Ultrafast Experimental Results

Calculating the exact response of a semiconductor heterostructure to an ultrafast laser pulse poses a daunting challenge. Fortunately, several approximate methods have been developed that encompass most of the dominant physical effects. In this work a model Hamiltonian approach is adopted to make contact with previous advances in quantum control theory. This method can be systematically improved to obtain agreement with existing experimental results. One of the main goals of this research is to evaluate the validity of the model, and to discover the conditions under which it can be reliably applied. 

Easy availability of ultrafast high intensity lasers has fuelled the dream of their use as molecular scissors to cleave selected bonds (1-3). Theoretical approaches to laser assisted control of chemical reactions have kept pace and demonstrated remarkable success (4,5) with experimental results (6-9) buttressing the theoretical claims. The different tablished theoretical approaches to control have been reviewed recently (10). While the focus of these theoretical approaches has been on field design, the photodissociation yield has also been found to be extremely sensitive to the initial vibrational state from which photolysis is induced and results for (11), HI (12,13), HCl (14) and HOD (2,3,15,16) reveal a crucial role for the initial state of the system in product selectivity and enhancement. This critical dependence on initial vibrational state indicates that a suitably optimized linear superposition of the field free vibrational states may be another route to selective control of photodissociation. 

From the discussion presented in previous sections, vibrational relaxation (Appendix II) plays a very important role in the initial ET in photosynthetic RCs. This problem was first studied by Martin and co-workers [4] using Rb. capsulatas Dll. In this mutant, the ultrafast initial ET is suppressed and the ultrafast process taking place in the ps range is mainly due to vibrational relaxation. They have used the pumping laser at Xpump = 870 nm and probed at A.probe = 812 nm at 10 K. The laser pulse duration in this case is 80 fs. Their experimental results are shown in Fig. 16, where one can observe that the fs time-resolved spectra exhibit an oscillatory build-up. To analyze these results, we use the relation... 

Fig. 7.6 Effect of sequential addition of the ultrafast component of the solvent orientational motion on the values in MeOH at 298 K. Closed circles are experimental results curves (1), (2) and (3) were obtained by the molecular theory by including the first (slowest) one, the first two, and all three Debye relaxations the dashed line is by the Zwanzig theory [30 b].

Our objective is to understand how the noncovalent interactions responsible for nucleic acid secondary structure (i.e. base stacking and base pairing) affect the photophysics of these multichromophoric systems. Here we describe initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides... 

N. Nandi, S. Roy and B. Bagchi, Ultrafast solvation dynamics in water isotope effects and comparison with experimental results, J. Chem. Phys., 102 (1995) 1390-7. 

Some serious discrepancies however exist, as follows (1) The total Stokes shifts from nonequilibrium calculations, 17.7 kJ/mole for isomer 1 and 22.5 kJ/mol for isomer 2, are significantly larger than the experimental result, 9.5 kJ/mol (2) no ultrafast decay (or inertial motion) in less than 1 ps in the experiments... 

Considerable work has already been carried out using ab initio calculations to predict the photodissociation dynamics of gas-phase metal carbonyls (45). This is a fertile area for computational work, given the extensive experimental results available, which include the use of ultrafast methods to characterize the short time behavior in photoexcited states. There is considerable evidence that surface crossings, especially of a spin-forbidden nature, play a considerable part in the dynamics. Much of the theoretical work so far has focused on reduced-dimensionality models of the PESs, which have been used in quantum mechanical smdies of the nonadiabatic nuclear dynamics, in which spin-forbidden transitions are frequently observed (45). Here, too, the potential benefits to be derived from a proper understanding of the spin-state chemistry are considerable, due to the importance of light-induced processes in organometallic and bioinorganic systems. 

Using fs resolution, two residence times of water at the surface of two proteins have been reported (Fig. 7.6) [21]. The natural probe tryptophan amino acid was used to follow the dynamics of water at the protein surface. For comparison, the behavior in bulk water was also studied. The experimental result together with the theoretical simulation-dynamical equilibrium in the hydration shell, show the direct relationship between the residence time of water molecules at the surface of proteins and the observed slow component in solvation dynamics. For the two biological systems studied, a bimodal decay for the hydration correlation function, with two primary relaxation times was observed an ultrafast time, typically 1 ps or less, and a longer one typically 15-40 ps (Fig. 7.7) [21]. Both times are related to the residence period of water at the protein surface, and their values depend on the binding energy. Measurement of the OH librational band corresponding to intermolecular motion in nanoscopic pools of water and methanol... 

Figure 3.14. The eomparison of the values of the limiting ionic conductivity (Aq) in water of rigid monopositive ions with the prediction of the molecular theory that takes into account the ultrafast sub-50 fs SD in liquid water. Here (Aq) is plotted as a function of the inverse ionic radius in water at 298 K. The solid line represents the predictions of the microscopic theory. The open circles denote the experimental results. Here, the tetra-alkylammonium ions are represented by C1-C4, where C = (C H2 +i)4N, n being 1,2,3, or 4. Adapted with pennission from cc. Chem. Res., 31(1998), 181. Copyright (1998) American Chemical Society.

In addition, the ultrafast OKE of the PBN film was studied. Because of the much higher molecule number density of the film than of a solution, the OKE signal in the film is stronger. We have also observed a subpicosecond time-resolved optical Kerr effect in samples of PNBN and PABN. The experimental results are similar to those for PBN. We found that the values of x in and 7, were comparable with that of PBN. Table 4 shows some of the experimental results. 

These complementary experimental results can be explained to a great extent by quantum dynamical simulations of the real-time experiments. In Sect. 3.2.2, first the results obtained by means of two-dimensional (2d) ab initio potential-energy surfaces are briefly summarized. Even more sophisticated calculations are performed on three-dimensional (3d) ab initio potential-energy and transition dipole surfaces (Sect. 3.2.3). There, all three vibrational degrees of freedom of the Nas molecule are included in the theoretical treatment. The time-dependent wave packet dynamics elucidate the effect of ultrafast state preparation on the molecular dynamics. Extensive theoretical calculations indicate the possibility of initiating the molecular dynamics predominantly in selected modes during a certain time span by variation of the pump pulse duration (Sect. 3.2.4). 

For the bound Naa B system, 1.25 ps and 120 fs one-color real-time TPI spectroscopy, at moderate intensities, yielded preferential excitations of the relatively slow pseudorotation (3 ps) and the fast symmetric stretch mode (310 fs) respectively. Three-dimensional quantum chemical and quantum dynamical ab initio investigations fully corresponded to these experimental results. The time-dependent wave packet dynamics elucidate the effect of ultrafast state preparation on the molecular dynamics. Hence, these experiments manifest efficient control of molecular dynamics using the pulse duration as a control parameter. Since known cw spectra show the pseudorotation features only, this result demonstrates also that cw and femtosecond spectroscopy have complementary sensitivities for the excitation of different vibrational modes, thus neatly confirming the original conjecture of Zewail (see e.g. [425]). 

The formation and dissociation of chemical bonds is of prime concern in chemistry, and with the advent of ultrafast high-intensity lasers, selective bond dissociation using appropriate laser pulses as molecular tweezers and scissors has received extensive and intense attention. - Theoretical approaches to laser-assisted control of chemical reactions have kept pace and demonstrated remarkable success, with experimental results reinforcing the theoretical ideas. The development of... 

The remainder of this chapter is organized in four parts. ELECTRON SOLVATION TIMES IN POLAR LIQUIDS briefly summarizes the electron solvation data available from various laboratories by the NATO ASI 1987 conference date FEMTOSECOND LASER SPECTROSCOPY outlines the novel femtosecond laser spectroscopy techniques for studying ultrafast molecular motion EXPERIMENTAL RESULTS ON FEMTOSECOND KERR RESPONSES presents the femtosecond nonlinear optical data recently obtained by us for several simple organic liquids and, in concluding, examines how these ultrafast responses could be linked to future experiment and theory of electron localization and solvation. 

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