Ruby pressure scale

Ruby spectra R1, R2 lines

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.

Principles

Ruby is chromium-doped corundum (Al₂O₃). The Cr³⁺ 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 Cr³⁺ are energized by lasers, the excited electrons would go to ⁴T₂ and ²T₂ levels. Later they return to ²E levels and the R₁, R₂ lines come from luminescence from ²E levels to ⁴A₂ ground level. The energy difference of ²E levels are 29 cm⁻¹, corresponding to the splitting of R₁, R₂ lines at 1.39 nm.

Development

Ruby fluorescence spectra has two strong sharp lines, R₁ and R_2. R₁ refers to the stronger intensity and lower energy (longer wavelength) excitation and is used to gauge pressure.

Pressure is calculated as: $P(Mbar)=\frac{a}{b}[{\left(\frac{\lambda }{{\lambda }_0}\right)}^b-1]$, where λ₀ is the R₁ 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:

Year	First Author	a	b	Primary pressure standard used	Temperature	Pressure range	Pressure transmitting medium	References
1972	R. A. Forman	-	-, linear	Transitions of CCl4, H2O, C2H5Br, n-C7H16	Room temperature	2.2 GPa	See standards used
1978	H. K. Mao	1904	5	Ag, Cu, Mo, Pd	Room temperature	6 - 100 GPa	M-E, H2O
1986	H. K. Mao	1904	7.665	Cu, Ag, Ar	Room temperature	80 GPa	Ar
2004	A. Dewaele	1904	9.5	Al, Cu, W	Room temperature	153 GPa	Helium
2005	A. D. Chijioke	1876(6.7)	10.71(0.14)	Au, Pt	Room temperature	150 GPa	Helium, Hydrogen, Ar
2008	S.D Jacobsen	1904	10.32(7)	MgO	Room temperature	118 GPa	Helium
2012	H. Yamaoka	1762*ln(λ/λ0)		Ruby R1	Low temperature to 16K	26 GPa	M-E, Silicone oil
2020	G. Shen	1870	5.63(3)	MgO, Mo, Cu, Diamond	Room temperature	150 GPa	Helium