  The diffusion Monte Carlo technique is used to calculate and analyze the
excitation spectrum of $^3$He atoms bound to a cluster of $^4$He atoms, by
using a previously determined optimum filling of single-fermion orbits with
well defined orbital angular momentum $L$, spin $S$ and parity quantum numbers.
The study concentrates on the energies and shapes of the three kinds of states
for which the fermionic part of the wave function is a single Slater
determinant: maximum $L$ or maximum $S$ states within a given orbit, and fully
polarized clusters. The picture that emerges is that of systems with strong
shell effects whose binding and excitation energies are essentially determined
over configuration at fixed number of particles and spin, i.e., by the monopole
properties of an effective Hamiltonian.
