Mean Life of theP23Metastable Argon Level

Using a previously described light absorption technique, the mean life of the metastable ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ level of argon has been measured in pure argon as a function of pressure at 77°K and 300°K, and in mixtures of neon and argon at 300°K. The coefficient of diffusion of metastable argon atoms in the parent gas falls much more rapidly with decreasing temperature than the $T^{\frac{3}{2}}$ dependence predicted using the hard-sphere model. The cross section for the quenching of metastable atoms by two-body collisions with normal atoms changes slightly with temperature, in a manner inconsistent with the assumption of collision-induced transitions to the radiating ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ level. As the temperature is reduced from 300°K to 77°K, the frequency of quenching by three-body collisions increases from $13.5p^2$ to $440p^2$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{}\left(\mathrm{mm}\mathrm{of}\mathrm{Hg}\right)\mathrm{}}^2$. At a given total pressure, the two-body quenching collision cross section is much reduced and the three-body quenching collisions disappear as the ratio of partial pressures of neon to argon is increased.