  We have experimentally studied the renormalized effective mass m* and Dingle
temperature T_D in two spin subbands with essentially different electron
populations. Firstly, we found that the product (m*T_D) that determines damping
of quantum oscillations, to the first approximation, is the same in the
majority and minority subbands even at the spin polarization degree as high as
66\%. This result confirms the theoretical predictions that the interaction
takes place at high energies ~ E_F rather than within a narrow strip of
energies E_F\pm k_BT. Secondly, to the next approximation, we revealed a
difference in the damping factor of the two spin subbands, which causes
skewness of the oscillation lineshape. In the absence of the in-plane magnetic
field, the damping factor (m*T_D) is systematically smaller in the
spin-majority subband. The difference, quantified with the skew factor \gamma =
(T_{D\downarrow}-T_{D\uparrow})/2T_{D0} can be as large as 20%. The skew factor
tends to decrease as B_\parallel or temperature grow, or perpendicular field
decreases; for low electron densities and high in-plane fields the skew factor
even changes sign. Finally, we compared the temperature and magnetic field
dependencies of the magneto-oscillations amplitude with predictions of the
interaction correction theory, and found, besides some qualitative
similarities, several quantitative and qualitative differences. To explain
qualitatively our results, we suggested an empirical model that assumes the
existence of easily magnetized triplet scatterers on the Si/SiO_2 interface.
