Femtosecond pumps

Femtosecond pump-probe experiments have burgeoned in the last ten years, and this field is now connnonly referred to as laser femtochemistry [26, 27, 28 and 22],... 

Pollard W T, Lee S-Y and Mathies R A 1990 Wavepacket theory of dynamic absorption spectra in femtosecond pump-probe experiments J. Chem. Phys. 92 4012... 

It is interesting to note that the use of correlation functions in spectroscopy is an old topic, and has been used to derive, for example, infrared (IR) spectra, from classical trajectories [134,135]. Stock and Miller have recently extended this approach, and derived expressions for obtaining electronic and femtosecond pump-probe spectra from classical trajectories [136]. 

Figure 10-4. Temporal behavior of the pholoinduccd transmission changes in LPPP alter excitation with a femtosecond pump pulse at 400 nnt. The two curves correspond to probe photon eneigies of 2.48 eV (dotted line) and 1.91 eV (solid line). At 2.48 eV the transmission change is positive due to stimulated emission (SE) while a photoin-dueed absorption (PIA) is observed at 1.91 eV (according to Ref.(24J).

Nechay, B.A., Siegner, U.,Achermann, M., Bielefeldt, H. and Keller, U. (1999) Femtosecond pump-probe near-field optical microscopy. Rev. Sci. Instrum., 70, 2758-2764. 

The results for the A state show that a different mechanism is operative. A series of femtosecond pump-probe experiments were performed at wavelengths corresponding to the Rydberg states A (v = 0,1,2) of ammonia molecules.64-66,68,69 The wavelengths used to access these vibrational levels were 214 nm, 211 nm, and 208 nm for the pump laser and 321 nm, 316.5 nm, and 312 nm for the probe laser, respectively. 

Fig. 7 Femtosecond pump-probe data for TD 2765 (molecular structure shown left). The probe wavelength is set at 670 nm and the pump wavelengths are (1) 710 nm, (2) 700 nm, and (3) 690 nm. See [62] for additional details...

Describing complex wave-packet motion on the two coupled potential energy surfaces, this quantity is also of interest since it can be monitored in femtosecond pump-probe experiments [163]. In fact, it has been shown in Ref. 126 employing again the quasi-classical approximation (104) that the time-and frequency-resolved stimulated emission spectrum is nicely reproduced by the PO calculation. Hence vibronic POs may provide a clear and physically appealing interpretation of femtosecond experiments reflecting coherent electron transfer. We note that POs have also been used in semiclassical trace formulas to calculate spectral response functions [3]. 

Figure 20.3. The time evolution of photoexcited sodium iodide. The signal intensity proportional to covalent ([Na—I] ) stmctures oscillates following the femtosecond pump pulse the [Na—I] vibrates between covalent and ionic structures before crossing to the lower potential energy surface and dissociating to give Na and I atoms. [Reproduced with pemtission from A. H. Zewail, J. Phys. Chem. A. 2000, 104, 5660.]...

Importantly, Cl s seems to be involved in many classes of physical, chemical and biological processes, from pericyclic reactions to the complex light harvesting and energy conversion functions of chromophores in proteins (See in this volume) and others amply described in this conference. In contrast, direct experimental information on the passage of the vibrational wavepacket through or near Cl s is less abundant. It mostly concerns, femtosecond pump-probe experiments on isolated organic molecules in the gas phase. 

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]. 

A simple approach for the calculation of femtosecond pump-probe spectra for electronically nonadiabatic systems... 

Femtosecond pump-probe investigation of primary photoinduced processes in C60/Sn nanostructures... 

In short, a Ti Sapphire laser system in combination with two four-pass OPAs were used to create the femtosecond pump and probe pulses at variable wavelengths. The wavelength of the pump laser was chosen to be 530 nm for all experiments presented in the following. For the probe laser we used the fundamental output at 800 nm as well as wavelengths between 300 and 400 nm. The light scattered from the oriented single crystal samples was analyzed by a monochromator equipped with a photomultiplier tube. A more detailed description of the experimental setup is given in ref. 1. 

A simple approach lor the calculation of Femtosecond pump-probe spectra for electronically nonndiiihuuc systems... 

Figure 6. Potential energy curves relevant for the pump-probe experiment on the 2 1 double minimum state of Na2- A femtosecond pump pulse forms a wavepacket at the inner turning point above the barrier. A second probe pulse (540 nm) can either only ionize (excitation of the ground state of Na2+) or ionize and fragment (excitation of the repulsive state) the molecule depending on the location of the wavepacket.

My question to Prof. Gerber is the following Could you please explain the different virtues of femtosecond pump-pulse experiments versus ultrashort zero electron kinetic energy (ZEKE) spectroscopy Do they yield complementary information on the molecular dynamics or are there specific domains where one of them should be preferred with respect to the other ... 

G. Gerber At moderate laser intensities we do see in femtosecond pump-probe experiments a very similar slow time and long time dynamics in all cluster sizes n > 5 up to n = 50 (largest size investigated up to now) irrespective of the charge state of the particular Hg cluster. From single-pulse TOF mass spectrometry we infer that the... 

There are three quite distinct perturbations that could have caused the vibrational motion to delocalize. The first is that the potential that governs the vibrational motion is not harmonic and it is not harmonic either in the simulations or, of course, in the real molecule. In our experience, under most circumstances this is the effect that comes in at the earliest times. We have seen this to be the case not only for a chemical reaction but also in simulations of femtosecond pump-probe experi-... 

The wavepacket calculation for the femtosecond pump-probe experiment presented in Fig. 16 (bottom) is the result of the first consistent ab initio treatment for three coupled potential-energy surfaces in the complete three-dimensional vibrational space of the Naa molecule. In order to simulate the experimental femtosecond ion signal, the experimental pulse parameters were used duration A/fWhm = 120 fs, intensity I - 520 MW/cm2, and central... 

R. D. Levine As emphasized by the speakers on femtosecond pumping schemes, an important point is that the initial excitation is localized within the Franck-Condon regime. The question is whether the sheer localization can be used to advantage to induce laser-selective chemistry (K. L. Kompa and R. D. Levine, Acc. Chem. Res. 27, 91 (1994)]. As we understand better the topography of potential-energy surfaces for polyatomic molecules, it may be possible to launch the system with such initial conditions that it will, of its own accord, proceed to cross a particular transition state and so exit toward a particular set of products. 

Ellington et al.40) used femtosecond pump-probe spectroscopy to probe directly the arrival of electrons injected into the TiOz film with near- and mid-IR that probe the absorption at 1.52 jum and in the range of 4.1-7.0 jUm. Their measurements indicate an instrument limited 50 fsec upper limit on the electron injection time. These observations suggest that electron injection from Dye 2 to... 

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. 

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