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 4${\mathrm{f}}^7$5s6p ${\mathrm{}}^8$ ${\mathrm{P}}_{9/2}$ and strontium 5${\mathrm{s}}^2$ ${\mathrm{}}^1$ ${\mathrm{P}}_1$ atoms. The transition was verified by measurement of the wavelength dependence in the quasistatic wings of the cw-laser–excited 4595.3-Å line in Eu broadened by Sr ground-state atoms.