Spectroscopic studies fast time-resolved

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. 

Unlike the case with silenes, most of the kinetic data that exist on the reactions of disilenes have been obtained with stable derivatives, and relatively few transient disilenes have yet been studied by fast time-resolved spectroscopic methods. Although much of this work has been summarized in previous reviews of disilene chemistry7,8 11 14-16 132, it will be re-covered here in order to establish a context for more recently published work on the kinetics of disilene reactions. 

Only four transient disilenes have been studied to date by fast time-resolved spectroscopic techniques l,l,2-trimethyl-2-phenyldisilene (103), ( )- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (104) and tetrakis(trimethylsilyl)disilene (35). The first three compounds were generated by photolysis of the 7,8-disilabicyclo[2.2.2]octa-2,5-diene derivatives 101 and 102 (equation 76)148 while 35 was generated, together with 106, by photolysis of the 1,2-disilacyclobutane derivative 33 (equation 77)68. 

In Section 2.3.5 we saw that 2.67, an iridium analogue of 3.10, on photolysis loses a CO and activates an alkane by oxidative addition (see (2.3.5.2)). Intermediates that precede oxidative addition are expected to be short-lived. Their spectroscopic identification therefore requires an inert solvent and a technique fast enough to identify such short-lived intermediates. Low-temperature ultraviolet (UV) flash photolysis of 3.10 in liquid krypton doped with cyclohexane or neopentane has been studied by time-resolved IR. These experiments show that transient intermediates 3.11 and 3.12 are generated under these conditions. 

The photophysics of solid salicylic acid (SA) has been studied by using steady-state and time-resolved spectroscopic techniques [207,208], Dimers of SA form in two possible structures (59 and 60) due to fast ground-state double proton transfer. Dual fluorescence is observed at 380 nm and 440 nm, which are ascribable to the excited-state double proton transfer between different dimeric structures of SA. The enol form is more stable in the ground state. However, in the excited singlet state, the keto form has a lower potential energy [207], This excited enol-keto tautomerism has a barrier height of -1250 cm as is calculated from the dependence of dual fluorescence on excitation wavelength in the... 

In a bimolecular solution reaction, the reactants A and B diffuse to a point close to one another at which reaction is possible. This process is called formation of the precursor complex. At this point, rearrangement of bond lengths and bond angles in the two reactants, and of the surrounding solvent molecules, can occur to form an activated complex or transition state between the reactants and products. As one would expect, the nature of this process depends on the specific reaction involved. It is the focus of the development of the theory of the elementary step in the reaction and the associated energy requirements. In some cases it has been studied experimentally using very fast laser spectroscopic techniques which provide time-resolved information about the elementary step in the femtosecond range. 

The technique of representing the intensities of spectral lines as a function of time is referred to as time-resolved spectroscopy. Time resolution of spectroscopic information has been applied to many problems, such as the kinetics of fast decay phosphorus, radiation from fast photolysis sources, and exploding wire phenomena. Of most importance to analytical spectroscopy is the use of time-resolved spectroscopy to study the characteristics of ac spark and ac arc discharges of the type normally used for analytical emission spectral analysis, since such information may be useful in optimizing operating conditions. 

Many of the spectroscopic studies were performed to demonstrate the capability of the technique and of a number of variants which are specific for the combination of laser spectroscopy with fast beams of ions or atoms. An example has already been discussed in Section 3.3 Resonant two-photon exdtation becomes possible by adjusting the Doppler shifts for interaction with the direct and retroreflected laser beam to the atomic transition energies. Other features include the preparation of otherwise inaccessible atomic states, the separation of hyperfine structures from different isotopes by the Doppler shift, or the observation of time-resolved transient phenomena along the beam. The extensive research on nuclear moments and radii from the hyperfine structure and isotope shift constitutes a self-contained program, which will be discussed separately in Section 5. 

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