Three-photon absorption spectroscopy allows determination of the
nf2F25,270 energy levels of alkali metals ($\ensuremath{\Delta}L=+3$). The
3d2D23,25 levels of KI were selectively excited using a narrow-band dye laser. The $3d^{2}D_{\frac{3}{2},\frac{5}{2}}\ensuremath{\rightarrow}\mathrm{nf}^{2}F_{\frac{5}{2},\frac{7}{2}}^{0}$ absorption lines were observed in the background spectrum of another dye laser, and the
nf2F25,270 energy levels were deduced, from
n=10 to
n=14 inclusive, with an accuracy better than 0.1 ${\mathrm{cm}}^{\ensuremath{-}1}$. Previously, an experiment was performed for the
7f2F25,270 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 $4s^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}3d^{2}D_{\frac{3}{2},\frac{5}{2}}$ probabilities.