Pumping reactions pulse evolution

Figure 3 System for gas chromatographic measurement of Oz evolution from superconductors dissolved in acid (29) (A) enlarged view of reaction flask, (B) circulating system including flask (G), NaOH U-tube (T), three-way valves (S), 1 mL loop (L), pulse pump (PP) and vacuum system (V), At the right is the gas chromatograph (G.C.). (From Reference 29. With Permission.)...

On the experimental side, the chemical dynamics on the state-to-state level is being studied via molecular-beam and laser techniques [2]. Alternative, and complementary, techniques have been developed in order to study the real-time evolution of elementary reactions [3]. Thus, the time resolution in the observation of chemical reactions has increased dramatically over the last decades. The race against time has recently reached the ultimate femtosecond resolution with the direct observation of chemical reactions as they proceed along the reaction path via transition states from reactants to products. This spectacular achievement was made possible by the development of femtosecond lasers, that is, laser pulses with a duration as short as a few femtoseconds. In a typical experiment two laser pulses are used, a pump pulse and a probe... 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

Femtosecond time-resolved methods involve a pump-probe configuration in which an ultrafast pump pulse initiates a reaction or, more generally, creates a nonstationary state or wave packet, the evolution of which is monitored as a function of time by means of a suitable probe pulse. Time-resolved or wave... 

Before proceeding to discuss these various methods it is perhaps helpful to clarify terminology with regard to the terms time-resolved and transient RR. The latter term is used in the case where a single laser pulse is used to both photolyze and probe the sample. The term time-resolved is appropriate in cases where the beam used to probe the photo-lyzed sample is temporally delayed with respect to the photolysis pulse (the pump pulse). In such an experiment the reaction is initiated by a short photolysis pulse, and the RR spectrum is acquired using a probe pulse which is delayed with respect to the pump pulse. The temporal evolution of the system can thus be documented by performing a series of such experiments where the delay time between the pump and probe pulses is varied. 

Two pulses with a FWHM of 11 and 180 fs were selected for excitation. As expected from the previous S /So studies, the prepared excited wavepackets show different subsequent time evolutions. The longer pulse results in a broader wavepacket distribution on the surface, spreading over large areas of the reaction surface, whereas the short fs-pulse excitation results in a more localized wavepacket, dominantly accelerated and focused into the C 2-Coln. Following the relaxation transfer of this localized wavepacket reveals that nearly the complete transfer occurs through the symmetric conical intersection. Thus the influence of changing only one parameter, the pump-pulse duration, points to the possibility to control the molecular dynamics and thereby enhance the transfer through one of the conical intersections. 

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