Level, single energy

Electrons must occupy all the orbitals of a given energy level singly with the same spin before pairing begins (Hund s Rule). 

The elfect of temperature on the absolute values of a, cr,-, and cr as a function of the incident photon energy. All the cross-section data shown in this chapter were measured for molecules in the gas phase at room temperature and thus, the target molecules do not lie in a single energy level as an initial level. This means that the measured cross sections seem to be dependent on gas temperature, which is important in various applications of the cross-section data. 

Fig. 16. Structures involved in the decomposition of the adduct intermediate resulting from the reaction of [Fe(CN)r,NO]2 with NH3. (a) Only the formation of the linear, q1-N2 complex is considered, (b) The formation of T 2-N2 as an intermediate step is also included. The structures correspond to single points on the potential hypersurface, calculated at the B3LYP/6-31G level. Relative energies are not drawn to scale. Arrows indicate changes in the molecule that lead to the next step.

In most semiconductors, there are, in addition to the allowed energy levels for electrons in the conduction and filled bands of the ideal crystal, discrete levels with energies in the forbidden gap which correspond to electrons localized at impurity atoms or imperfections. In zinc oxide, such levels arise when there are excess zinc atoms located interstitially in the lattice. At very low temperatures the interstitial zinc is in the form of neutral atoms. However, the ionization energy of the interstitial atoms in the crystal is small and at room temperature most are singly ionized, their electrons being thermally excited into the conduction band. These electrons give rise to the observed A-type conductivity. 

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