Femtosecond spectroscopy, applications

Seidner L, Stock G and Domcke W 1995 Nonperturbative approach to femtosecond spectroscopy - general theory and application to multidimensional nonadiabatic photoisomerization processes J. Chem. Phys. 103 4002... 

Mialocq J.-C. and Gustavsson T. (2001) Investigation of Femtosecond Chemical Reactivity by Means of Fluorescence Up-Conversion, in Valeur B. and Brochon J. C. (Eds), New Trends in Fluorescence Spectroscopy. Applications to Chemical and Life Sciences, Springer-Verlag, Berlin, pp. 61-80. 

For the future, it is clear that dynamics methods are almost essential if one is going to examine the interesting results that are coming from femtosecond spectroscopy and to study quantum yields. These methods are just beginning to be exploited, and this is an exciting new direction for quantum chemistry. We have not commented on the role of the solvent or the role of the environment provided by a biochemical system. There are no special problems related to excited state chemistry for the former, and one can look forward to applications to biochemical systems to appear in the near future. 

Ultrafast spectroscopy is usually distinguished from fast spectroscopy in that the former studies processes which occur in less than 10 ps. The commercial availability of solid-state femtosecond lasers, such as Ti-sapphire, makes femtosecond spectroscopy available for a wide range of applications [29]. 

Many new boxed essays have been added that emphasize applications of chemistry in the real world and show how new techniques such as STM and femtosecond spectroscopy are providing revolutionary insights into the nature of matter. 

During the past three decades, we have seen dramatic progress in the generation of ultrafast laser pulses and their application to the study of phenomena in many disciplines [10, 13, 137-150]. As a consequence a kind of Temtofascination [14] has arisen and forms the basis of the so-called femtosecond spectroscopy . 

With the exception of the femtosecond spectroscopy, which requires a lot of experimental and technical effort, most of the other experiments can be performed with a conventional macro- or micro-Raman setup and a cryostat. With the same setup, luminescence measurements can be performed, which give further information about the electronic states. All these points confirm the fast and easy applicability of Raman spectroscopy for the structural investigation of II-VI-semiconductor nanostructures. 

H. Bleier, Kinetics of charged excitations in conjugated polymers an example for the application of picosecond- and femtosecond spectroscopy, in Organic Materials for Photonics Science end Technology, Z. Gerbi, ed. Amsterdam North Holland, 1993 77-102, and other articles in the same source. 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

Both molecular dynamics studies and femtosecond laser spectroscopy results show that molecules with a sufficient amount of energy to react often vibrate until the nuclei follow a path that leads to the reaction coordinate. Dynamical calculations, called trajectory calculations, are an application of the molecular dynamics method that can be performed at semiempirical or ah initio levels of theory. See Chapter 19 for further details. 

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

Structure, then the time-resolved photoelectron spectra [20, 21] could reveal signatures of two different intermediate structures, representing two different pathways on the PES. Transient absorption spectroscopy and other femtosecond time-resolved techniques may also be applicable to this problem. 

In addition to the natural improvements expected in the accuracy of the measurements, and the increased scope in the types of systems examined, new techniques go beyond the issue of thermochemistry to allow for very detailed studies of reaction dynamics. The investigation by Zewail and co-workers of the reactivity of planar COT" on the femtosecond time scale is likely only the beginning. Time-resolved photoelectron spectroscopy, for example, has recently been used to map the potential energy surfaces for the dissociation of simple ions IBr and l2. " Although applications in the field of organic reactive molecules are likely far off, they are now possible. 

It is very likely that the metal-insulator transition, the unusual catalytic properties, the unusual degree of chemical reactivity, and perhaps even some of the ultramagnetic properties of metal clusters are all linked intimately with the dynamic, vibronic processes inherent in these systems. Consequently, the combination of pump-probe spectroscopy on the femtosecond time scale with theoretical calculations of wavepacket propagation on just this scale offers a tantalizing way to address this class of problems [5]. Here we describe the application of these methods to several kinds of metal clusters with applications to some specific, typical systems first, to the simplest examples of unperturbed dimers then, to trimers, in which internal vibrational redistribution (IVR) starts to play a central role and finally, to larger clusters, where dissociative processes become dominant. 

Gruebele, M., Roberts, G., Dantus, M., Bowman, R.M., and Zewail, A.H. (1990). Femtosecond temporal spectroscopy and direct inversion to the potential Application to iodine, Chem. Phys. Lett. 166, 459-469. 

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