Nitrogen buffer gas experiments in mercury trapped ion frequency standards

Practical, continuous operating mercury trapped ion frequency standards have traditionally used a helium buffer gas to increase loading efficiency and cool ions to near room temperature. The fractional frequency shift of the 40,507,347.9968x Hz clock transition due to collisions with helium is measured to be (df/dP/sub He/)(1/f)=+1.2/spl times/10/sup -10//Pa. The use of a nitrogen buffer gas is considered for low power and mass applications where unattended operational life must be greater than 10 years. The nitrogen pressure shift is measured to be (df/dP/sub N2/)(1/f)=-8.7/spl times/10/sup -9//Pa. Nitrogen would allow long operation with a only small ion pump but require increased pressure regulation to achieve the ultra-high stability obtained using helium in multipole Hg+ standards.