  Clusters of single spins form the relevant spin entities in the formation of
long-range magnetic order in spin cluster Mott insulators. Such type of spin
order bears resemblance to molecular crystals, and we therefore may expect a
prototypical spin wave spectrum which can be divided into low-energy external
and high-energy internal cluster spin wave modes. Here, we study high-energy
spin cluster excitations in the spin cluster Mott insulator Cu$_{2}$OSeO$_{3}$
by means of spontaneous Raman scattering. Multiple high-energy optical magnon
modes are observed, of which the Raman-activity is shown to originate in the
Elliot-Loudon scattering mechanism. Upon crossing the long-range ordering
transition temperature the magnetic modes significantly broaden, corresponding
to scattering from localized spin excitations within the spin clusters.
Different optical phonon modes show a strong temperature dependence, evidencing
a strong magnetoelectric coupling between optical phonons and the high-energy
spin cluster excitations. Our results support the picture that
Cu$_{2}$OSeO$_{3}$ can be regarded as a solid-state molecular crystal of spin
nature.
