Stark spectroscopy of the 6s5d ¹D₂ – 5d^{2 1}D₂ forbidden transition in atomic barium

We report an observation of the 6s5d ¹D₂ – 5d^{2 1}D₂ forbidden transition in Ba atom by means of high-resolution atomic-beam laser spectroscopy. Stark shifts and splittings of this forbidden transition were measured for ¹³⁶Ba and ¹³⁸Ba using the Stark-induced interference method, increasing as the applied electric field increases. From measured spectra at various electric fields, scalar and tensor polarizabilities of the forbidden transition were obtained and found to be much larger than those of other levels. The forbidden transition with large electric polarizabilities, resulting from strong mixing between the two closely lying levels of 5d^{2 1}D₂ and 5d6p ¹D₂, provides a good candidate for atomic parity non-conservation study.