The 6S(0)-6P(0) transition in thallium isotope ion Tl: A superior atomic clock.

Experimental and theoretical studies of the forbidden J = 0 --> J' = 0 optical transition 6(1)S(0)-6(3)P(0) in the odd Hg isotopes are in the literature. From this work natural width and quadratic Zeeman effect have been estimated for the same line in the isoelectronic, even thallium ion (204)Tl(+), which has a very small nuclear magnetic moment. For the m(F) = 0 --> 0 component a linewidth of <0.01 Hz and a quadratic Zeeman shift <10(-18) at 0.1 Gauss are found, compared to 1 Hz and approximately 10(-8) for the hydrogen maser. Electronic quadrupole moments vanish for both J and J' states and with them shifts due to electric field gradients. All shifts are orders of magnitude smaller than for Hg(+) and Ba(+), which have been studied as atomic clock elements.