  Theoretical treatment of the electron spin echo envelope modulation (ESEEM)
spectra from polarons in a semiconducting $\pi$- conjugated polymer is
presented. The contact hyperfine coupling and the dipolar interaction between
the polaron and proton spins are found to have distinct contributions in the
ESEEM spectra. However, since the two contributions are spaced very closely,
and the dipolar contribution is dominant, the detection of the contact
hyperfine interaction is difficult. To resolve this problem, a recipe of
probing the contact hyperfine and dipolar interactions selectively is proposed,
and a method for detecting the polaron contact hyperfine interaction is
formulated. The ESEEM decay due to the polaron random hopping is analyzed, and
the robustness of the method against this decay is verified. Moreover, this
decay is linked to the transport properties of polarons, providing an auxiliary
probe for the polaron transport.
