Collision-Induced Transitions within Excited Levels of Neon

The interaction of an intense optical-maser field with excited atoms in a gas discharge leads to a considerable change in the population of those levels which are resonant at the maser frequency. This will cause a change in the population of other levels that are connected to the maser levels through collision-induced nonradiative as well as radiative transitions. Using the 1.15-μ optical maser, these effects have been studied in detail for the $2s$ levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the $2s_2$ and $2s_3$ levels. In pure neon this has been found to be ${\sigma }_{23}=(2.3\mathrm{}\pm 0.3)\times {10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. For a helium-neon mixture the measured cross section is ${\sigma }_{23}=(1.8\mathrm{}\pm 0.3)\times {10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2s_2\to 2p_{10}$ and $2s_3\to 2p_{10}$. The measured ratio is $\frac{A_2}{A_3}=3.0\mathrm{}\pm 0.4$.