Comparison experiments of neon and helium buffer gases cooling in trapped ¹⁹⁹Hg⁺ ions linear trap

The influences of different buffer gas, neon and helium, on ¹⁹⁹Hg⁺ clock transition are compared in trapped ¹⁹⁹Hg⁺linear trap. By the technique of time domain's Ramsey separated oscillatory fields, the buffer gas pressure frequency shifts of ¹⁹⁹Hg⁺ clock transition are measured to be (df/d P_Ne)(1/f)=1.8×10^-8 Torr^-1 for neon and (df/d P_He)(1/f)=9.1×10^-8 Torr^-1 for helium. Meanwhile, the line-width of ¹⁹⁹Hg⁺ clock transition spectrum with the buffer gas neon is narrower than that with helium at the same pressure. These experimental results show that neon is a more suitable buffer gas than helium in ¹⁹⁹Hg⁺ ions microwave frequency standards because of the ¹⁹⁹Hg⁺ clock transition is less sensitive to neon variations and the better cooling effect of neon. The optimum operating pressure for neon is found to be about 1.0×10^-5 Torr in our linear ion trap system.