Prospects in rare-gas hydride ions for terahertz frequency standards

A proposal is given to use the $X^1\Sigma (v,J,F)=(0,0,1/2)-(0,1,1/2)$ transition of the HeH⁺ (2.0 THz), NeH⁺ (1.0 THz), and ArH⁺ (0.62 THz) molecular ions as novel frequency standards in the terahertz region. This transition is free from the electric-quadrupole shift, and the DC Stark and Zeeman shifts are much less than those of the CaH⁺ and MgH⁺ molecular ions. These transitions are useful for locking the frequencies of oscillators like quantum cascade lasers (QCL) or resonant tunnel diodes (RTD) with uncertainties of 10⁻¹⁵. (paper)