomerization reactions are fundamental processes in many relevant biological phenomena and have profound implications in the design of new devices in nanotechnology. From a mechanistic point of view, these photoreactions take place via a Coin structure, which mediates the ultrafast conversion between the isomers exposed to light. The important role of Coins in photoisomerizations has been recently supported by experiments employing sophisticated techniques which combine high temporal resolution with a broad spectral observation window. In that work, the authors have found an excellent... 

It was clear that in large organic molecules internal conversion from upper to lower excited states occms on a subpicosecond scale. In the many organic molecules that exhibit no emission, such ultrafast conversion evidently occurred all the way to the ground electronic state, in some cases accompanied by a chemical transformation, in others not. When a chemical transformation did occur, it seemed to be almost always accompanied... 

It has been shown that photoexcitation of the guanine-cytosine (G-C) base pair leads to proton transfer [231], Watson-Crick (WC) base pairs have excited state lifetimes much shorter than other non-WC base pairs indicating once again that the natural occurring WC base pairs are more photostable than other alternative configurations [115, 118, 232-235], Much work has been done in the gas phase where many different base pair isomers exist. The ultrafast relaxation of the WC base pair has also been confirmed in solution using fluorescence up-conversion measurements [117]. 

Pecourt J-ML, Peon J, Kohler B (2000) Ultrafast internal conversion of electronically excited RNA and DNA nucleosides in water. J Am Chem Soc 122 9348... 

Kang H, Jung B, Kim SK (2003) Mechanism for ultrafast internal conversion of adenine, J Chem Phys 118 6717... 

Zgierski MZ, Patchkovskii S, Fujiwara T, Lim EC (2005) On the origin of the ultrafast internal conversion of electronically excited pyrimidine bases. J Phys Chem A 109 9384-9387... 

Merchan M, Serrano-Andres L (2003) Ultrafast internal conversion of excited cytosine via the lowest it it electronic singlet state. J Am Chem Soc 125 8108... 

Chen H, Li SH (2005) Theoretical study toward understanding ultrafast internal conversion of excited 9H-adenine. J Phys Chem A 109 8443-8446... 

Zgierski MZ, Patchkovskii S, Fujiwarab T, Lim EC (2007) The role of out-of-plane deformations in subpicosecond internal conversion of photoexcited purine bases absence of the ultrafast decay channel in propanodeoxyguanosine. Chem Phys Lett 440 145-149... 

Macpherson, A. N and T. Gillbro. 1998. Solvent dependence of the ultrafast S2-Sj internal conversion rate of (3-carotene. J. Phys. Chem. A 102 5049-5058. 

Litvinenko KL, Webber NM, Meech SR (2001) An ultrafast polarisation spectroscopy study of internal conversion and orientational relaxation of the chromophore of the green fluorescent protein. Chem Phys Lett 346 47-53... 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

From the above discussion, we can see that the purpose of this paper is to present a microscopic model that can analyze the absorption spectra, describe internal conversion, photoinduced ET, and energy transfer in the ps and sub-ps range, and construct the fs time-resolved profiles or spectra, as well as other fs time-resolved experiments. We shall show that in the sub-ps range, the system is best described by the Hamiltonian with various electronic interactions, because when the timescale is ultrashort, all the rate constants lose their meaning. Needless to say, the microscopic approach presented in this paper can be used for other ultrafast phenomena of complicated systems. In particular, we will show how one can prepare a vibronic model based on the adiabatic approximation and show how the spectroscopic properties are mapped onto the resulting model Hamiltonian. We will also show how the resulting model Hamiltonian can be used, with time-resolved spectroscopic data, to obtain internal... 

Various ultrafast phenomena occuring in the femtosecond time-scale in the condensed phase have been studied by fluorescence up-conversion (for a review, see Mialocq and Gustavsson, 2001). As already mentioned in Chapter 7 (Box 7.1),... 

Seel, M., and Domcke, W. (1991), Femtosecond Time-resolved Ionization Spectroscopy of Ultrafast Internal-Conversion Dynamics in Polyatomic Molecules Theory and Computational Studies, J. Chem. Phys. 95,7806. 

Reflecting personal preferences, we focus in this review on the modeling of ultrafast bound-state processes following photoexcitation such as electron transfer, internal-conversion via conical intersections, and nonadiabatic... 

Parameters of Model II, Which Represents a Three-State Eive-Mode Model of the Ultrafast C — B — X Internal-Conversion Process in the Benzene Cation [179, 180] ... 

Here we first describe the ultrafast fluorescence microscope, which nses the fluorescence up-conversion method. This microscope simnltaneonsly achieves femtosecond time resolution and snbmicron space resolution (Fnjino and Tahara 2003, 2004). 

The second example of the application of fluorescence up-conversion microscope is imaging of organic microcrystals based on ultrafast fluorescence dynamics (femtosecond fluorescence dynamics imaging) (Fujino et al. 2005a). In this measurement, the site-specific energy transfer rate in a tetracene-doped anthracene microcrystal was measured, and the crystal was visualized based on the observed local ultrafast dynamics. 

As described in the previous section, the femtosecond fluorescence up-conversion microscope enabled us to visualize microscopic samples based on position-depen-dent ultrafast fluorescence dynamics. However, in the imaging measurements using the fluorescence up-conversion microscope, XY scanning was necessary as when using FLIM systems. To achieve non-scanning measurements of time-resolved fluorescence images, we developed another time-resolved fluorescence microscope. 

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... 

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