Spectroscopic Property of the Passive Oxide

The extinction coefficient, 2, in Fig 24 is seen to increase with decrease of wavelength. The k2 value can be converted to the light absorption coefficient, a, of the passive oxide by the following equation. 

The Ught absorption at the neighbor of the absorption edge (i.e., the Band gap energy) is approximately described as the following equation, 

From the intercept the Sg is estimated to be 2.6 eV. Searson et al. replotted the absorption coefficient estimated from the data in Fig. 24 in (ahvf = (Av -sae) to evaluate the band gap energy of 1.75 eV for the indirect transition. Such band gap energy has been evaluated from the photo-excited cd measured as a function of photon energy under an assumption that the cd was proportional to the absorption coefficient. The absorption edge was estimated from the photo-excited cd to be a range from 2 to 3 eV. The photo-excited current will be discussed in the following section. 

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The multi-wavelength ellipsometiy (i.e., spectroscopic elhpsome-try) can characterize spectroscopic property of the passive oxide. Figure 24 indicates spectra of the complex refractive index, N2 = 2 - j 2, of the passive oxide formed on iron at 1.43 V vs. reversible hydrogen electrode at the same solution (RHE) in pH 8.4 borate solution and in pH 3.1 phosphate solution for 1 In Fig. 24, the thickness of the passive oxide was estimated at each wavelength of incident light. The measurement and estimation were made by the 3-parameter method. Similar results were also reported by Cahan et al. " ... 

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