Impact broadening of the 4p-7s transitions in potassium by helium and argon

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Abstract
The impact broadening of the potassium 4p2P1/2-7s2S1/2 (5782 AA) and 4p2P3/2-7s2S1/2 (5802 AA) lines by helium and argon foreign gases has been studied interferometrically in the emission spectrum of a low-current electrical discharge. Unlike the resonance lines and other transitions from low-lying states, the broadening and shift rates here are so large that very significant effects are observable with conventional spectroscopy at gas pressures as low as 10 Torr. The observed effects are interpreted with impact broadening calculations based on the pseudopotential interactions computed by Pascale and Vandeplanque (1974), and on the Fermi-type potential proposed by Roueff (1970). The pseudopotentials underestimate considerably the observed broadening and shift, while the Fermi potential gives an improved, although not entirely satisfactory, agreement. Potentials deduced for the 7s2S1/2 state by fitting power-law interactions to the observed broadening and shift rates have a long-range attraction for argon and a long-range repulsion for helium. These fitted interaction potentials are about an order of magnitude stronger than the pseudopotential calculations for the internuclear separations of approximately 20 AA that contribute most significantly to the broadening and shift.