Avoided-crossing spectroscopy technique based on detection of atoms in metastable states

We present a highly efficient technique for observing an avoided-crossing signal resulting from modified decay branching ratios of excited atomic states in the vicinity of avoided crossings (ACs) formed in tunable external fields. The technique is based on detection of atoms in metastable (MS) states, and we apply it to the case of photoexcited helium atoms which cascade to the singlet and triplet 1s2s states in a dc electric field. We show that narrow structures present in the MS atom yield are due to the increased probability to cascade to the triplet MS states at the field strengths at which the ACs emerge. The resolution of the present technique is not limited by the excitation bandwidth, only by the extent to which a homogeneous field can be produced over a limited region of space. Using results of high-precision calculations which include relativistic and QED corrections, we are able to reproduce the dependence of the measured MS atom yield on the electric-field strength.