*4.2. Non-Bridging Oxygen Hole Centers*

The non-bridging oxygen hole center (NBOHC) is the simplest oxygen-related intrinsic defect in silica. It corresponds to a Si atom bonded to an O atom having a dangling bond, i.e., an unpaired electron in a 2*p*-like non-bonding orbital [65]. As mentioned above, there are two major pathways through which NBOHCs can be created. The first is called the intrinsic mechanisms and consists of the photolysis of a strained Si–O–Si bond, as given in reaction (1). The second is the extrinsic mechanism, which corresponds to the homolytic dehydrogenation of a silanol group bond, shown in reaction (5). The predominance of one mechanism over the other is determined by the presence of pre-existing defects, their concentration, and the energy of the excitation beam [85].

The electronic structure of NBOHCs was fully described by Suzuki et al. using *ab initio* cluster calculations [86]. The proposed energy level diagram is illustrated in Figure 6. Apart from the bonding and antibonding *σ* orbitals, the diagram shows a series of O non-bonding orbitals whose degeneracy is lifted by the interaction of the defect with the surrounding atoms of the amorphous network. In particular, the interaction splits the non-bonding orbitals of the bridging oxygens into two sets of multiply-degenerate *np<sup>x</sup>* (OB) and *np<sup>z</sup>* (OB) orbitals, and the lone-pair orbitals of the non-bridging oxygen into two *np<sup>x</sup>* (ONB) and *np<sup>y</sup>* (ONB) levels. The highest occupied molecular orbital (HOMO) is the singly-occupied *np<sup>y</sup>* (ONB) orbital whereas the lowest unoccupied molecular orbital (LUMO) coincides with the antibonding *σ* ∗ *pz* (Si-ONB) orbital. The promotion of an electron from the ground-state levels to the HOMO gives rise to three OA bands corresponding to three distinct electronic transitions [87,88]:


**Figure 6.** Energy level diagrams of a non-bridging oxygen hole center. Vertical arrows correspond to optical transitions between bondind and non-bonding orbitals, while grey boxes represent multiply degenerated levels. Adapted from Suzuki et al. [86].

The decay of the excited state created by the above transitions gives rise to a photoluminescence (PL) band at 1.91 eV with an FWHM = 0.17 eV and a lifetime of ∼14 µs [80,89]. As was generally expected, the excitation from the *σ* orbital to the HOMO should imply a weakening of the Si O bond and a lengthening of the mean bond distance. Instead, experimental results indicate that the frequencies of the Si O symmetric stretching mode in the ground (890 cm−<sup>1</sup> ) and the excited state (860 cm−<sup>1</sup> ) are almost the same and the Stokes shift between the excitation and emission bands is as small as 0.06 eV [90–92]. This anomalous behaviour is caused by the interaction of the doubly occupied *np<sup>y</sup>* (ONB) orbital in the excited state with a symmetry-adapted combination of the three empty *σ* ∗ *py* (Si-OB) orbitals [86]. This so-called "negative hyperconjugation" is responsible for the partial delocalization of the electron density from the oxygen to the silicon atom and for the resulting stabilization of the *σp<sup>z</sup>* (Si-ONB) MO in electronically excited NBOHCs. In this way, the small electron–phonon coupling typical of NBOHC and the almost-equal Si O bond length in the ground and excited state can be explained.
