State femtosecond

Less pronounced thermal diffusion provides better lateral and depth resolution and is the basis of successful application of femtosecond pulses in material processing and microstructuring [4.231, 4.232]. All-solid-state femtosecond lasers with a pulse duration of 100-200 fs and a pulse energy of approximately 1 mj have recently become commercially available [4.233, 4.234]. 

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]. 

The conventional narrowband CARS process probes one particular vibrational mode selectively. Conversely, so-called broadband CARS measurements, using ultrashort pulsed laser sources, can probe multiple RS-active vibrational modes simultaneously [19, 29-31]. In the case of two-beam broadband CARS method, one of the two beams has a narrow bandwidth and the other a broad bandwidth. Therefore, the technical issue is how to generate these beams from a single laser source. Typically, subpicosecond pulses from a conventional solid-state femtosecond laser... 

During the last decade, solid-state lasers captured the market and substituted the complex dye systems more and more (Fig. 1). The breakthrough for solid-state femtosecond oscillators was connected with the development of the Kerr lens mode-locking technique for the Tiisapphire laser [7]. The simple Tiisapphire cavity contains the active medium (Tiisapphire rod) and dispersive elements. Kerr lensing in a Tiisapphire rod develops due to an intensity-dependent refractive index across the spatial beam profile yielding a self-focusing of the laser beam. With an additional aperture in the beam... 

In 1997, the old dye laser record with respect to the shortest pulses was improved by a Ti sapphire laser. Pulses with a duration of 5 fs could be generated [9]. Despite the many advantages of solid-state femtosecond laser systems compared to dye lasers (power, compactness, no toxic dyes and solvents etc.), one drawback should be noted. The passive mode-locking process is not self-starting in many cases. 

If there is an ultimate method for the direct observation of transition states femtosecond spectroscopy could well qualify for it. This method, largely developed by Zewail and co-workers, is now more and more used to study - even complex - uni- and bimolecular processes. It employs an extremely short pulse from a femtosecond laser to initiate a... 

The use of the lasers with femtosecond duration opens the way to overcome the problem of fractionation in the most radical way. The first generation of all-solid-state femtosecond lasers is now commercially available, although expensive. For such a fast energy deposition the non-thermal mechanisms of material removal (e g., ion repulsions due to Coulomb forces) can be significant. The non-thermal character of material removal with fs pulses enables the depth profiling of a solid sample with a few nm resolution, which was impossible with ns pulses because of melting and mixing of different layers. 

THEORETICAL EXPLORATION OF SINGLE AND MULTI STATE FEMTOSECOND NUCLEAR DYNAMICS OF SMALL METALLIC CLUSTERS USING THE DF METHOD... 

In summary, a fast photodissociation process coupled with a rather low oscillator strength might be the reason why cw spectroscopy was unable to detect this electronic state. Femtosecond spectroscopy with high peak power and broad spectral width of the exciting laser pulses, combined with the probing of the excited electronic state within a few picoseconds, however, opens a time window to efficiently detect this state and its dynamics. 

The progress of both technology and applications in the field of ultrafast processes within the last 20 years has been of remarkable dimensions. Not least because of the advent of all-solid-state femtosecond laser sources and because the extension of laser wavelengths by frequency conversion techniques has provided a variety of high-performance sources for extremely short light pulses. These excellent sources have enabled researchers all over the world extensively and quite successfully to investigate ultrafast phenomena in physical, chemical, and biological systems. 

Because the carboxylates have relatively long-lived photoexcited states, femtoseconds TRIR studies allow one to probe the charge distribution in the SI and T1 states as a function of time. Particularly helpful in this regard are ligands that have reporter IR groups that are located in a region of the infrared spectrum that is devoid of other vibrations. Thus, C=X bonds, where X = 0, N, or C, that are IR active in the region 2000-2200 cm" are ideal. 

Much of the previous section dealt with two-level systems. Real molecules, however, are not two-level systems for many purposes there are only two electronic states that participate, but each of these electronic states has many states corresponding to different quantum levels for vibration and rotation. A coherent femtosecond pulse has a bandwidth which may span many vibrational levels when the pulse impinges on the molecule it excites a coherent superposition of all tliese vibrational states—a vibrational wavepacket. In this section we deal with excitation by one or two femtosecond optical pulses, as well as continuous wave excitation in section A 1.6.4 we will use the concepts developed here to understand nonlinear molecular electronic spectroscopy. 

Lee S-Y 1995 Wave-packet model of dynamic dispersed and integrated pump-probe signals in femtosecond transition state spectroscopy Femtosecond Chemistry ed J Manz and L Wdste (Heidelberg VCH)... 

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

Schinke R and Huber J R 1995 Molecular dynamics in excited electronic states—time-dependent wave packet studies Femtosecond Chemistry Proc. Berlin Conf. Femtosecond Chemistry (Berlin, March 1993) (Weinheim Verlag Chemie)... 

Seifert F, Petrov V and Woerner M 1994 Solid-state laser system for the generation of midinfrared femtosecond pulses tunable from 3.3-Mu-M to 10-Mu-M Opt. Lett. 19 2009-11... 

Plenary 7 7. P M Champion et al, e-mail address champ neu.edu (TRRRS). Femtosecond impulsive preparation and timing of ground and excited state Raman coherences in heme proteins. Discovery of coherence transfer along a de-ligation coordinate. See above for fiirther connnent. 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Zewail A H 1991 Femtosecond transition-state dynamics Faraday Discuss. Chem. Soc. 91 207-37... 

Dantus M, Rosker M J and Zewail A H 1987 Real-time femtosecond probing of transition states in chemical reactions J. Chem. Phys. 87 2395-7... 

Pedersen S, Herek J L and Zewail A H 1994 The validity of the Diradical hypothesis direct femtosecond studies of the transition-state structures Science 266 1359-64... 

Kovalenko S A, Ernsting N P and Ruthmann J 1996 Femtosecond hole-burning spectroscopy of the dye DCM in solution the transition from the locally excited to a charge-transfer state Chem. Phys. Lett. 258 445-54... 

Edington M D, Riter R E and Beck W F 1997 Femtosecond transient hole-burning detection of interexciton-state radiationless decay in allophycocyanin trimers J. Phys. Chem. B 101 4473-7... 

Dexheimer S L, Wang Q, Peteanu L A, Pollard WT, Mathies R A and Shank C V 1992 Femtosecond impulsive excitation of nonstationary vibrational states in bacteriorhodopsin Chem. Phys. Lett. 188 61-6... 

Wynne K, Haran G, Reid G D, Moser 0 0, Dutton P L and Hochstrasser R M 1996 Femtosecond infrared spectroscopy of low-lying excited states in reaction centers of Rhodobacter sphaeroides J. Rhys. Chem. 100 5140-8... 

Weiner A M and Ippen E P 1985 Femtosecond excited state relaxation of dye molecules in solution Chem. Phys. Lett. 114 456-60... 

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