SENSITIZED FLUORESCENCE IN VAPORS OF ALKALI METALS: VI. ENERGY TRANSFER IN COLLISIONS BETWEEN RUBIDIUM AND INERT GAS ATOMS

Abstract

The total cross sections for inelastic collisions between rubidium and inert gas atoms, leading to a transfer of excitation between the 5 2 P 1/2 and 5 2 P 3/2 levels in rubidium, have been determined in a study of sensitized fluorescence in rubidium – inert gas mixtures. The experiments were performed at a rubidium vapor pressure of 1.5 × 10 −6 mm Hg, at which there is no imprisonment of resonance radiation, and over a range of inert gas pressures extending from 1 to 350 mm Hg. The cross sections Q 1 (5 2 P 1/2 → 5 2 P 3/2 ) and Q 2 (5 2 P 1/2 ← 5 2 P 3/2 ) are as follows: Rb–He: 7.6 × 10 −18 and 10.3 × 10 −18 cm 2 ; Rb–Ne: 1.7 × 10 −19 and 2.3 × 10 −19 cm 2 ; Rb–A: 1.0 × 10 −19 and 1.6 × 10 −19 cm 2 ; Rb–Kr: 6.4 × 10 −20 and 1.5 × 10 −19 cm 2 ; Rb–Xe: 7.9 × 10 −20 and 2.1 × 10 −19 cm 2 . The cross sections are subject to two apparent anomalies: they do not vary monotonically with the polarizabilities of the inert gases and not all of their ratios Q 1 /Q 2 have values predicted by the principle of detailed balancing. These properties of the cross sections, as well as a partial disagreement with Franck's rule, are interpreted qualitatively in terms of a "free particle" interaction between the inert gas atoms and the 5 2 P orbital electron in rubidium. A competing process involving three-body collisions and the possible formation of Rb – inert gas van der Waals' molecules is also suggested.