Reflection spectra of NaClO 3 , NaBrO 3 , and LiIO 3 gyrotropic crystals in the vacuum UV region
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ICAL PROPERTIES OF CRYSTALS
Reflection Spectra of NaClO3, NaBrO3, and LiIO3 Gyrotropic Crystals in the Vacuum UV Region V. I. Burkov and V. N. Makhov Moscow Institute of Physics and Technology, Dolgoprudnyі, Moscow oblast, 141700 Russia email: [email protected] Lebedev Physical Institute, Russian Academy of Sciences, Leninskiі pr. 53, Moscow, 119991 Russia Received March 23, 2009
Abstract—The reflection spectra in the fundamentalabsorption region, 5–25 eV (250–40 nm), of optically active crystals with cubic symmetry (NaClO3 , NaBrO3) and uniaxial optically active crystal (LiIO3) have been investigated. It is shown that the reflection spectra of cubic crystals have a similar structure, which is determined by the electronic transitions in the XO3 group. The comparison of these spectra with the corre sponding spectrum of lithium iodide made it possible to determine the type of transition in the spectra of cubic crystals. Using the projection operator method, it was shown that the sign of optical rotation of cubic crystals with symmetry T is independent of the screw axis sign. Possible reasons for the unprecedentedy large optical rotation of lithium iodide crystal in the optical axis direction are considered. DOI: 10.1134/S1063774510020197
INTRODUCTION
RESULTS AND DISCUSSION
The optical activity of NaClO3 and NaBrO3 crys tals has been repeatedly discussed in the literature. In particular, it was noted that the optically active sodium chloride and sodium bromide crystals of cubic sym metry (sp. gr. Р213 (Т 4)) with identical absolute con figuration, which are transparent in the spectral range of 230–3000 nm, have an optical rotation of a differ ent sign in this spectral range [1–3]. The optical rota tory dispersion of these crystals was measured in [4, 5], and the results obtained were generalized in [6].
A study of the absorption spectra of the crystals showed that the absorption edges of sodium chloride and sodium bromide lie near 220 and 260 nm, respec tively. An inflection is observed at the absorption edge of the former near λ ∼ 238 nm. A similar inflection is observed in the spectra of sodium bromide and lithium iodide crystals at λ ∼ 270 and 330 nm, respectively [6]. These transitions barely manifest themselves in the circular dichroism (CD) spectra, whereas the CD sig nal sharply increases near the fundamental absorption edge. The measurement of absorption and CD spectra does not yield data on the position of excited elec tronic states of the crystals (more specifically, on the energies of excited states of ClO3− and BrO3− chro mophores) due to the strong absorption. At the same time, the sign of CD spectra near the absorption edge indicated that the crystals studied had the same absolute configuration. The results of measuring the optical rotation sign confirmed this conclusion. For this reason we investigated the spectra of mirror reflection of the crystals in the energy range of 5–25 eV (250–40 nm) to determine the differences in their electronic structure. The reflection spectra of NaCl
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