Linear and nonlinear optical measurements of the Lorentz local field

We have studied the resonant optical response of a dense atomic potassium vapor under conditions such that the response is strongly influenced by local-field effects. Our experimental results are in good agreement with the predictions of theory. We have also discovered a collision-induced shift of the potassium resonance lines given by Δ${\mathrm{\omega }}_{\mathrm{c}\mathrm{o}\mathrm{l}=}$βN, where β=-5.0×${10}^{\mathrm{-}8}$ ${\sec }^{\mathrm{-}1}$ ${\mathrm{cm}}^3$ for the 4 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{S}}_{1/2}$⇆4 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{P}}_{1/2}$ transition and where β=-3.0×${10}^{\mathrm{-}8}$ ${\sec }^{\mathrm{-}1}$ ${\mathrm{cm}}^3$ for the 4 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{S}}_{1/2}$⇆4 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{S}}_{1/2}$⇆4 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{P}}_{3/2}$ transition.