  We present the derivation of a new model to describe neutron spin echo
spectroscopy and quasi-elastic neutron scattering data on liposomes. We compare
the new model with existing approaches and benchmark it with experimental data.
The analysis indicates the importance of including all major contributions into
modeling of the intermediate scattering function. Simultaneous analysis of the
experimental data on lipids with full contrast and tail contrast matched
samples, reveals highly confined lipid tail motion. A comparison of their
dynamics demonstrates the statistical independ-ence of tail-motion and
height-height correlation of the membrane. A more detailed analysis indi-cates
that lipid tails are subject to relaxations in a potential with cylindrical
symmetry, in addition to the undulation and diffusive motion of the liposome.
Despite substantial differences in the chemis-try of the fatty acid tails, the
observation indicates a universal behavior. The analysis of partially
deuterated systems confirms the strong contribution of the lipid tail to the
intermediate scattering function. Within the time range from 5 to 100 ns, the
intermediate scattering function can be de-scribed by the height-height
correlation function. The existence of the fast-localized tail motion and the
contribution of slow translational diffusion of liposomes determines the
intermediate scattering function for t < 5 ns and t > 100 ns, respectively.
Taking into account the limited time window lowers the bending moduli by a
factor of 1.3 (DOPC) to 2 (DMPC) compared to the full range.
