Nanosecond time-resolved experiments

Nanosecond time-resolved experiments allowed for the rationalization of the lower efficiency of the cobalt-based couples with respect to iodide/iodine and for the clarification of their structure-dependent performance. Figure 17.23a shows the decay of the photogenerated N3 dye cation, observed at 480 nm in the presence of Co (DTB)32 + 0.1M and AI0.1M in both cases, 2/ of about 0.35 /rs indicates that dye regeneration by iodide and by Co(II) occurs at a very similar rate. Upon Li+... 

G. Gerber In our time-resolved experiments on the NaafZ ) state we observe the symmetric stretch even for long delay times. From nanosecond laser and CW laser spectroscopy it is well known that the B state does not decay on femtosecond or picosecond time scales. So I do not see how the decay in the picosecond experiment by Prof. Woste can be understood and how the evolution of the B state symmetric stretch into the pseudorotation and the radial motion can occur. 

We have also prepared BLMs from polymerizable surfactants and polymerized them in situ (26). Extents of polymerization have been followed by nanosecond, time-resolved fluorescence spectroscopy and anisotropic measurements (26). Experiments have been initiated for realizing the different biological transport mechanisms in polymerized and partially-polymerized BLMs and for studying their mechanisms by simultaneous electrical and spectroscopic measurements. 

With commercially available equipment, time-resolved experiments (flash CIDNP) with a time resolution in the submicrosecond range can be performed. That this is possible at all results from the fact that the polarizations are generated during the lifetime of the paramagnetic intermediates (on the order of nanoseconds) but persist in the diamagnetic products for a time on the order of Tx (seconds for protons). For applications of this method, see, for example, Sections IV.C, V.A.2, and V.D.l. 

The population probabilities Pn t) defined in Eqs. (8)-(13) should not be confused with the population probabilities which have been considered in the extensive earlier literature on radiationless transitions in polyatomic molecules, see Refs. 28 and 29 for reviews. There the population of a single bright (i.e. optically accessible from the electronic ground state) zero-order Born-Oppenheimer (BO) level is considered. Here, in contrast, we define the electronic population as the sum of all vibrational level populations within a given (diabatic or adiabatic) electronic state. These different definitions are adapted to different regimes of time scales of the system dynamics. If nonadiabatic interactions are relatively weak, and radiationless transitions relatively slow, the concept of zero-order BO levels is useful the populations of these levels can be prepared and probed using suitable laser pulses (typically of nanosecond duration). If the nonadiabatic transitions occur on femtosecond time scales, the preparation of individual zero-order BO levels is no longer possible. The total population of an electronic state then becomes the appropriate concept for the interpretation of time-resolved experiments. ° ... 

Weller [21-23] has pointed out the competition between the rates of proton transfer and the deactivation in the excited state. In fact, it has been shown that proton-induced fluorescence quenching competitive with the proton-transfer reaction is present in the excited state of naphthylamines, that is, simple acid-base equilibrium cannot be accomplished in the excited state of aromatic amines, and that a dynamic analysis containing the quenching process is, therefore, needed in order to obtain the correct values [32,33]. The dynamic analyses by means of nanosecond time-resolved spectroscopy with fluorimetry have been applied to 1-aminopyrene [34,35], 1-aminoanthracene [36], phenanthrylamine [37,38], and naphthols [39]. This method to determine the values of naphthylamines has been used by Hafner et al. [40], and similar experiments for excited naphthols have been carried out by Harris and Selinger [41]. On the other hand, establishment of prototropic equilibrium has been reported in the case of 2-hydroxynaphthalene-6,8-disulfonate [42]. 

The last technique employed by these authors is very useful because it allows to do femtosecond time resolved experiments simply by using incoherent nanosecond laser pulses. 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

In conventional nanosecond pump-probe dispersive TRIR experiments, also described previously, kinetic data are collected at one frequency at a time. These data can then be used to construct a series of time-resolved IR spectra. Thus, in the dispersive experiment kinetic data are used to construct spectra, and in the step-scan experiment spectral data are used to derive kinetics. 

Diphenylmethylene was the first carbene to be studied using fast, time-resolved spectroscopic methods (Closs and Rabinow, 1976). Since then both nanosecond and picosecond laser techniques have been used to probe this intermediate (Eisenthal et al., 1980, 1984 Hadel et al., 1984a,b Griller et al., 1984b Langan et al., 1984 Sitzmann et al., 1984). The results of these experiments are essentially undisputed, but the interpretation of them still remains somewhat controversial. 

With the exception of thermodynamically stabilized [64] or sterically protected [65] carbenes, these species and their hetero-analogs, nitrenes, are very reactive and therefore special conditions are required for their direct observation. Fast spectroscopic techniques capable of characterizing species with lifetimes of a few picoseconds have been used [1-3]. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of microseconds and even nanoseconds [4-6]. 

Time-resolved fluorescence from sub-picosecond to the nanosecond time-scale of dye molecules like coumarins has been widely used to study solvation dynamics in liquids [1], As the dye is photoexcited, its dipole moment abruptly changes. Then by monitoring the time-dependent fluorescence energy one can have access to the solvent dynamical response to the charge reorganization in the dye. The microscopic interpretation of these experiments has greatly benefited from Molecular Dynamics (MD) studies [2], Recently, few experimental [3-5] and theoretical [6,7] works have been performed on solvation dynamics in liquid mixtures. A number of new phenomena can arise in mixtures which are not present in pure solvents, like association, mutual diffusion and preferential solvation [6]. We present here a... 

Research based on time-resolved XAS in an optical pump-x-ray probe scheme has first been implemented at synchrotron radiation sources. Mills et al. [2] used a 20 Hz repetition rate Nd YAG laser to photolyse carbonmonomyoglobin (MbCO) and monitor the photolysis product with time-resolved XAS around the K-edge of the iron atom. Other studies were carried out on different types of photolyzed systems in liquids, by Thiel et al. [3], Clozza et al. [4], Chance et al. [5,6] and Chen et al. [7,8,9]. All these studies were limited to the nanosecond or longer time domain. We recently reported on time-resolved XANES studies of a Ruthenium complex in water solution reaching the picosecond time scale [10]. This work allows us to evaluate the feasibility of future time-resolved XAS experiments, which we present below together with our new results. 

As has been shown by time-resolved flash photolysis measurements in colloidal titanium dioxide suspensions trapping is a very fast process. Rothenberger et al. performed picosecond and nanosecond transient absorption experiments on titanium dioxide and observed that the electron trapping time was faster than 30 ps, the time resolution of their laser system [4e]. The trapping time for holes was estimated to be < 250 ns. In a recent picosecond study by Serpone et al. on titanium dioxide colloids solutions of varying diameters it was observed that the spectra of trapped electrons as well as of trapped holes are fully developed after a laser... 

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