Ruby pressure scale

The ruby fluorescence pressure scale is an optical method to measure pressure within a sample chamber of a diamond anvil cell apparatus.^[1] 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 spectra R1, R2 lines

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.^[2] The energy difference of ²E levels are 29 cm⁻¹, corresponding to the splitting of R₁, R₂ lines at 1.39 nm.^[2]

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: ${\displaystyle P(Mbar)={\frac {a}{b}}[\left({\frac {\lambda }{\lambda _{0}}}\right)^{b}-1]}$, where λ₀ is the R₁ wavelength measured at 1atm, a and b are constants. (e.g. a = 19.04, b = 5^[3])

Since first demonstrated by Forman and colleagues in 1972,^[4][5] 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	[4]
1978	H. K. Mao	1904	5	Ag, Cu, Mo, Pd	Room temperature	6 - 100 GPa	M-E, H2O	[3]
1986	H. K. Mao	1904	7.665	Cu, Ag, Ar	Room temperature	80 GPa	Ar	[6]
2004	A. Dewaele	1904	9.5	Al, Cu, W	Room temperature	153 GPa	Helium	[7]
2005	A. D. Chijioke	1876(6.7)	10.71(0.14)	Au, Pt	Room temperature	150 GPa	Helium, Hydrogen, Ar	[8]
2008	S.D Jacobsen	1904	10.32(7)	MgO	Room temperature	118 GPa	Helium	[9]
2012	H. Yamaoka	1762*ln(λ/λ0)		Ruby R1	Low temperature to 16K	26 GPa	M-E, Silicone oil	[10]
2020	G. Shen	1870	5.63(3)	MgO, Mo, Cu, Diamond	Room temperature	150 GPa	Helium	[11]