Optical methods, high-pressure experiments

Even though the rates of initiation span almost a 10-fold range, the values of k, show a standard deviation of only 4%, which is excellent in view of experimental errors. Note that the rotating sector method can be used in high-pressure experiments and other unusual situations, a characteristic it shares with many optical methods in chemistry. 

The NEXAFS experiments reported here were carried out at the U1 Beamline of the National Synchrotron Light Source, Brookhaven National Laboratory. Details concerning the optics of the beamline, as well as the UHV chamber with facilities for high pressure reactions, have been described previously.7,8 In our experimental set-up, NEXAFS spectra can be recorded by measuring either the electron yield or fluorescence yield. While the electron yield method is sensitive only to the top few atomic... 

The low-energy end of the optical spectrum, the mid and far i.r. region (wavenumbers of 1000 to 10 cm ), allows observation of phonon (intermolecular) modes in solids. This is a domain where high-pressure methods, although potentially quite useful, have been comparatively little used. The small number of experiments which have been performed in this region were mostly on molecular compounds using large-volume, low-pressure cells with... 

The AV data of Fig. 5.1 that are satisfactorily accounted for by Eqs (5.5)-(5.8) are fewer in number than the anomalous cases of Table 5.1. This is a rather unsatisfactory situation, even though most of the anomalies can be explained away - indeed, deviations from the predictions of Eqs (5.5)-(5.8) can often provide important mechanistic information. More AV data are clearly desirable, but the prospects for further successful experiments are poor. The measurements of AV summarized in Fig. 5.1 and Table 5.1 were obtained at high pressures by radiochemical tracer methods for the slowest reactions [12, 17, 25], NMR linebroadening techniques for the faster cases [11, 13, 15, 19-22, 34], and stopped-flow circular dichroism [13, 14, 18] for moderately rapid reactions of reactants that could be prepared as resolved enantiomers. There are, however, many self-exchange reactions that are inaccessible to these techniques. For example, rates of electron transfer in couples where both reactants have unpaired electrons generally cannot be studied by NMR methods, while other couples that undergo electron transfer at intermediate rates may not be resolvable into optical isomers or be amenable to radiochemical sampling procedures under pressure. 

Optical Methods. Optical methods may conveniently be considered in three categories, (a) Methods suitable up to 10 to 20 MPa in which the coexisting liquid and vapour phases at predetermined pressures and temperatures are analysed. Such methods combine the advantages of the optical and analytical methods, (b) Methods suitable up to 10 to 20 MPa in which the composition of the mixture studied is controlled by the filling of the cell at the outset of the experiment, (c) Methods suitable for high-pressure studies above 10 MPa in which the composition of the mixture studied is controlled by the filling of the cell at the outset of the experiment. Such methods are suitable for studying either or both liquid-liquid or so-called gas-gas immiscibility. 

As noted above, a combination of high pressure diamond anvil techniques, all utilizing the ruby fluorescence method of pressure measurement, were used to carry out these experiments. These include (1) Fourier transform infrared (FTIR) spectroscopy for the kinetic measurements [9-11], (2) energy dispersive x-ray powder diffraction for crystallographic identification of the observed polymorphic forms and also compression measurements [12], (3) optical polarizing microscopy... 

---