glasses are insulator and charge-up problem is unavoidable. The author's research group has revealed that metal-mesh screen was effective to remove inhomogeneous charge distribution of glass surface. $ ^{4)} $ By using a Ni-mesh screen, high resolution measurement has become possible; in the case of $ SiO_{2} $ glass, peak width of O1s signal is broad (3.8 eV) without using any charge control, and it becomes quite sharp (1.4 eV) by combination of the Ni-mesh screen and an irradiation of low energy electron flux.

Then, XPS measurements have been carried out for various glass systems, such as $ B_{2}O_{3} $ , $ SiO_{2} $ , $ P_{2}O_{5} $ , $ GeO_{2} $ , $ TeO_{2} $ and $ Bi_{2}O_{3} $ .⁵) The results of alkali binary glasses are shown in Fig. 1, where O1s binding energy, $ E_{\mathrm{B}}(\mathrm{O1s}) $ is plotted against optical basicity, $ \Lambda(\chi) $ which is calculated from the following equation.³)

$$ \left\{\begin{aligned}\Lambda(\chi)=1-\sum_{i}\frac{z_{i}\ r_{i}}{2}\left(1-\frac{1}{\gamma_{i}}\right)\\ \gamma_{i}=1.36\big(\chi_{i}-0.26\big)\end{aligned}\right. $$

where $ z_{i} $ is the oxidation number of the cation i, and $ r_{i} $ is the ionic ratio with respect to the total number of oxides. $ \gamma_{i} $ is the basicity moderating parameter and is empirically given by using the Pauling electronegativity $ \chi_{i} $ . Equation (1) is derived empirically based on the optical basicity measurements for various multi component glasses. It is quite convenient because basicity $ \Lambda(\chi) $ is calculated directly from the glass composition.

Fig. 1. Correlation between experimentally obtained O1s binding energy, $ E_{\mathrm{B}}(\mathrm{O1s}) $ and the optical basicity numerically-estimated from electronegativity, $ \Lambda(\chi) $ . $ ^{5)} $

As shown in Fig. 1, a linear correlation is confirmed between $ E_{\mathrm{B}}(\mathrm{O1s}) $ and $ \Lambda(\chi) $ in the typical glass systems of $ B_{2}O_{3} $ , $ SiO_{2} $ , $ P_{2}O_{5} $ and $ GeO_{2} $ , where $ E_{\mathrm{B}}(\mathrm{O1s}) $ decreases with increasing $ \Lambda(\chi) $ . The same correlation is seen in $ TeO_{2} $ system, and $ E_{\mathrm{B}}(\mathrm{O1s}) $ is, however, lower than that of $ P_{2}O_{5} $ and $ B_{2}O_{3} $ systems at similar $ \Lambda(\chi) $ . In the case of $ Bi_{2}O_{3} $ system, different trend is confirmed, that is, $ E_{\mathrm{B}}(\mathrm{O1s}) $ increases with increasing $ \Lambda(\chi) $ . The unique trends found in $ TeO_{2} $ and $ Bi_{2}O_{3} $ systems are probably due to the vacant d orbitals of Te and Bi ions. In the case of $ TeO_{2} $ glasses, electrons are delocalized by O→Te back-donation through Te5d–O2p $ \pi $ orbital, $ ^{(6)} $ and it is hence expected that wave functions of neighboring oxide ions overlap largely, resulting in higher electron density of O2p orbitals.

The correlations of $ E_{\mathrm{B}}(\mathrm{O1s}) $ and the other expressions of basicity were also examined. $ ^{5)} $ However, expression applicable to all the glass systems was not found. Then, a novel expression for the O1s chemical shift was proposed. $ ^{5)} $ As shown in Fig. 2, O1s chemical shifts of $ TeO_{2} $ and $ Bi_{2}O_{3} $ systems are in a linear correlation with other glass systems of $ B_{2}O_{3} $ , $ SiO_{2} $ , $ P_{2}O_{5} $ and $ GeO_{2} $ .

Fig. 2. Correlation between experimentally obtained O1s binding energy, $ E_{\mathrm{B}}(\mathrm{O1s}) $ and the numerically estimated chemical shift parameter, CSP. $ ^{5)} $

#### 3. Relation between basicity and glass structure 3.1 Glass structure and XPS chemical shift

Before the establishment of surface charge control method, it was also difficult to determine absolute value of binding energy in XPS measurements. In the case of silicate glass, for example, O1s binding energy has been corrected as reference to Si2p signal, assuming that Si2p binding energy was constant and insensitive to glass composition. $ ^{7)} $ After the establishment of binding energy correction method, it was revealed that Si2p peak also shifted chemically as well as O1s peak. $ ^{8)} $ As shown in Fig. 3, it is worth noting that not only $ E_{\mathrm{B}}(\mathrm{O1s}) $ but also $ E_{\mathrm{B}}(\mathrm{Si2p}) $ and $ E_{\mathrm{B}}(\mathrm{Na1s}) $ decrease with increasing $ Na_{2}O $ content. If ionicity of chemical bonds increases along with $ Na_{2}O $ addition, oxygen atoms get more electrons from neighboring Si and Na atoms, resulting in the negative shift of O1s and