## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Photosensitized Generation of Singlet Oxygen
The quantum theory describes the ground energy state of a O\({}_{2}\) molecule as two different molecular orbitals located at two different oxygen atoms with the total spin equals to 1, which is denoted as \({}^{3}\Sigma\) state. The electronically excited states are singlet ones and they are characterized by the energy excess of about 0.98 eV (\({}^{1}\Delta\) state) and 1.63 eV (\({}^{1}\Sigma\) state) in respect to the ground one. The electron transition from the \({}^{(3)}\)O\({}_{2\text{ state}}\) to \({}^{(1)}\)O\({}_{2}\) one is a spin-flip process and the direct state conversion via absorption of photons is spin-forbidden according to the spin-selection rule. Since the photon absorption and emission are mirror-like processes the pure radiative decay lifetimes of the \({}^{1}\Delta\) and \({}^{1}\Sigma\) states are extremely long--about 45 min and 7 s respectively [66]. Note, the observed lifetimes of the singlet states are considerably shorter in condensed phases (\(\sim\)10\({}^{-9}\) s and \(\sim\)10\({}^{-3}\) s for the \({}^{1}\Sigma\) and \({}^{1}\Delta\) states respectively) because of collisions of \({}^{(1)}\)O\({}_{2}\) with other molecules [68]. For the first time, the strong interaction of oxygen molecules with photoexcited nc-Si in por-Si was observed at cryogenic temperatures [64] and it was further studied in more details at different excitation conditions and temperatures including room temperature [65]. The investigated samples of por-Si were prepared by standard electrochemical etching of c-Si wafers in a solution of hydrofluoric acid [77]. In References [64; 65] free standing por-Si layers were dried in air for several hours and a part of the prepared por-Si was milled to get powder of MPs assembled from nc-Si and their agglomerates. The samples were kept in a vacuum chamber in oil-free vacuum with a residual pressure of about 10\({}^{-5}\) Torr. The exciton PL spectrum of nc-Si measured in vacuum at low temperature is characterized by a broad, featureless emission band located in the visible region (dashed line in Figure 21). The spectral position and bandwidth reflect the wide band-gap distribution of the nc-Si assembly. Intrinsic defects commonly attributed to Si dangling bonds (DBs) are responsible for the infrared PL band below 1 eV [61]. The adsorption of O\({}_{2}\) on nc-Si surface resulted in a strong suppression of both the exciton PL and the defect PL band [65]. The PL quenching by O\({}_{2}\) adsorption is maximal for the PL photon energies above 1.63 eV (Figure 21). The latter value corresponds to the \({}^{3}\Sigma\rightarrow{}^{1}\Sigma\) transition energy in O\({}_{2}\) molecule. At the same time, the PL line at 0.98 eV, which is related to the radiative transition \({}^{1}\Delta\rightarrow{}^{1}\Sigma\), appeared after oxygen adsorption in por-Si (inset in Figure 21). These spectroscopic data manifest the \({}^{(1)}\)O\({}_{2}\) photosensitization by the energy transfer from excitons in nc-Si [65]. Detailed description of the energy-transfer mechanism responsible for the photosensized generation of \({}^{1}\)O\({}_{2}\) can be obtained from the spectral dependence of the PL quenching strength, which can be defined as follows [65]:  
\[Q=I_{\rm PL}{}^{\rm vac}/I_{\rm PL}{}^{\rm ox}, \tag{14}\]  
where \(I_{\rm PL}{}^{\rm vac}\) and \(I_{\rm PL}{}^{\rm ox}\) are the exciton PL intensities of nc-Si in vacuum and in oxygen ambient, respectively. The spectra of \(Q(ln\
u_{PL})\) and its second derivative are shown in Figure 22. Besides the strongest quenching at 1.63 eV there is an additional maximum at \(h\
u_{PL}\) of about 57 meV below the \({}^{1}\Sigma\) state energy and numerous maxima separated by energy of about 63 meV. These maxima correspond to the energy transfer from nc-Si with band gaps, which do not satisfy to the resonance with the excitation energies of \({}^{(1)}\)O\({}_{2}\).