  We show that electronic Raman scattering affords a window into the essential
properties of the pairing potential $V_{\mathbf{k},\mathbf{k^{\prime}}}$ of
iron-based superconductors. In Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ we observe band
dependent energy gaps along with excitonic Bardasis-Schrieffer modes
characterizing, respectively, the dominant and subdominant pairing channel. The
$d_{x^2-y^2}$ symmetry of all excitons allows us to identify the subdominant
channel to originate from the interaction between the electron bands.
Consequently, the dominant channel driving superconductivity results from the
interaction between the electron and hole bands and has the full lattice
symmetry. The results in Rb$_{0.8}$Fe$_{1.6}$Se$_2$ along with earlier ones in
Ba(Fe$_{0.939}$Co$_{0.061}$)$_2$As$_2$ highlight the influence of the Fermi
surface topology on the pairing interactions.
