High-precision Phase Shift Spectroscopy of the weak 1283 nm M1 Transition in a Thallium Atomic Beam

Using a thallium atomic beam apparatus, we are undertaking a series of laser spectroscopy measurements with the goal of providing precise, independent cross-checks on the accuracy of new calculations of parity nonconservation in thallium\footnote{M.\ Kozlov {\em et al.}, Phys Rev.\ A64,\ 053107 (2001); A.\ Derevianko, private comm.}. In our apparatus, a laser beam interacts transversely with a 2-cm-wide thallium beam of density $\sim$4x10$^{11}$ cm$^{-3}$ and reveals roughly tenfold Doppler narrowing of the absorption profile. In the current experiment we study the very weak 1283\,nm $6P_{1/2}-6P_{3/2}$ transition using an interaction region which includes high-voltage field plates and a high-finesse confocal Fabry-Perot cavity to study phase-shifts induced by interaction with the atomic beam. We seek to determine both Stark shift components, as well as the various components of the Stark-induced amplitude within this mixed M1/E2 transition.For these studies a Lasersystem with a frequency stability in the order of $\Delta \nu$=1MHz is essential. This stability is achieved by means of a a new method for laser frequency stabilization using high-resolution detection of thallium Faraday rotation in magnetic fields of a few gauss. This phase-shift detection technique is also being used in a bi-directional ring cavity interaction geometry to search for possible long-range T-violating forces in thallium.