Optical properties and London dispersion interaction of amorphous and crystalline SiO2 determined by vacuum ultraviolet spectroscopy and spectroscopic ellipsometry
The interband optical properties of crystalline (quartz) and amorphous SiO2 in the vacuum ultraviolet (VUV) region have been investigated using combined spectroscopic ellipsometry and VUV spectroscopy. Over the range of 1.5-42 eV the optical properties exhibit similar exciton and interband transitions, with crystalline SiO2 exhibiting larger transition strengths and index of refraction. Crystalline SiO2 has more sharp features in the interband transition strength spectrum than amorphous SiO2 , the energy of the absorption edge for crystalline SiO2 is about 1 eV higher than that for amorphous SiO2 , and the direct band-gap energies for X -cut and Z -cut quartz are 8.30 and 8.29 eV within the absorption coefficient range 2-20cm-1 . In crystalline SiO2 we report different interband transition peaks at 16.2, 20.1, 21, 22.6, and 27.5 eV, which are in addition to those lower energy transitions previously reported at 10.4, 11.6, 14, and 17.1 eV. We find the bulk plasmon energy in X - and Z -cut crystalline quartz and amorphous SiO2 to be at 24.6, 25.2, and 23.7 eV, respectively. The oscillator strength (f) sum rules of the interband transitions for crystalline SiO2 is 10-10.8 electrons per formula unit for transition energies up to 45 eV. These differences in the electronic structure and optical properties, and the physical densities of crystalline and amorphous SiO2 , can be attributed to differences in the intermediate-range order (IRO) and long-range order (LRO) of the different forms of SiO2 . The intimate relationship between the electronic structure and optical properties and the London dispersion interaction has attracted increased interest recently, and the role of amorphous silica and other structural glass formers as a fluid in high-temperature wetting and materials processes means a detailed knowledge of the optical properties and London dispersion interaction in SiO2 is important. Hamaker constants for the London dispersion interaction of the configuration of two layers of c-SiO2 or a-SiO2 separated by an interlayer film have been determined, using full spectral methods, from the interband transition strength. The London dispersion interaction is appreciably larger in c-SiO2 than a-SiO2 due to the increased physical density, index of refraction, transition strengths, and oscillator strengths in quartz.