Bond disorder

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

This disorder decrease on alloying is in principle unexpected, because alloying introduces chemical disorder in addition to bond disorder. However, Silva et al. [14] reported results on the joint density of states as determined by EELS which showed a clear decrease in the density of tail states at 7-at.% N and showed a further increase for higher nitrogen content. 

H. Holsaas, C. M. Sebastian, H. Feldermann, R. Merk, C. Ronning, in M. P. Segal, W. J. Milner, J. E. Jaskie (Eds.), Symposium Proceedings, Covalently Bonded Disordered Thin Film Materials, Vol. 498, Boston, USA, December 2-4, Materials Research Society, 1997, p. 129. 

Cycloaddition of buta-1,3-diene to the C2 ligand affords 356, containing complexed cyclohex-l-en-4-yne, this time attached to an Ru3 face.538 With cyclopenta-1,3-diene, formal insertion of one of the C2 carbons into a C=C double bond occurs (possibly via a three-membered ring and ringopening) to give 357.539 Two molecules of the cyclic diene have been incorporated into the organic ligand, which shows C=C double bond disorder. 

The tandem hydrogen-bond configuration may also be an example of hydrogen bond disorder, since the bonds involved are also hydroxyls and the hydrogen to hydrogen separation is less than 2.4 A. 

Two mechanisms for hydrogen-bond disorder have to be considered. One is the... 

In the proteins and nucleic acids the configurational mechanism has to be invoked to explain any hydrogen-bond disorder involving )N-H or NH groups where there is no orientational flexibility. This is shown in the two examples, below (the second describing amino/imino and keto/enol tautomeric states) ... 

One conclusion from the structure studies in Chapter 2 is that the bonding disorder of a-Si H is relatively small. The silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of about 10% and a much smaller bond length disorder. Fig. 

Since the slope, E, of the Urbach absorption reflects the shape of the valence band tails, it follows that varies with the structural disorder. For example, one measure of the disorder is the average bond angle variation, which is measured from the width of the vibrational spectrum using Raman spectroscopy (Lannin 1984). Fig. 3.22 shows an increasing E with bonding disorder, which is caused by changes in the deposition conditions and composition (Bustarret, Vaillant and Hepp 1988 also see Fig. 3.20). The defect density is another measure of the disorder and also increases with the band tail slope (Fig. 3.22). A detailed theory for the dependence of defect density on is given in Section 6.2.4. 

Eq. (3.35) relates the broadening of the Urbach edge and the shift of the band gap, both of which originate from the thermal and bonding disorder. 

The bonding disorder of a glass suggests that a decay with a single time constant is not expected, but instead an average over the structural configurations. One possibility is a local variation in decay rates described by a distribution of time constants i t), so that... 

The liquid phase of water is ringed by a series of hydrogen bond disordered ice phases (Fig. 1). How hydrogen bond disorder arises is illustrated in Fig. 2. The constraints of the ice rules - each water molecule required to form two H-bonds as a donor and two as an acceptor - do not fix the orientation of the water molecules because each molecule has six ways to point its two hydrogens in two tetrahedral directions and lone pairs in two other directions. 

We now turn to the interaction between the hydroxide ion and the surrounding random H-bond disordered medium. To illustrate the effect, two configurations of a 96-water (95 waters and a hydroxide) unit cell are shown in Fig. 3. The hydroxide and L-defects in Fig. 3 The H-bond arrangement of the other water molecules differs, while, except for the defects, still maintaining the ice rules (each water donates to two H-bonds, and accept two other H-bonds). 

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