Ultrafast

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

Mohrschladt R, Schroeder J, Troe J, Vdhringer P and Votsmeier M 1994 Solvent influence on barrier crossing in the S. -state of c/s- and trans- st ff stiibene Ultrafast Phenomena /Xed P F Barbara et al (New York Springer) pp 499-503... 

Many of the most interesting current developments in electronic spectroscopy are addressed in special chapters of their own in this encyclopedia. The reader is referred especially to sections B2.1 on ultrafast spectroscopy. Cl.5 on single molecule spectroscopy, C3.2 on electron transfer, and C3.3 on energy transfer. Additional topics on electronic spectroscopy will also be found in many other chapters. 

Biswas N and Umapathy S 1998 Resonance Raman spectroscopy and ultrafast chemical dynamics Curr. Sol. 74 328-40... 

Plenary 11. W Kiefer et al, e-mail address wolfgang.kiefer mail.imi-wue.de (TR CARS). Ultrafast impulsive preparation of ground state and excited state wavepackets by impulsive CARS with REMPI detection in potassium and iodine duners. 

Morita N and Yajima T 1984 Ultrafast-time-resolved coherent transient spectroscopy with incoherent light Rhys. Rev. A 30 2525-36... 

Albrecht A C, Smith S P, Tan D, Schaertel S A and DeMott D 1995 Ultrasharp spectra and ultrafast timing from noisy coherence in four wave mixing Laser Rhys. 5 667-75... 

Vanden Bout D and Berg M 1995 Ultrafast Raman echo experiments in liquids J. Raman Spectrosc. 26 503-11... 

Muller L J, Vanden Bout D and Berg M 1993 Broadening of vibrational lines by attractive forces ultrafast Raman echo experiments in a CH2l CDCl2 mixture J. Chem. Phys. 99 810-19... 

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)... 

Especially with LTG GaAs, materials became available that were nearly ideal for time-resolved THz spectroscopy. Due to the low growth temperature and the slight As excess incorporated, clusters are fonned which act as recombination sites for the excited carriers, leading to lifetimes of <250 fs [45], With such recombination lifetunes, THz radiators such as dipole anteimae or log-periodic spirals placed onto optoelectronic substrates and pumped with ultrafast lasers can be used to generate sub-picosecond pulses with optical bandwidths of 2-4 THz. Moreover, coherent sub-picosecond detection is possible, which enables both... 

Wokosin D L, Centonze V, White J G, Armstrong D, Robertson G and Ferguson A I 1996 All-solid-state ultrafast lasers facilitate multiphoton excitation fluorescence imaging IEEE J. Sel. Top. Quantum Electron. 21051-65... 

STM has not as yet proved to be easily applicable to the area of ultrafast surface phenomena. Nevertheless, some success has been achieved in the direct observation of dynamic processes with a larger timescale. Kitamura et al [23], using a high-temperature STM to scan single lines repeatedly and to display the results as a time-ver.sn.s-position pseudoimage, were able to follow the difflision of atomic-scale vacancies on a heated Si(OOl) surface in real time. They were able to show that vacancy diffusion proceeds exclusively in one dimension, along the dimer row. 

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. 

The development of ultrafast spectroscopy has paralleled progress in the teclmical aspects of pulse fomiation [Uj. Because mode-locked laser sources are tunable only with diflSculty, until recently the most heavily studied physical and chemical systems were those that had strong electronic absorption spectra in the neighbourhood of conveniently produced wavelengths. 

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. 

The shortest optical pulses actually used so far (1998) in ultrafast spectroscopic experunents were obtained by Shank and co-workers from an amplified CPM laser [ ]. In these extraordinary experiments, a sequence of a pair of prisms... 

Time-resolved fluorescence is perhaps the most direct experunent in the ultrafast spectroscopist s palette. Because only one laser pulse interacts with the sample, the mediod is essentially free of the problems with field-matter time orderings that arise in all of the subsequently discussed multipulse methods. The signal... 

Shank C V 1986 Investigation of ultrafast phenomena in the femtosecond domain Science 233 1276-80... 

Raksi F, Wilson K R, Jiang Z, Iklef A, Cote C Y and Kieffer J-C 1996 Ultrafast x-ray absorption probing of a chemical reaction J. Chem. Phys. 104 6066-99... 

Katzenellenbogen N and Grischkowsky D 1991 Efficient generation of 380 fs pulses of THz radiation by ultrafast laser pulse excitation of a biased metal-semiconductor interface Appl. Phys. Lett. 58 222-4... 

Khundkar L R and Zewail A H 1990 Ultrafast molecular reaction dynamics in real-time progress over a decade Annu. Rev. Phys. Chem. 41 15-60... 

Gruebele M and Zewail A H 1990 Ultrafast reaction dynamics Phys. Today 43 24-33... 

Ippen E 1997 Characterizing optical components for ultrafast laser applications Optics 1997/98 Catalog (Irvine, CA Newport Corp.) pp 8-2-8-3... 

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