Femtosecond time-resolved experiments

The success of femtosecond time-resolved experiments depends on having lasers able to provide sufficiently short and intense pulses, together with an effective integration of all the other instrumentation, optical components, and computers necessary for the enterprise, and on the susceptibility of starting material to photoexcitation. These requirements are readily understood. Success also depends on coherent formation of the excited system through the pump pulse, a consideration not so readily grasped. 

Several recent reviews have presented broad overviews of ultrafast time-resolved spectroscopy [3-6], We shall concentrate instead on a selected, rather small subset of femtosecond time-resolved experiments carried out (and to a very limited extent, proposed) to date. In particular, we shall review experiments in which phase-coherent electronic or, more often, nuclear motion is induced and monitored with time resolution of less than 100 fs. The main reason for selectivity on this basis is the rather ubiquitous appearance of phase-coherent effects (especially vibrational phase coherence) in femtosecond spectroscopy. As will be discussed, nearly any spectroscopy experiment on molecular or condensed-phase systems is likely to involve phase-coherent vibrational motion if the time scale becomes short enough. Since the coherent spectral bandwidth of a femtosecond pulse often exceeds collective or molecular vibrational frequencies, such a pulse may perturb and be perturbed by a medium in a qualitatively different manner than a longer pulse of comparable peak power. The resulting spectroscopic possibilities are of special interest to these reviewers. 

In a nonperturbative treatment of radiationless decay, Pn t) is generally not simply an exponential function the nonperturbative rate kn t) is thus time-dependent. It should be stressed that Pn t) and thus kn t) can be measured explicitly, at least in principle, in a femtosecond time-resolved experiment. There is thus no need to invoke a long-time limit or to perform an average over time to obtain time-independent rates. 

To observe ultrafast fragmentation of excited alkali clusters the appropriate tool is real-time MPI spectroscopy. This technique allows the mass-selected detection of the ultrafast photodissociation with high sensitivity. In 1992 Gerber and coworkers presented the first femtosecond time-resolved experiments in cluster physics [32, 131, 132], showing differences in the fragmentation behavior of Nan<2i clusters dependent on the excitation at different wavelengths. 

The last technique employed by these authors is very useful because it allows to do femtosecond time resolved experiments simply by using incoherent nanosecond laser pulses. 

Employing all of the TR results in conjunction with results from femtosecond time-resolved transient absorption (fs-TA) and femtosecond time-resolved Kerr gated fluorescence (fs-KTRF) experiments enables a reaction mechanism to be developed... 

After the introduction of frequency resolved CARS by Maker and Terhune [1], time resolved experiments became possible with the invention of high power lasers with femtosecond resolution. Leonhardt [2] and for example Hayden [3] performed femtosecond CARS experiments in liquids. A first femtosecond time resolved CARS experiment in gas phase was performed by Motzkus et. al. [4] where the wave packet dynamics of the dissociation of Nal was monitored. The first observation of wave packet dynamics in gaseous iodine was reported by Schmitt et al. [5]. They were able to observe dynamics in both, the ground and excited state with the same experiment. A summary of high resolution spectroscopy in gas phase by nonlinear methods is given by Lang et al. [6]. 

G. Gerber In our time-resolved experiments on the NaafZ ) state we observe the symmetric stretch even for long delay times. From nanosecond laser and CW laser spectroscopy it is well known that the B state does not decay on femtosecond or picosecond time scales. So I do not see how the decay in the picosecond experiment by Prof. Woste can be understood and how the evolution of the B state symmetric stretch into the pseudorotation and the radial motion can occur. 

AT DNA double strand oligomers with sodium counterions and a length of 20 base pairs were obtained from Biotherm, and were dissolved in water and dried on a CaF2 window at 293 K in an atmosphere of 52% relative humidity (saturated solution of NaHSC. IDO at 20° Celsius [60]). This results in DNA samples with approximately 4 to 6 water molecules per base pair [37] (sample thickness 6.5 pm). It has been reported that under these conditions AT DNA oligomers adopt the B -form [35], Femtosecond time-resolved IR pump-probe experiments were performed with two independently tunable femtosecond pulses generated by parametric conversion processes pumped by a regenerative Ti sapphire laser system (800 nm, repetition rate 1 kHz, pulse duration 100 fs) [61]. The central frequency of the pump pulse was varied from 1630 to 1760 cm-1 and the probe was centred around 1650 cm-1 or 3200... 

To help completely understand the ultrafast interface ET between dye molecules and semiconductor nanoparticles, it is desirable to experimentally measure the femtosecond time-resolved spectra (i.e., probing signal in the pump-probe experiment) at various pumping wavelengths. Changing laser pulse-durations will also be useful. 

Photoionization always produces two species available for analysis the ion and the electron. By measuring both photoelectrons and photoions in coincidence, the kinetic electron may be assigned to its correlated parent ion partner, which may be identified by mass spectrometry. The extension of the photoelectron-photoion-coincidence (PEPICO) technique to the femtosecond time-resolved domain was shown to be very important for studies of dynamics in clusters [131, 132]. In these experiments, a simple yet efficient permanent magnet design magnetic bottle electron spectrometer was used for photoelectron... 

Since the pulse time is so short (see Sec. 3.6.2.2.3) one can coherently excite many vibrational modes at a time and monitor relaxation processes in real time. The first reported femtosecond time-resolved CARS experiments (Leonhardt et al., 1987 Zinth et al., 1988) showed beautiful beating patterns and fast decays of the coherent signal for several molecular liquids. The existence of an intermolecular coherence transfer effect was suggested from the analysis of the beating patterns (Rosker et al., 1986). Subsequent studies by Okamoto and Yoshihara (1990) include the vibrational dephasing of the 992 cm benzene mode. A fast dephasing process was found that is possibly related to... 

Ru(bipy)3 + is the prototype of a very large family of MLCT species. In the standard model of the photoprocesses of this compound, a photon excites the molecule to an initial Frank-Condon singlet state, MLCT, that rapidly transforms to a triplet, MLCT, with a quantum yield of near unity. Femtosecond pump probe experiments have established a half-life of about 100 fs for the formation of the triplet state. Recent studies utilizing femtosecond time-resolved fluorescence emission spectroscopy has observed fluorescence emission from the Frank-Condon state itself and the hfetime of this state has been estimated to be 40 15 fs. ... 

This review describes some of the recent developments in materials which exhibit enhanced two-photon absorption that can initiate photopolymerization or up-converted emission. Various optical methods including femtosecond time-resolved pump-probe experiments to characterize the two-photon properties are discussed. Finally, the applications of two-photon processes to optical power limiting, up-converted lasing, 3-D data storage, 3-D micro-fabrication, two-photon fluorescence microscopy and bio-imaging, and two-photon photodynamic therapy are presented. 

Abstract The density matrix method is a powerful theoretical technique to describe the ultrafast processes and to analyze the femtosecond time-resolved spectra in the pump-probe experiment. The dynamics of population and coherence of the system can be described by the evolution of density matrix elements. In this chapter, the applications of density matrix method on internal conversion and vibrational relaxation processes will be presented. As an example, the ultfafast internal conversion process of Jt jt nn transition of pyrazine will be presented,... 

Most recently, the earlier generation of femtosecond time-resolved absorption measurements has been superseded by an extensive set of new experiments on Rb.sphaeroides (up to... 

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