Ultrafast pump-probe spectroscopy

Diffey W M and Beck W F 1997 Rapid-scanning interferometer for ultrafast pump-probe spectroscopy with phase-sensitive detection Rev. Sci. Instrum. 3296-300... 

AI-water complexes with more than three waters have received less attention because it is believed that such large complexes cannot be directly involved in the tautomerization. Moreover, these complexes are difficult to be spectroscopically assigned due to the complexity of their electronic [11] and vibrational [10] structures. 7AI with four waters was studied by Fohner et al. [27] using ultrafast pump-probe spectroscopy combined with theoretical calculations. Their results revealed that the proton-transfer rate increases compared to that of 7AI with two and three waters. Their deuteration studies provided proof for the occurrence of proton transfer (PT), although it was not conclusively confirmed that the proton transfer resulted in a complete tautomerization of the 7AI monomer. For even bigger clusters of 7AI with five waters, there are no experimental investigations available only a theoretical study was reported on the second hydration shell effect [45]. 

A theory for the ultrafast pump-probe spectroscopy of large polyatomic molecules in condensed phases was developed in the work [15]. A multimode Brownian oscillator model was used to account for high-frequency molecular vibrations and local intermolecular modes as well as collective solvent motions. A semiclassical picture was provided using the density matrix in Liouville space. Conditions for the observation of quantum beats, spectral diffusion, and solvation dynamics (dynamic Stokes shift) are specified. 

Yan, Y.J., Fried, L.E., Mukamel, S. Ultrafast pump-probe spectroscopy femtosecond dynamics in Liouville space. J. Phys. Chem. 93, 8149-8162 (1989)... 

The conceptual framework for the - semiclassical simulation of ultrafast spectroscopic observables is provided by the Wigner representation of quantum mechanics [2, 3]. Specifically, for the ultrafast pump-probe spectroscopy using classical trajectories, methods based on the semiclassical limit of the Liouville-von Neumann equation for the time evolution of the vibronic density matrix have been developed [4-8]. Our approach [4,6-8] is related to the Liouville space theory of nonlinear spectroscopy developed by Mukamel et al. [9]. It is characterized by the ability to approximately describe quantum phenomena such as optical transitions by averaging over the ensemble of classical trajectories. Moreover, quantum corrections for the nuclear dynamics can be introduced in a systematic manner, e.g. in the framework of the entangled trajectory method [10,11]. Alternatively, these effects can be also accounted for in the framework of the multiple spawning method [12]. In general, trajectory-based methods require drastically less computational effort than full quantum mechanical calculations and provide physical insight in ultrafast processes. Additionally, they can be combined directly with quantum chemistry methods for the electronic structure calculations. 

The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. In cyclohexane population transfer from the optically excited La to the Lb state happens within 7 ps. In ethanol ultrafast state reversal is observed, followed by population transfer from the Lb to the La state within 6 ps. In water ultrafast branching occurs between the fluorescing state and the charge-transfer-to-solvent state. Presolvated electrons, formed together with indole radicals within our time resolution, solvate on a timescale of 350 fs. 

Coherent nuclear motion of reacting excited-state molecules in solution observed by ultrafast two color pump-probe spectroscopy... 

Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give us a clue to reveal its significance in chemical reactions. In this study, we employed two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigated the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state. 

Stock, G. and Domcke, W. (1990). Theory of femtosecond pump-probe spectroscopy of ultrafast internal conversion processes in polyatomic molecules, J. Opt. Soc. Am. B 7, 1971. 

Abstract A challenging task in surface science is to unravel the dynamics of molecules on surfaces associated with, for example, surface molecular motion and (bimolecular) reactions. As these processes typically take place on femtosecond time scales, ultrafast lasers must be used in these studies. We demonstrate two complementary approaches to study these ultrafast molecular dynamics at metal surfaces. In the first, the molecules are studied after desorbing from the surface initiated by a laser pulse using the so called time-of-flight technique. In the second approach, molecules are studied in real time during their diffusion over the surface by using surface-specific pump-probe spectroscopy. 

Virgili T, Cerullo G, Luer L, Lanzani G, Gadermaier C, Bradley DDC (2003) Understanding fundamental processes in poly(9,9-dioctylfluorene) light-emitting diodes via ultrafast electric-field-assisted pump-probe spectroscopy. Phys Rev Lett 90(24) 247402... 

Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast Internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example . The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge... 

Gradinaru CC, Pascal AA, van Mourik F, Robert B, Horton P, van Grondelle H and van Amerongen H (1998) Ultrafast evolution ofthe excited states in the chlorophyll a/b complex CP29 from green plants studied by energy-selective pump-probe spectroscopy. Biochemistry 37 1143-1149... 

Ultrafast continuum pump-probe spectroscopy has led to the discovery of subpicosecond processes in bacteriorhodopsin. This protein contains a retinal chromophore and acts as a light-induced proton pump that creates a potential difference across a membrane in Halobacterium halobium, which can be used as a free-energy source. Changes in the bleaching, SE, and ESA were observed and were used in concert to describe, assign, and model the observed spectral changes and kinetics. 

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