Electron-impact excitation of the potassium atom

Abstract

Absolute optical electron-impact excitation functions for 24 transitions of the sharp, principal, diffuse, and fundamental spectral series of potassium have been measured. The determination of the density of the potassium vapor in the collision chamber was made by measuring the degree of transmission, by the vapor, of potassium resonance radiation generated externally in a fluorescence cell. From the measured optical excitation functions direct excitation functions have been determined for 14 states ($5S$, $6S$, $7S$, $8S$, $4P$, $5P$, $6P$, $7P$, $3D$, $5D$, $6D$, $5F$, $6F$, and $7F$) with the aid of known radiative-transition probabilities compiled in the literature. Theoretical calculations of these same 14 excitation functions, as well as $4D$ and $4F$, were carried out by means of the Born approximation. The $4P$, $5P$, $5S$, $3D$, and $4D$ direct excitation functions at intermediate energies (10-25 eV) were also calculated by the method of multistate close coupling, neglecting projectile-target-electron exchange. The high-energy (above 100 eV) Born-approximation cross sections agree with the experimental results for $4P$ and for all $S$ states, but are lower than experimental results, by 30-40%, for the $D$ and $F$ states. At intermediate energies the close-coupling excitation calculations agree well with the experimental excitation functions for $4P$ and $5P$, but are significantly higher than experimental values for $5S$ and $3D$. The discrepancies between the experimental and theoretical results are probably due to a combination of systematic experimental errors, errors in the available transition-probability values, and errors in the theoretical excitation functions introduced by the use of approximate excited-state wave functions (Hartree-Fock-Slater), or, in the case of intermediate or low energies, by the neglect of projectile-target-electron exchange. The polarization of the $4P\ensuremath{-}4S$ and $3D\ensuremath{-}4P$ radiation produced by electron impact was measured, and the results were used to determine the direct excitation functions of the separate magnetic sublevels of the $4P$ state.