We have used the acoustic Faraday effect in superfluid ${}^3\mathrm{He}$ to perform high resolution spectroscopy of an excited state of the superfluid condensate, called the imaginary squashing mode. With acoustic cavity interferometry we measure the rotation of the plane of polarization of a transverse sound wave propagating in the direction of the magnetic field from which we determine the Zeeman energy of the mode. We interpret the Landé $g$ factor, combined with the zero-field energies of this excited state, using the theory of Sauls and Serene, to calculate the strength of $f$-wave interactions in ${}^3\mathrm{He}$.

• Received 30 August 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.015301