Measurement of the atomic Na(3P) lifetime and of retardation in the interaction between two atoms bound in a molecule

From molecular spectroscopy of the Na-2 purely long range O-g(-), state we determine the Na(3P) lifetime and measure the predicted but previously unobserved effect of retardation in the interaction between two atoms. Our lifetime tau(P-3/2) = 16.230(16) ns helps to remove a longstanding discrepancy between experiment and theory. Electron cloud overlap is unimportant in the 0(g)(-) state (R > 55 alpha(0)) and the spectrum is calculated, ab initio, from atomic properties. By measuring the binding energies the 120 MHz correction due to retardation of the resonant-dipole R(-3) interaction is confirmed.