Isotope Effect in the Imprisonment of Resonance Radiation

On theoretical grounds it is expected that the decay time of imprisoned resonance radiation in the vapor of a single even isotope of mercury should be some six times larger than that observed with the natural samples of mixed isotopic constitution. To investigate this effect experimentally, decay measurements were carried out with samples of Hg¹⁹⁸ (3 percent Hg¹⁹⁹ and 0.1 percent Hg²⁰⁰). For vapor densities N10¹⁴cc, the predicted effect was verified. For N>10¹⁴cc, the observed decay time drops below that predicted theoretically for pure Hg¹⁹⁸. This secondary effect is here attributed to the transfer of excitation from Hg¹⁹⁸ to Hg¹⁹⁹ and Hg²⁰⁰ by collisions of the second kind. Comparison of experimental data with an appropriately generalized theory permits estimation of the cross section for such collisions. The value so obtained is 10^-13 cm², some twenty times the gas-kinetic cross section, and in order-of-magnitude agreement with theoretical expectations.