SENSITIZED FLUORESCENCE IN VAPORS OF ALKALI METALS: II. ENERGY TRANSFER IN POTASSIUM–ARGON COLLISIONS

Sensitized fluorescence in potassium, induced by argon at pressures between 0.01 mm Hg and 350 mm Hg, was investigated to determine the cross sections for collisions of the second kind between potassium atoms in the 4 2P states and argon atoms in their ground states. The experiment was carried out at a potassium vapor pressure of 2 × 10−6 mm Hg at which there is no trapping of resonance radiation. The collision cross sections at very low argon pressures, at which the collision frequencies do not exceed one per average lifetime of the excited states, are Q1 = 10 × 10−15 cm2 for the 4 2P1/2 → 4 2P3/2 transition and Q2 = 7.2 × 10−15 cm2 for the 4 2P1/2 ← 4 2P3/2 transition. At higher pressures, where the collisions are much more frequent, Q1 = 1.8 × 10−15 cm2, Q2 = 1.1 × 10−15 cm2, and Q1/Q2 = 1.6, in exact agreement with the principle of detailed balancing. The variation of the collision cross sections with argon pressure is ascribed to different transfer mechanisms predominating in the various pressure regions.