SENSITIZED FLUORESCENCE IN VAPORS OF ALKALI METALS: VII. ENERGY TRANSFER IN RUBIDIUM–CESIUM COLLISIONS

Abstract

Sensitized fluorescence in cesium vapor induced by collisions with excited rubidium atoms was investigated in order to determine the total cross sections for inelastic collisions between excited rubidium atoms and cesium atoms in their ground states. The partial pressure of the rubidium vapor in the Rb–Cs mixture was kept below 2 × 10 −5 mm Hg in order to eliminate effects due to the trapping of the Rb resonance radiation. The collision cross sections for the various excitation transfer processes are as follows: Q 12′ (Rb 5 2 P 1/2 → Cs 6 2 P 3/2 ) = 1.5 Å 2 ; Q 11′ (Rb 5 2 P 1/2 → Cs 6 2 P 1/2 ) = 0.5 Å 2 ; Q 22′ (Rb 5 2 P 3/2 → Cs 6 2 P 3/2 ) = 0.9 Å 2 ; Q 21′ (Rb 5 2 P 3/2 → Cs 6 2 P 1/2 ) = 0.3 Å 2 . The fact that the cross sections are considerably smaller than those for collisions between similar atoms indicates that the Rb–Cs interactions probably involve van der Waals' forces with a much shorter range than exchange forces, which play a dominant role in Rb–Rb or Cs–Cs collisions.