Experimental determination of highly excited F levels in potassium by three-photon absorption spectroscopy

Three-photon absorption spectroscopy allows determination of the nf²F5/2,7/2⁰ energy levels of alkali metals (ΔL=+3). The 3d²D3/2,5/2 levels of KI were selectively excited using a narrow-band dye laser. The 3d²D3/2,5/2→nf²F5/2,7/2⁰ absorption lines were observed in the background spectrum of another dye laser, and the nf²F5/2,7/2⁰ energy levels were deduced, from $n=10$ to $n=14$ inclusive, with an accuracy better than 0.1 cm^-1. Previously, an experiment was performed for the 7f²F5/2,7/2⁰ levels in order to verify the method. Shifts and systematic errors were entirely negligible. For all lines, the relative intensities of the absorbed doublet peaks are in good agreement with the estimates of the 4s²S1/2→3d²D3/2,5/2 probabilities.