Experimental determination of ¹²⁹Xe–Cs/Rb spin-exchange efficiencies due to binary collisions and van der Waals molecules in ¹²⁹Xe–N₂ mixtures

The relative spin-exchange efficiencies of Cs and Rb remain an important question in the field of hyperpolarized ¹²⁹Xe; however, direct experimental comparisons are still limited. We present measurements of the spin-exchange efficiency for ¹²⁹Xe–Cs/Rb in ¹²⁹Xe:N₂ = 1: 50 mixtures. Under low-magnetic-field and short-lifetimes van der Waals molecular regime, we determine the spin-exchange efficiencies of ¹²⁹Xe–Cs/Rb by measuring the spin-exchange rate coefficient for ¹²⁹Xe–Cs/Rb and the Xe-induced spin-relaxation rate of Cs/Rb. Under identical gas composition and pressure conditions, the spin-exchange efficiency of Cs is approximately 1.2–1.5 times higher than Rb. We also derive an expression that enables the prediction of the spin-exchange efficiency for ¹²⁹Xe–Cs/Rb in arbitrary ¹²⁹Xe–N₂ mixtures.