Fluorescence and absorption of rhodamine 6G solutions as pressure standards for a diamond-anvil cell

The absorption and fluorescence spectra of rhodamine 6G (R6G) dissolved in water, methanol, ethanol and isopropanol have been applied to calibrate the pressure in a diamond anvil cell (DAC). Owing to the strong initially nearly linear temperature-independent bathochromic shift of 7 nm GPa^-1 found in absorption and emission bands for methanol R6G solutions, they are ideal for precise pressure calibration to over 3.4 GPa, when methanol freezes. Analogous bathochromic shifts as a function of pressure are revealed for aqueous, ethanolic and isopropanolic solutions. The pressure-induced shifts are considerably reduced in the crystallized solution. Practical protocols are presented to calibrate high pressures with R6G solutions.