Radio-Frequency Spectroscopy of a Mass-Imbalanced Fermi-Fermi Mixture: Measuring Atom-Dimer Interactions*

We use radio-frequency spectroscopy to investigate a mixture of $^{40}$K atoms and $^{6}$Li$^{40}$K Feshbach molecules on the repulsive side of a $^{6}$Li-$^{40}$K Fermi-Fermi interspecies Feshbach resonance. The shifts in the peak positions in our spectra indicate an attractive interaction between the $^{40}$K atoms and $^{6}$Li$^{40}$K dimers, related to a $p$-wave atom-dimer scattering resonance. The measured attraction agrees well with a mean field description of the atom-molecule interaction, even in the strongly-interacting regime. Strong tails in our spectra point to the importance of momentum corrections to the wavefunction of the interacting K atoms and provide information about the excited states of our system.