Interferometric measurements in the spectrum of Ge I by Kaufman and Andrew show differences in wavelengths for 4p5s-4p6p lines emitted from different electrodeless discharge tubes containing germanium compounds and inert gases. The pattern of the shifts suggests that Stark effect is involved. This is illustrated on the basis of the theory for weak-field Stark effect for multiplets. The 6p level most sensitive to Stark effect is 6p3P0. Through interaction with 5d3D1, a field strength of 1.5 kV/cm shifts 6p3P0 by 0.01 cm-1.
LENNART MINNHAGEN, "Correlation between Observed Wavelength Shifts Produced in Electrodeless Discharge Tubes and Predicted Stark-Effect Shifts in the Spectrum of Neutral Germanium (Ge I)," J. Opt. Soc. Am. 54, 320-321 (1964)