Transition from second- to first-order dipole-dipole interaction in the excited EuSr molecule

For the first time a transition from a second- to first-order dipole-dipole interaction (van der Waals to resonance interaction) has been observed in a study of the excited EuSr molecule which dissociates into europium 4f⁷5s6p ⁸{P}9/2$ and strontium 5${s}²$ ${}¹P₁ atoms. The transition was verified by measurement of the wavelength dependence in the quasistatic wings of the cw-laser--excited 4595.3-A{} line in Eu broadened by Sr ground-state atoms.