UV band structure

Spectra as in Fig. 3.17 can also be obtained as a function of polar and azimuthal angle, and with polarized UV light, enabling one to probe band structures in all directions [18]. In this chapter we limit ourselves to angle-integrated measurements of the electron density of states. 

As the number of silicon atoms in the delocalized backbone cr-electron system increases, the number of HOMO and LUMO states increases, resulting in a band structure for high molecular weight polymers. Electronic absorptions from the HOMO (cr) to LUMO (essentially a ) are responsible for the characteristic UV absorption of polysilanes observed between 300 and 400 nm, the transition moment for which is in the direction of the Si chain.198 Polysilanes are... 

The empirical approach [7] was by far the most fruitful first attempt. The idea was to fit a few Fourier coefficients or form factors of the potential. This approach assumed that the pseudopotential could be represented accurately with around three Fourier form factors for each element and that the potential contained both the electron-core and electron-electron interactions. The form factors were generally fit to optical properties. This approach, called the Empirical Pseudopotential Method (EPM), gave [7] extremely accurate energy band structures and wave functions, and applications were made to a large number of solids, especially semiconductors. [8] In fact, it is probably fair to say that the electronic band structure problem and optical properties in the visible and UV for the standard semiconductors was solved in the 1960s and 1970s by the EPM. Before the EPM, even the electronic structure of Si, which was and is the prototype semiconductor, was only partially known. 

Luminescence spectra of hardystonite under 266 nm laser excitation reveal an extremely strong, rather narrow UV band at 355 nm, with a very short decay time of 25 ns (Fig. 4.20b). Usually such bands in minerals are attributed to Ce luminescence. However as another band was already confidently ascribed to this center (Fig. 4.20a) assignment appears problematic. In principle it is possible that several different Ce " centers occur in a structure, which are formed, for example, as a result of substitutions on Ca and Zn positions or because of different types of charge compensations. The first possibihty may be excluded based on the large differences in ionic radii of Ce " (115 ppm) and Zn " " in tetrahedral coordination (74 ppm), while the second possibihty may be taken into consideration. 

The absolute values of the absorption cross sections of HCHO have been somewhat controversial. This appears to be due to a lack of sufficient resolution in some studies as discussed in Chapter 3.B.2, if the spectral resolution is too low relative to the bandwidth, nonlinear Beer-Lambert plots result. The strongly banded structure means that calculations of the photolysis rate constant require actinic flux data that have much finer resolution than the 2- to 5-nm intervals for which these flux data are given in Chapter 3 or, alternatively, that the measured absorption cross sections must be appropriately averaged. One significant advantage of the highly structured absorption of HCHO is that it can be used to measure low concentrations of this important aldehyde in the atmosphere by UV absorption (see Sections A.ld and A.4f in Chapter 11.). 

As discussed in the chapter, UV-visible spectroscopy provides sensitive detection for many atmospheric gases. However, a criterion for applying DOAS is that the absorption have a banded structure. Why do so few molecules of interest have a detailed, banded structure in the UV-visible compared, for example, to their infrared spectra ... 

The bulk of UPS literature has appeared since 1970, and it has concentrated in two areas (1) interpreting spectra of organic vapors and (2) studing transition metal electronic band structure changes caused by sorption of simple gases on the metal surfaces. Interpreting uv photoelectron spectra is very difficult and until a data base of spectral measurements is accumulated, it will be used infrequently in surface chemical studies. 

The usefulness of UV-excitation for these experiments was already stressed on page 2. In many cases UPS and XPS give valuable complementary information S2), therefore an instrument for band structure analysis should permit different modes of excitation of both gases and solids. However, in the present article we have to restrict ourselves to studies on solids by means of x-ray photons. 

When UV photons are used, the available energy provides only the possibility of studying the outer electron shells. Therefore UPS (Ultraviolet Photoelectron Spectroscopy) studies the valence band structures of materials. 

Pristine SWCNTs and their fluorinated derivatives, F-SWCNTs, were reacted with organic peroxides to functionalize their sidewalls covalently by attachment of free radicals (Scheme 1.15). The tubes reactivity towards radical addition was compared with that of corresponding polyaromatic and conjugated polyene JT-systems [150, 151]. The characterization of the functionalized SWCNTs and F-SWCNTs was performed by Raman, FT-IR and UV/Vis/NIR spectroscopy and also by TGA/MS, TGA/FT-IR and with TEM measurements. The solution-phase UV/Vis/NIR spectra showed complete loss of the van Hove absorption band structure, typical of functionalized SWCNTs [150]. 

---