Up-down asymmetry of the electrons ejected from barium $6p_{1/2}nk$ autoionizing states*

The Ba $6p_{1/2}nk$ autoionizing Stark states do not have well-defined parities but have the Rydberg electrons localized in the upfield or downfield side of the atom. As a result when they autoionize the angular distributions of ejected electrons are not up/down symmetric with respect to the laser polarization direction. In this experiment, we measure the ejected electron signals from Ba $6p_{1/2}nk$ autoionizing Stark states, of n=28 and 29, produced by linearly polarized laser excitation in weak electric fields of less than 15 V/cm. In the weak electric field region, the energy gained by the ejected electrons is comparable to the initial energy of the ejected electron. This makes the observation of the up/down asymmetry of the ejected electrons possible. We observe that the electrons from Ba $6p_{1/2}nk$ autoionizing red states are ejected preferentially in the upfield direction while the electron from Ba $6p_{1/2}nk$ autoionizing blue states are ejected preferentially in the downfield direction.