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}ωcol={β}N, where {β}=-5.0×{}10^-8 sec^-1 cm³ for the 4 ²{S}1/24²P1/2 transition and where {β}=-3.0×{}10^-8 sec^-1 cm³ for the 4 ²{S}1/24²S1/2{}4 ²{P}3/2$ transition.