Helium pressure shift of the hyperfine clock transition in 201Hg+

There are two stable odd isotopes of mercury with singly ionized hyperfine structure suitable for a microwave atomic clock: ¹⁹⁹Hg+ and ²⁰¹Hg⁺. We are investigating the viability of a trapped ion clock based on ²⁰¹Hg⁺ in a configuration that uses a buffer gas to increase ion loading efficiency and counter ion heating from rf trapping fields. Traditionally, either helium or neon is used as the buffer gas at ~10^-5 torr to confine mercury ions near room temperature. In addition to the buffer gas, other residual background gasses such as H₂O, N₂, O₂, CO, CO₂, and CH₄ may be present in trace quantities. Collisions between trapped ions and buffer gas or background gas atoms/molecules produce a momentary shift of the ion clock transition frequency and constitute one of the largest systematic effects in this type of clock. Here we report an initial measurement of the He pressure shift in ²⁰¹Hg⁺ and compare this to ¹⁹⁹Hg⁺.