SODIUM 3P FINE-STRUCTURE EXCITATION TRANSFER INDUCED BY COLLISIONS WITH RUBIDIUM AND CESIUM ATOMS

The cross sections for the excitation energy transfer in the collisional processes Na*(3P(1/2)) + M(nS(1/2)) <--> Na*(3P(3/2)) + M(nS(1/2)), M being ground-state Rb (n = 5) or Cs (n = 6) atom, have been measured by laser fluorescence method. With either of the two sodium 3P fine-structure states optically excited, the intensity ratio of the sensitized to direct fluorescence was measured. The polarization effects due to partially polarized and anisotropic direct fluorescence were investigated. The measured cross sections (T = 530 K) for the sodium 3P fine-structure excitation transfer induced by collisions with Rb are: sigma(1/2-->3/2) = 170 Angstrom(2) and sigma(3/2-->1/2) = 94 Angstrom(2), with an overall accuracy of 20%. The corresponding values in the Cs case are: sigma(1/2-->3/2) = 140 Angstrom(2), sigma(3/2-->1/2) = 83 Angstrom(2). The theoretical calculations reveal the decisive role of the dipole-quadrupole interaction. The obtained values are in satisfactory agreement with the experiment only if both the dipole-dipole and dipole-quadrupole interactions are taken into account, while the calculations involving the dipole-dipole term alone yield the cross section values which are as much as one order of magnitude lower than the experimentally observed ones.