Femtosecond pump-probe techniques

Figure 3.23 Schematic illustration of the femtosecond pump-probe technique used to initiate and follow the reactions of adsorbed species SHG, second harmonic generation SFG, sum frequency generation IRAS, infrared reflection-adsorption spectroscopy PPE, photons per event A R, change in reflectivity...

Figure 16.6 Femtosecond transition-state spectroscopy the figure shows the photodissociation of ICN femtosecond pump-probe technique (see text for details). Data adapted from Rosker et at, J. Chem. Phys., 6113, with permission of the American Institute of Physics...

Resonantly enhanced multiphoton ionization (REMPI), more generally called multiphoton ionization (MPI), of small molecules and clusters has been studied in recent years by a variety of techniques and is rather well understood. Different alkali molecules and clusters have been investigated by applying real-time MPI spectroscopy. The time evolution of the MPI signals is obtained by means of either picosecond or femtosecond pump probe techniques followed by mass-selective detection of the ionized aggregates. 

The outlook is good for applications of these picosecond methods to an increasing number of studies on reactive intermediates because of the limitations imposed by the time resolution of nanosecond methods and the generally greater challenges of the use of a femtosecond spectrometer. The pump-probe technique will be augmented in more widespread applications of the preparation-pump-probe method that permits the photophysics and photochemistry of reactive intermediates to be studied. 

Following another experimental approach, GWgoire et al [9] have tried to understand the influence of an increasing number of solvent molecules on the femtosecond dynamics of diatomic molecules, including the dimers Nal and Csl. Due to its relative simplicity, the isolated Nal molecule has been studied extensively with pump-probe techniques both experimentally [10], and theoretically [11,12], In this report, we investigate theoretically the femtosecond pump-probe ionization of Nal and Csl when aggregated with a molecule of acetonitrile CH3CN. 

Experimentally it has proven very difficult to investigate bimolecular reaction kinetics. Although optical techniques have been developed with femtosecond time resolution, the bimolecular nature of the reactions precludes standard femtosecond pump-probe experiments, as a common starting time for the reaction is not readily established. Here we initiate the fast bimolecular acid-base reaction by converting a very weak base, NO3 to a weak base ONOO" and follow the reaction ONOO + H+ as a function of [ft]. 

We have employed this phase-sensitive pump-probe technique to further investigate the multiphoton ionization of Na2 with 618-nm femtosecond pulses as discussed in the previous paragraph and have observed the interference of the A E and 2 Tlg wavepackets created by the first pulse and those created by the second pulse in the Na2+ signal. The amplitude of the high-frequency oscillations in the Na2+ signal was obtained as a function of pump-probe delay by filtering the transient with the laser frequency. It is shown in Fig. 8 (top). Below the averaged Na2+ transient of Fig. 4 is... 

Ultrafast molecular elimination of iodine from IF2C-CF2I has been studied using the velocity map ion imaging technique in combination with femtosecond pump-probe laser excitation.51 By varying the femtosecond delay between pump and probe pulse, it has been found that elimination of molecular iodine is a concerted process, although the two carbon-iodine bonds are not broken synchronously. 

Our goals were to elucidate important physical concepts in energy-angle resolved TRPES and to illustrate the range of its applicability to problems in molecular dynamics. We discussed general aspects of femtosecond pump-probe experiments from both the wave packet and the frequency domain point of view. Experimentalists are, in principle, free to choose a final state in which to observe the wave packet dynamics of interest. We emphasized the critical role of the choice of the final state in determining both the experimental technique (e.g., collection of photons or particles) and the information content of an experiment (averaged or state-resolved). The molecular ionization continuum has a rich structure that can act as a template onto which multidimensional wave packet dynamics may be projected. The set of electronic states of the cation are sensitive to both the electronic population... 

While the first experiments of time-resolved IR spectroscopy were conducted with pulse durations exceeding 10 ps, the improved performance of laser systems now offers subpicosecond (12) to femtosecond (13-15) pulses in the infrared spectral region. In addition, the pump-probe techniques have been supplemented by applications of higher-order methods, e.g., IR photon echo observations (16). 

A simplified view of the early processes in electron solvation is given in Figure 7. Initially, electron pulse radiolysis was the main tool for the experimental study of the formation and dynamics of electrons in liquids (Chapter 2), first in the nanosecond time range in viscous alcohols [23], later in the picosecond time range [24,25]. Subsequently, laser techniques have achieved better time resolution than pulse radiolysis and femtosecond pump-probe laser experiments have led to observations of the electron solvation on the sub-picosecond to picosecond time scales. The pioneering studies of Migus et al. [26] in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 8). The electron solvation process could thus be depicted by sequential stepwise relaxation cascades, each of the successive considered species or... 

The validity of the physics that adopts the point of view of decaying states depends on the characteristics of the process of excitation-preparation. Specifically, one must assume that the duration of the pulse of excitation energy is much shorter than the lifetime of the unstable state. This implies that indeed the system is prepared in a nonstationary state at f = 0, i.e., in the localized state (T o/ Eo)/ while losing memory of the excitation step. For long-lived unstable states, this is expected to be achievable easily. For shortlived unstable atomic or molecular states, say of the order of 10 s, this is also achievable, in principle, via modern pump-probe techniques with time-delays in the range of a few femtoseconds or of a couple of hundreds of attoseconds. 

The femtosecond pump-probe absorption spectroscopy was used for the investigation of the SI-photoisomerization of cis-stilbene in compressed solvents [20]. The authors of the work [21] demonstrated a technique for femtosecond time-resolved optical pump-probe spectroscopy that allowed to scan over a nanosecond time delay at a kilohertz scan rate without mechanical delay line. Two mode-locked femtosecond lasers with 1 GHz repetition rate were linked at a fixed difference frequency of =11 kHz. One laser delivers the pump pulses, the other provides the probe pulses. The techniques enabled high-speed scanning over a 1-ns time delay with a time resolution of 230 fs. 

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