Developing a Portable Optical Frequency Standard with Atomic Mercury

A two-photon excitation strategy is proposed to couple the $^{1}$S$_{0}$ and $^{3}$P$_{0}$ levels of mercury atoms with zero nuclear spin and in the presence of zero external magnetic field.~ This excitation strategy could allow for a portable optical frequency standard based on a thermal mercury vapor with a fractional frequency resolution at the $\sim $10$^{-15}$ level.~ The $^{1}$S$_{0}\to ^{3}$P$_{0}$ transition requires two photons at 531 nm, which are generated by a compact solid state laser system.~ Detection of population in the $^{3}$P$_{0}$ state can be accomplished via subsequent excitation to higher-lying excited states, e.g. $^{3}$P$_{0}\to ^{3}$S$_{1}$ can be driven with a diode laser at 405 nm.~ We also discuss a preliminary detection technique involving collisions between mercury atoms in the $^{3}$P$_{0}$ state with ground state ammonia molecules.