  A non-intrusive method to measure particle interaction using only the thermal
motion of the particles is applied to a vertically aligned dust particle pair
in a complex plasma. The scanning mode spectra (SMS) are obtained by tracking
the thermal motion of the grains, with the interaction strength then determined
from the frequencies and the eigenvector configuration of the normal modes. The
interaction of the bottom particle acting on the top particle is shown to be
Yukawa-like with the screening length suppressed against the ion flow. The
interaction of the top particle acting on the bottom particle is repulsive in
the vertical direction and attractive in the horizontal direction. The vertical
interaction from the top to bottom particle is stronger than that from the
bottom to top particle, agreeing with an extended ion wake tail as predicted by
the inhomogeneous ion wake theory. Determination of the horizontal attraction
strength serves as a direct verification and quantification of the ion wake
effect. Heating of the lower particle in both the vertical and horizontal
directions is observed and quantitatively related to the nonreciprocity of the
interaction. The in situ confinement strength at the position of the bottom
particle is found to be consistently lower than at the position of the top
particle, caused by decharging of the lower particle by 10-30% while inside the
ion wake of the top particle.
