  The multichannel Na-Cs interactions are characterized by a series of
measurements using two atoms in an optical tweezer, along with a multichannel
quantum defect theory (MQDT). The triplet and singlet scattering lengths are
measured by performing Raman spectroscopy of the Na-Cs motional states and
least-bound molecular state in the tweezer. Magnetic Feshbach resonances are
observed for only two atoms at fields which agree well with the MQDT. Our
methodology, which promotes the idea of an effective theory of interaction, can
be a key step towards the understanding and the description of more complex
interactions. The tweezer-based measurements in particular will be an important
tool for atom-molecule and molecule-molecule interactions, where high densities
are experimentally challenging and where the interactions can be dominated by
intra-species processes.
