The ruby fluorescence pressure scale is an optical method to measure pressure within a sample chamber of a diamond anvil cell apparatus. Since it is an optical method, which fully make use of the transparency of diamond anvils and only requires an access to a small scale laser generator, it has become the most prevalent pressure gauge method in high pressure sciences.
Ruby is chromium-doped corundum (Al<sub>2</sub>O<sub>3</sub>). The Cr<sup>3+</sup> in corundum's lattice forms an octahedra with surrounding oxygen ions. The octahedral crystal field together with spin-orbital interaction results in different energy levels. Once 3d electrons in Cr<sup>3+</sup> are energized by lasers, the excited electrons would go to <sup>4</sup>T<sub>2</sub> and <sup>2</sup>T<sub>2</sub> levels. Later they return to <sup>2</sup>E levels and the R<sub>1</sub>, R<sub>2</sub> lines come from luminescence from <sup>2</sup>E levels to <sup>4</sup>A<sub>2</sub> ground level. The energy difference of <sup>2</sup>E levels are 29 cm<sup>âÂÂ1</sup>, corresponding to the splitting of R<sub>1</sub>, R<sub>2</sub> lines at 1.39 nm.
Ruby fluorescence spectra has two strong sharp lines, R<sub>1</sub> and R<sub>2.</sub> R<sub>1</sub> refers to the stronger intensity and lower energy (longer wavelength) excitation and is used to gauge pressure.
Pressure is calculated as: , where û<sub>0</sub> is the R<sub>1</sub> wavelength measured at 1atm, a and b are constants. (e.g. a = 19.04, b = 5)
Since first demonstrated by Forman and colleagues in 1972, many scientists have contributed to the establishment of accurate ruby pressure scale in various experimental conditions.
A likely incomplete summary of is given below: