Ultrafast processes

Free-electron lasers have long enabled the generation of extremely intense, sub-picosecond TFlz pulses that have been used to characterize a wide variety of materials and ultrafast processes [43]. Due to their massive size and great expense, however, only a few research groups have been able to operate them. Other approaches to the generation of sub-picosecond TFlz pulses have therefore been sought, and one of the earliest and most successfid involved semiconducting materials. In a photoconductive semiconductor, carriers (for n-type material, electrons)... 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Cong P, Simon J D and Yan Y 1995 Probing the molecular dynamics of liquids and solutions Ultrafast Processes in Chemistry and Photobiology ed M A El-Sayed, I Tanaka and Y Molln (Oxford Blackwell) pp 53-82... 

El-Sayed M A, Tanaka I and Molin Y (eds) 1995 Ultrafast Processes in Chemistry and Photobiology (Oxford Blackwell)... 

Ultrafast TRCD has also been measured in chemical systems by incoriDorating a PEM into the probe beam optics of a picosecond laser pump-probe absorjDtion apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

This chapter discusses the apphcation of femtosecond lasers to the study of the dynamics of molecular motion, and attempts to portray how a synergic combination of theory and experiment enables the interaction of matter with extremely short bursts of light, and the ultrafast processes that subsequently occur, to be understood in terms of fundamental quantum theory. This is illustrated through consideration of a hierarchy of laser-induced events in molecules in the gas phase and in clusters. A speculative conclusion forecasts developments in new laser techniques, highlighting how the exploitation of ever shorter laser pulses would permit the study and possible manipulation of the nuclear and electronic dynamics in molecules. 

An important point is that these advances have been complemented by the concomitant development of innovative pulse-characterisation procedures such that all the features of femtosecond optical pulses - their energy, shape, duration and phase - can be subject to quantitative in situ scrutiny during the course of experiments. Taken together, these resources enable femtosecond lasers to be applied to a whole range of ultrafast processes, from the various stages of plasma formation and nuclear fusion, through molecular fragmentation and collision processes to the crucial, individual events of photosynthesis. 

Kinetics of chemical reactions at liquid interfaces has often proven difficult to study because they include processes that occur on a variety of time scales [1]. The reactions depend on diffusion of reactants to the interface prior to reaction and diffusion of products away from the interface after the reaction. As a result, relatively little information about the interface dependent kinetic step can be gleaned because this step is usually faster than diffusion. This often leads to diffusion controlled interfacial rates. While often not the rate-determining step in interfacial chemical reactions, the dynamics at the interface still play an important and interesting role in interfacial chemical processes. Chemists interested in interfacial kinetics have devised a variety of complex reaction vessels to eliminate diffusion effects systematically and access the interfacial kinetics. However, deconvolution of two slow bulk diffusion processes to access the desired the fast interfacial kinetics, especially ultrafast processes, is generally not an effective way to measure the fast interfacial dynamics. Thus, methodology to probe the interface specifically has been developed. 

Nielsen SB, Soiling TI (2005) Are conical intersections responsible for the ultrafast processes of adenine, protonated adenine, and the corresponding nucleosides . Chem Phys Chem 6 1276... 

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. 

Note that the usage of 10-fs laser pulse leads to rich oscillatory components as well as these rapid kinetics in their pump-probe time-resolved profiles. Obviously in this timescale, the temperature T will have no meaning except for the initial condition before the pumping process. In addition, such oscillatory components may be due not only to vibrational coherence but also to electronic coherence. A challenging theoretical question may arise, for such a case, as to how one can describe these ultrafast processes theoretically. 

From the discussion presented in previous sections, vibrational relaxation (Appendix II) plays a very important role in the initial ET in photosynthetic RCs. This problem was first studied by Martin and co-workers [4] using Rb. capsulatas Dll. In this mutant, the ultrafast initial ET is suppressed and the ultrafast process taking place in the ps range is mainly due to vibrational relaxation. They have used the pumping laser at Xpump = 870 nm and probed at A.probe = 812 nm at 10 K. The laser pulse duration in this case is 80 fs. Their experimental results are shown in Fig. 16, where one can observe that the fs time-resolved spectra exhibit an oscillatory build-up. To analyze these results, we use the relation... 

Using this optical Keir cell the authors developed a technique to measure the lifetimes of atomic and molecular transitions on a picosecond time scale 138a), See also the reviews by Rentzepis 138b) about ultrafast processes and Merkelo 138c),... 

Quantum-dynamical modeling of ultrafast processes in complex molecular systems multiconfigurational system-bath dynamics using Gaussian wavepackets... 

Solvation in water was extensively studied and processes on different timescales were described ranging from 30 fs to several ps [8]. Due to our experimental resolution the shortest decay time we measure contains various superimposed contributions from the ultrafast processes presumably the inertial response of water and initial librational motions of molecules in the first solvation layer. 

Femtosecond Chemistry and Physics of Ultrafast Processes, M- Chergui, Ed., World... 

D. M. Jonas and G. R. Fleming, in Ultrafast Processes in Chemistry and Photobiology (Chemistry in the 21st Century IUPAC), M. A. El-Sayed, I. Tanaka, and Y. Molin, Eds., Blackwell Science, Oxford, 1995, p. 225. 

B. Reischl, Chem. Phys. Lett. 239, 173 (1995) R. de Vivie-Riedle, J. Gaus, V. Bonacic-Koutecky, J. Manz, B. Reischl, S. Rutz, E. Schreiber, and L. Woste, in Femtosecond Chemistry and Physics of Ultrafast Processes, M. Chergui, Ed., World Scientific, Singapore, 1996 B- Reischl, R. de Vivie-Riedle, S. Rutz, E. Schreiber, J. Chem. Phys. 104, 8857 (19%). 

A. Goldberg and J. Jortner, in Femtochemistry, Physics and Chemistry of Ultrafast Processes in Molecular Systems, M. Chergui, Ed., World Scientific, Singapore, 1996, p. 15. 

In conclusion, although the increased propensity for photodamage by femtosecond pulses and the requirement for an additional delayed laser pulse can be disadvantageous, time-resolved CARS microspectroscopy not only provides a means for efficient and complete nonresonant background suppression but also offers the prospect for monitoring ultrafast processes of molecular species inside a sub-femtoliter sample volume [64, 152-154]. 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

Fig. 3.8 Generic layout of a system suitable for studying very fast and ultrafast processes. Appropriate radiation sources may be a flash lamp, a laser or an electron accelerator, while optical, conductivity, or ESR detection systems may be employed.

Keywords ultrafast processes, photo-induced absorption, photoluminescence, organic ma-... 

A main feature of ultrafast processes under consideration takes place in the time scale shorter than picoseconds. Thus, it is necessary to employ the laser with pulse-duration 10 fsec to study these ultrafast processes. From the uncertainty principle AE At h/2 it can be seen that using this pulse-duration, numerous vibronic states can be coherently pumped (or excited) and thus the probing signal in a pump-probe experiment will contain the information of the dynamics of both population and coherence (or phase). In other words, in order to obtain the information of ultrafast dynamics it is... 

The presented theoretical approach to large molecular systems such as bacterial photosynthetic RCs can provide microscopic details of ultrafast radiationless transition taking place faster than 100 fsec. In particular, this approach establishes a standard model for treating such ultrafast processes of RCs. It is possible to analyze and provide similar details for wild-type RCs or other mutant RCs for example, for wild-type RCs of Rb. sphaeroides the electronic coupling of radiationless transition from the B band to the higher excitonic band and that from the higher excitonic band to the lower one are found to be 105.5 and 123 cm-1. For R26.Phe-a mutant RCs, the former coupling is 105 cm-1 and the latter is 123.7 cm-1. 

In summary, we have combined state of the art optical multichannel analyzer techniques with well established low repetition rate picosecond laser technology to construct an instrument capable of measuring transient spectra with unprecedented reliability. It is, in its present form, a powerful tool for the investigation of ultrafast processes in biological, chemical, and physical systems. We foresee straightforward extension of the technique to the use of fourth harmonic excitation (at 265 nm) and also a future capability to study gaseous as well as condensed phase samples over a more extended spectral range. 

Having posed the problem in this fashion, we must deal with the knotty question of what it is that would constitute a satisfactory answer. The orientations, locations, and even identities of the participating solvent molecules are constantly changing. So how can there be any kind of definitive mechanism to find The short answer is that for a general liquid process there is no such detailed mechanism, at least not one with any claim to generality. Ultrafast processes, however, are a different story — and vibrational friction is, in fact, an ultrafast process. 

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