Polarised band intensity

E2x E2 A l +A2 +E j, and/Sj x E2 - Ex +E2, the results of Table 12 are easily found. Clearly therefore a polarised spectrum measurement, coupled with data relating to the appropriate band intensities (and their proportionality to the coth (hvj2 kT) function) at various temperatures, would provide considerable insight into the vibronic coupling mechanism for metallocene systems. 

Figure 2. Extinction spectra of original sample and p-polarisation bands for irradiated samples a) samples irradiated at 550 nm in the multi-shot regimes (200, 500 and 1000 pulses in single spot), peak pulse intensity was 0.42 TW/cm2 b) samples irradiated at 490, 560 and 610 nm in the multishot regime (100 pulses in single spot), peak pulse intensity was 1.8 TW/cm2.

It may be shown that the Raman measurements are capable of yielding information on both < cos 0 > and < cos d >. The availability of < cos 0 > data can be valuable is distinguishing between the differing types of stress deformation mechanisms that have been proposed. However, an interpretation of the band intensities in terms of and is possible only when the principal components of the derived polarisability tensor are known. This information is often not available and assumptions must then be made these then render the method non-absolute. Examples of this approach will be considered briefly below. 

In Raman spectroscopy, the direction of observation of the radiation scattered by the sample is perpendicular to the direction of the incident beam. Polarised Raman spectra may be obtained by using a plane polarised source of electromagnetic radiation (e.g. a polarised laser beam) and placing a polariser between the sample and the detector. The polariser may be orientated so that the electric vector of the incident electromagnetic radiation is either parallel or perpendicular to that of the electric vector of the radiation falling on the detector. The most commonly used approach is to fix the polarisation of the incident beam and observe the polarisation of the Raman radiation in two different planes. The Raman band intensity ratio, given by the perpendicular polarisation intensity, /j., divided by the parallel polarisation intensity, 7, is known as the depolarisation ratio, p. 

Normal vibrational spectroscopy generates information about the molecular frequency of vibration, the intensity of the spectral line and the shape of the associated band. The first of these is related to the strength of the molecular bonds and is the main concern of this section. The intensity of the band is related to the degree to which the polarisability is modulated during the vibration and the band shape provides information about molecular reorientational motion. 

In EMIRS and SNIFTIRS measurements the "inactive" s-polarlsed radiation is prevented from reaching the detector and the relative intensities of the vibrational bands observed in the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that vibrational coupling to adsorbate/adsorbent charge transfer (11) and also w electrochemically activated Stark effect (7,12,13) can lead to apparent violations of the surface selection rule which can invalidate simple deductions of orientation. 

The surface selection rule operates in addition to the normal IR selection rules in determining which vibrational modes are observed. As a result of the SSR the relative intensities of the fundamental IR adsorption bands of an adsorbed species can be used to give information on the orientation of the species with respect to the surface. Both S- and P-polarised light interact equally with the randomly oriented solution species. 

Some authors have asserted that the quasi-Fermi level model requires a threshold with respect to light intensity. This problem has been discussed for photoconversion systems such as photoelectrolysis of FI2O (Gregg and Nozik, 1993 Shreve and Lewis, 1995). Since the discussion on the threshold problem has frequently led to misinterpretations, we want to clarify the situation by considering a simple charge-transfer between an n-type semiconductor and redox system, as illustrated in Fig. 2.23. The system is at equilibrium (i = 0) if the overvoltage is zero (rj = 0). Flere the quasi-Fermi levels of electrons and holes are both equal to Ap,redox (not shown). Assuming that the redox process occurs entirely via the valence band, then only the quasi-Fermi level of holes at the surface, Sp, is of interest. Anodic polarisation of the electrode in the dark produces a very small anodic current (lower i-rj curve in the centre of Fig. 2.23). As mentioned in the previous section, is practically pinned close to fip,redox (Fig. 2.23A) whereas p, differs from Ap,redox by qr]. On illumination, the anodic... 

---