Ultraviolet Absorption and Excitation Spectrum of Ruby and Sapphire

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Abstract
The absorption spectrum of thin samples of ruby with 0.5 to 0.005 wt. % of Cr2O3 has been measured up to 8.9 eV at room temperature with linearly polarized light. The spectrum shows an absorption peak at 7 eV followed by humps around 8 eV with absorption cross section and oscillator strength at the 7-eV peak ∼7×10−17 cm2 and ∼0.3, respectively. The absorption with E⊥c is generally larger than E| | c in the vacuum-uv region. The absorption between 6.8 and 8.6 eV is interpreted as a charge-transfer transition around the chromium ion because of its large oscillator strength and some similarity to the ultraviolet reflectance spectrum of Al2O3 crystal. The excitation spectrum of ruby cubes with several chromium concentrations has been measured to 11 eV at room temperature with linearly polarized light. Structure and polarization effects corresponding to those of the absorption spectrum are observed. The quantum efficiency in the region 7 to 9 eV is comparable to that in the blue absorption band of ruby and is essentially concentration independent. Above 9 eV the relative quantum efficiency drops about two orders of magnitude and is concentration dependent, with low quantum efficiency for samples of low chromium concentration. Similar spectra on sapphire are also included for comparison.

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