Femtosecond Photochemical Processes

As mentioned in the introduction, the ability to shape femtosecond laser pulses with unprecedented precision is the key to efficient control of photophysical and photochemical processes at the quantum level. In this section, we present the fundamentals of femtosecond pulse shaping and introduce specific pulse shapes that are used in the experiments and simulations presented in the following sections. We start with the electric field of a bandwidth-limited (BWL) femtosecond laser pulse written in terms of its positive frequency analytic signal... 

As will be seen below, much of the progress in understanding the intimate details of photochemical processes have come about because of ulfrahigh speed methods that have pushed photochemistry into the femtosecond time scale. The award of the Nobel Prize to Professor Ahmed H. Zewail for his contributions to these studies is tribute to the importance of this field. With the introduction of femtosecond spectroscopic techniques it is now possible to directly observe the excited... 

Few publications on the spectroscopic and isomerization properties of simple azo compounds have appeared in the last 15 years, as compared to the decades before. There is, however, one exception Ultrashort time-resolved spectroscopy of azobenzene and its relatives has opened new access to the dynamics following pico- and femtosecond excitation. The results are most relevant for the mechanisms of the photophysical and photochemical processes, which in azoaromatic compounds primarily are isomerizations. There is, however, a host of newer investigations into the isomerization of azobenzene and its family that are directed to applications in photoswitchable systems and devices. Some of them are relevant for the understanding of the parent molecules and therefore are included in this chapter. 

All these studies with femtosecond pulses on the primary photochemical processes of rhodopsin were done by means of transient absorption (pump probe) spectroscopy [10]. However, absorption spectroscopy may not be the best way to probe the excited-state dynamics of rhodopsin, because other spectral features, such as ground-state depletion and product absorption, are possibly superimposed on the excited-state spectral features (absorption and stimulated emission) in the obtained data. Each spectral feature may even vary in the femtosecond time domain, which provides further difficulty in analyzing the data. In contrast, fluorescence spectroscopy focuses only on the excited-state processes, so that the excited-state dynamics can be observed more directly. 

Ablation with femtosecond pulses is comprehensively reviewed by Kruger and Kautek in this issue. The present discussion focuses exclusively on photoproduct formation in the irradiation of Arl-doped systems with fs pulses. This examination indicates several subtle mechanistic possibilities. Upon irradiation with 500-fs pulses at 248 nm, only ArH-like product formation is observed as the fluence is raised above the ablation threshold (Fig. 17). Recombination (e.g., Nap2-type) or other by-products are not observed even for dopant concentrations as high as 4 wt% [83-84]. In contrast, at low flu-ences, a number of by-products are observed after irradiation with few laser pulses. In this case, the accumulating radicals evidently react with each other to produce ill-defined products. Most interestingly, ill-defined species are not observed above the threshold even after extensive irradiation (Fig. 17). It is clear that photochemical processes in the ablation with fs pulses must differ distinctly from those in ns ablation. 

Figure 19.1 Diagram showing the arrangement for closed-loop learning control. Following a femtosecond laser pulse, the products of the photochemical process are detected and compared with the user-defined objectives stored on the computer. A learning algorithm then calculates the modified electric fields required to shape the laser pulse and further optimize the yield of the desired product. Cycling through the loop many times gives the optimum pulse shape and best product yield. Adapted from Brixner et o/, Chem. Phys. Chem., 2003, 4 418, with permission of John Wiley Sons Ltd...

The photochemistry of phenyl azide and its simple derivatives have received the most attention in the literature. The results of early studies were summarized in a number of reviews. " Over the last decade, modem time-resolved spectroscopic techniques and high level ab initio calculations have been successfully applied and reveal the detailed description of aryl azide photochemistry. This progress was analyzed in recent reviews. Femtosecond time resolved methods have been recently employed to study the primary photophysical and photochemical processes upon excitation of aryl azides. The precise details by which aryl azide excited states decompose to produce singlet arylnitrenes and how rapidly the seminal nitrenes lose heat to solvent and undergo unimolecular transformations were detailed. As a result of the application of modem experimental and theoretical techniques, phenylnitrene (PhN) - the primary intermediate of phenyl azide photolysis, is now one of the best characterized of all known organic nitrenes. " 5 "-2° - ... 

Current photochemical research is strongly linked with the study of photophysical behavior of excited particles. Data on photophysical processes (such as luminescence, internal conversion, intersystem crossing, intramolecular energy dissipation) assist photochemists in the identification and interpretation of chemical deactivation modes. Most of the data related to the elementary steps within deactivation of excited particles have been obtained by fast flash techniques in nano-, pico-, and femtosecond time domains. Photophysics is, in general, as rich a branch of science as photochemistry, and both the parts of excited-state research deserve comparable attention and extent. In the present review, some results on photophysics will be mentioned where suitable and necessary. We will restrict our discussion, however, predominantly to photochemical behavior of metallotetrapyrroles. 

Figure 1 Schematic view of a course of a photochemical reaction. Processes can occur on a picosecond (ps) or femtosecond (fs) time scale.

For operation at telecommunication wavelengths, one must be concerned with two-photon excitation processes leading to photochemical decomposition mechanisms. Femtosecond pulse techniques, such as those described by Stro-hkendl et al. [185], prove useful in evaluating two-photon excitation cross sections and excited state energy transfer processes relevant to photochemical decay pathways. 

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