HE-3 IMPURITY EXCITATION SPECTRUM IN LIQUID-HE-4

We microscopically evaluate the excitation spectrum of the He-3 impuri ty in liquid He-4 at T=0 and compare it with the experimental curve at equilibrium density. The adopted correlated basis perturbative scheme includes up to two independent phonons, intermediate correlated state s, and the correlation operator is built up with two- and three-body c orrelation functions. The experimental spectrum is well described by t he theory along all the available momentum range. A marked deviation f rom the simple Landau-Pomeranchuck quadratic behavior is found and the momentum-dependent effective mass of the impurity increases by simila r to 50% at q similar to 1.7 Angstrom(-1) with respect to its q=0 valu e. No signature of rotonlike structures is found.