On the mechanism of selective population of the $3p_1$ level of the neon atom in $\mathrm{He}$–$\mathrm{Ne}$ plasma

In the spectroscopic studies of helium-neon plasma carried out earlier, a pronounced selectivity of the population of the $3p_1$ level (according to Paschen), the upper one from the group of levels of the $2p^54p$ configuration of the $\mathrm{Ne}$ atom, was revealed. As the helium pressure increased, up to $60$% of the intensity of all transitions $2p^54p\to 2p^53s$ was concentrated in the $352.05$ nm line $(3p_1\to1s_2)$. In the present work, a mechanism is proposed for the growth of the relative population of the $3p_1$ level with increasing $\mathrm{He}$ pressure, associated with the features of the collisional kinetics of the $3p_i$ states of the $2p^54p$ configuration. The reason for the significantly faster depletion of the lower $3p_i$ levels $(i\ge 2)$ compared to $3p_1$, apparently, is the features of the mutual arrangement of the adiabatic terms of the $\mathrm{Ne}(2p^54p)$ + $\mathrm{He}(1s^2\,{}^1S_0)$ system. The mechanism of formation of selective population of the 3p1 level considered in this work can be realized in a mixture of helium with neon and cannot be realized in pure neon, which corresponds to the results of spectroscopic studies. Model calculations of the part of the spectrum related to transitions ($3p_1$, $3p_4$, $3p_2\to1s_j$) were carried out for a fixed distribution of population flows $3p_1$, $3p_4$, $3p_2$ and different helium pressures. Good agreement between the measurement results and the results of numerical modeling is observed.