PCI effects in electron-impact excitation of the (3p$^{5}$4s$^{2})^{2}$P$_{J}$ levels in potassium.

We report preliminary results for the PCI lineshift in ejected-electron spectra corresponding to the decay of the (3p$^{5}$4s$^{2})^{2}$P$_{3/2,1/2}^{ }$ autoionizing levels in potassium atoms. The spectra were measured precisely for impact energies up to 7 eV above the corresponding excitation thresholds. The energy dependencies of the lineshift were fitted using a semi-classical approach [1], which ignores the potential influence of K$^{-}$ resonances. The autoionizing widths for the $^{2}$P$_{3/2 }$ and $^{2}$P$_{1/2 }$ levels obtained in this method were, respectively, 16 meV and 26 meV. These values exceed, by approximately a factor of three, previous experimental [2,3] results that were free of PCI effects. Hence this discrepancy may be due to the presence of the strong near-threshold K$^{-}$ resonances observed earlier in the excitation functions for the above levels [4]. [1] A. Niehaus, J. Phys. B \textbf{10} 1845 (1977). [2] R.D. Hudson and V.L. Carter, J. Opt. Soc. Amer. \textbf{57} 1471 (1967). [3] R.D. Driver, J. Phys. B \textbf{9} 143 (1976). [4] A.A. Borovik, A.N. Grum-Grzhimailo, K. Bartschat and O. Zatsarinny, J. Phys. B \textbf{38} 1081 (2005).