Widths of absorption bands

Absorption bands in crystal field spectra are not sharp lines. Instead, as the spectra illustrated in figs 3.1, 3.2 and 3.3 show, they contain rather broad envelopes approximating gaussian profiles which at half peak-height may have full widths ranging from 100 cm-1 to 1,000-2,000 cm-1. Several factors lead to broadened absorption bands and they are discussed below. 

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Since Fe2+ ions are concentrated in the acentric M4 sites of cummingtonite, the intense band around 10,000 cm-1 in the p spectrum (fig. 5.19a) arises from absorption by Fe2+ ions in this very distorted site. The increased intensity and width of absorption bands in the a and y spectra of grunerite and the broadening of the intense band in the P spectra result from increased occupancies of Fe2+ ions in the more regular Ml, M2 and M3 octahedral sites. 

System under study Amax of absorption bands (nm) (Half-width of absorption bands. A, cm )... 

It is well known from spectra in the visible and the ultra-violet that the width of absorption bands indicates the difference in the bonding character of the excited state and the ground state. Thus, the potential surfaces of the excited 4f i states of lanthanide complexes, or of some of excited 3ds states of manganese(II) complexes (75, 102) have the same intemuclear distances as the ground states, whereas in other cases, when the excited states prefer either much shorter or much longer inter-nuclear distances, the absorption bands (and emission bands of luminescence) are broad. 

The actual widths of absorption bands in the mid-infrared spectra of liquids and solutions depend strongly on the rigidity of the part of the molecule where the... 

In amorphous semiconductors, information about the width of the band tail states (or disorder) may also be extracted from the optical absorption spectra. For photon energies near bandgap energy, the optical absorption coefficient of amorphous semiconductors exhibit an exponential dependence on the photon energy, following the so-called Urbach relationship ... 

The width of a band in the absorption spectrum of a chromophore located in a particular microenvironment is a result of two effects homogeneous and inhomogeneous broadening. Homogeneous broadening is due to the existence of a continuous set of vibrational sublevels in each electronic state. Inhomogeneous broadening results from the fluctuations of the structure of the solvation shell... 

Widths of absorption or emission bands and their temperature dependence ... 

A third example can be taken from analytical chemistry. Absorption and resonance Raman spectra of phenol blue were measured in liquid and supercritical solvents to determine the solvent dependence of absorption bandwidth and spectral shifts. Good correlation between absorption peak shift and resonance Raman bands and between Raman bands and bandwidth of C-N stretching mode were observed while anomalous solvent effect on the absorption bandwidth occnrred in liquid solvents. Large band-widths of absorption and resonance Raman spectra were seen in supercritical solvents as compared to liquid solvents. This was dne to the small refractive indices of the supercritical solvents. The large refractive index of the liqnid solvents only make the absorption peak shifts withont broadening the absorption spectra (Yamaguchi et al., 1997). 

An important difference between atomic and molecular spectroscopy is the width of absorption or emission bands. Spectra of liquids and solids typically have bandwidths of — 100 nm, as in Figures 18-7 and 18-14. In contrast, spectra of gaseous atoms consist of sharp lines with widths of —0.001 nm (Figure 21-3). Lines are so sharp that there is usu-... 

In view of the possibility that existing bands may simply be smeared out at room temperature by the thermal disorder in the liquid and the resulting fluctuations in the structure of the electron trap, Arai and Sauer (4) have determined the absorption spectrum of the solvated electron in ethanol at —78° C. No structure was observed, so that evidence is lacking for a transition to a second level, Is - 3p, even at the lower temperature. The absorption maximum was, however, found to be shifted from 7000 A. at 23 ° C. to 5800 A. at — 78 ° C. It is interesting to note that the half-width of the band remained the same, about 1.5 e.v., at the lower temperature. 

Fig. 5.6. The absorption (gray bands) and emission (black bands) spectra of various fluorochromes. The wavelength widths of the bands for each fluorochrome indicate the range of wavelengths that will be absorbed and emitted. Laser (excitation) wavelengths are indicated at the bottom of the chart. From Shapiro (1995).

Spectral Manipulation Techniques. Many sophisticated software packages are now available for the manipulation of digitized spectra with both dedicated spectrometer minicomputers, as well as larger main - frame machines. Application of various mathematical techniques to FT-IR spectra is usually driven by the large widths of many bands of interest. Fourier self - deconvolution of bands, sometimes referred to as "resolution enhancement", has been found to be a valuable aid in the determination of peak location, at the expense of exact peak shape, in FT-IR spectra. This technique involves the application of a suitable apodization weighting function to the cosine Fourier transform of an absorption spectrum, and then recomputing the "deconvolved" spectrum, in which the widths of the individual bands are now narrowed to an extent which depends on the nature of the apodization function applied. Such manipulation does not truly change the "resolution" of the spectrum, which is a consequence of instrumental parameters, but can provide improved visual presentations of the spectra for study. 

In luminescence studies it can often be observed that intensities decrease with increasing pressure. A decreasing luminescence intensity can be ascribed to two main effects. On one hand, the excitation efficiency can decrease due to a pressure-induced shift of absorption bands away from a fixed excitation energy. This effect can be minimized either by a tunable excitation source or by exciting into a band, whose shift is negligible compared to its width. 

The most important use of energy level diagrams described in 3.5 is to interpret visible to near-infrared spectra of transition metal compounds and minerals. The diagrams provide qualitative energy separations between split 3d orbitals and convey information about the number and positions of absorption bands in a crystal field spectrum. Two other properties of absorption bands alluded to in 3.3 are their intensities and widths. 

Chapter 3 describes the theory of electronic spectra of transition metal ions. The three characteristic features of absorption bands in a spectrum are position or energy, intensity of absorption and width of the band at half peak-height. Positions of bands are commonly expressed as wavelength (micron, nanometre or angstrom) or wavenumber (cm-1) units, while absorption is usually displayed as absorbance, absorption coefficient (cm-1) or molar extinction coefficient [litre (g.ion)-1 cm-1] units. 

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