Ultrafast techniques

A historical perspective on these developments is given in the first chapter by Jonah. Janata offers a detailed account of the key technique of electron pulse radiolysis, then firmly placed on the modern stage of ultrafast techniques in the chapter by Belloni et al. By far the most common detection scheme is that of transient optical absorption, however chapters by Warman and de Haas (on microwave conductivity) and Le Caer et al. (on infrared spectroscopy) illustrate alternative approaches. Others, not explicitly addressed, but key to... 

Ultrafast techniques have really come into their own since the development over the past decade of reliable, relatively inexpensive femtosecond lasers, but in the 1980 s, ultrafast still meant picoseconds, and the chosen paper typifies work in this time-domain. The whole subject of course owes its inspiration to George Porter, who with Norrish pioneered flash photolysis, first in the milli-, then micro-, and ultimately nano- and picosecond time-domains. 

Ultrafast techniques have been used to observe energy transfer directly. For example, sub-picosecond time resolved intramolecular transfer examined in flexible bichromophoric coumarin molecules shows that exchange occurs within 1 to 20 ps depending polymethylene chain length . The distribution of interchromophoric distances in donor/acceptor coumarin supermolecules has been measured analysis of data ftom time resolved energy transfer . 

Excited state dynamics have been measured in polysilane by ultrafast techniques and site selective fluorescence studies made with polysilylenes. ... 

Ultrafast techniques are finding increasing applications in elucidating the mechanisms of photoreactions. For example, this powerful technique has been applied to photochemical ring-opening of cyclo-octatriene (Reed et al.). and the photo-cycloreversion of an aromatic endoperoxide (Emsting et al.). In the latter case, a C-0 bond in the S3 state ruptures within 0.35 ps of excitation. 

Details of the neutralization process following radiation-induced primary charge separation may be examined via the medium of ultrafast techniques now employed in studies of luminescence decay processes. As an example, the form of luminescence decay curves of dilute organic scintillator in aliphatic hydrocarbon solution excited by x-ray pulses of about 0.5-1.0 nsec, duration is attributed (in previous papers) to neutralization processes involving ions. The relation, t cc r3, for the time required for neutralization of an ion pair of initial separation r, when applied to such curves, leads to a distribution function of ion-pair separations. A more appropriate and desirable approach involves solution of a diffusion equation (which includes a Coulomb interaction term) for various initial conditions. Such solutions are obtained by computer techniques employed in analogy to corresponding electrical networks. The results indicate that the tocr3 law affords a fair description of the decay if the initial distribution can be assumed to be broad. 

The conrotatory electrocyclic ring opening of 1,3-cyclohexadiene can be observed by several ultrafast techniques. It is believed that the first step after excitation is attainment of a planar geometry, which occurs within 10 s. " The observations are consistent with an excitation followed by return to the ground state via a Cl that permits formation of either reactant or Z-l,3,5-hexatriene. The lifetimes of the two excited states are both shorter than 100 fs, and the product is formed within 200 fs. These results are summarized in Figure 12.21. 

Because NO3 is neither a reactant nor a product of the reaction—it is formed in one elementary reaction and consumed in the next—it is called an intermediate. Multistep mechanisms involve one or more intermediates. Intermediates are not the same as transition states, as shown in FIGURE 14.20. Intermediates can be stable and can therefore sometimes be identified and even isolated. Transition states, on the other hand, are always inherently unstable and as such can never be isolated. Nevertheless, the use of advanced ultrafast techniques sometimes allows us to characterize them. 

The contributions of ultrafast techniques are well illustrated by considering the successive levels of mechanistic insight attained as shorter and shorter flashes became available. These advances can be seen clearly in studies of the dissociation of iodine molecules (I2 21) and recombination of iodine atoms (21 I2). We outline the developments... 

One of the leading preoccupations of this book is that the development of ultrafast techniques in reaction kinetics, and of the Marcus model for elementary-reaction mechanisms, coupled with computerised molecular dynamics, have the capacity to transform our view of what can be expected in our understanding of what goes on in chemical reactions. These are landmark achievements, and their success depends on studies of fast reactions. Their claims to a special place in mechanistic chemistry may be outlined as follows. 

Experimental measurements on the course of a fast reaction, on the other hand, gain little from a knowledge of energy levels concerned in the reaction (except an indication of what species will be present after initiation). They do, however, lead (via molecular-dynamics calculations) to a vivid picture of the actual movements of molecules, and of atomic nuclei within them, during a molecular encounter. Such a picture can be manipulated, both mathematically and in the mind s eye it greatly helps the imagination. When the ultrafast techniques outlined in Chapter 7 are exploited, there is observed a mass of new detail, revealing aspects hitherto never eonsidered. 

Absorption and fluorescence are usually registered using a continuous irradiation of the sample, that is, in steady-state conditions. It is also possible to measure both absorption and fluorescence intensities at different wavelengths as a function of time. Ware and coworkers [33] followed the fluorescence spectra after excitation on the nanosecond timescale, and observed a redshift of the fluorescence maxima as a function of time. Ultrafast techniques have more recently been employed to measure the time-dependence of absorption and fluorescence spectra, and confirmed the generality of this phenomenon for charge-transfer transitions in condensed phases, known as the dynamic Stokes shift . 

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