  Correlation among particles in finite quantum systems leads to complex
behaviour and novel states of matter. One remarkable example is predicted to
occur in a semiconductor quantum dot (QD) where at vanishing density the
Coulomb correlation among electrons rigidly fixes their relative position as
that of the nuclei in a molecule. In this limit, the neutral few-body
excitations are roto-vibrations, which have either rigid-rotor or
relative-motion character. In the weak-correlation regime, on the contrary, the
Coriolis force mixes rotational and vibrational motions. Here we report
evidence of roto-vibrational modes of an electron molecular state at densities
for which electron localization is not yet fully achieved. We probe these
collective modes by inelastic light scattering in QDs containing four
electrons. Spectra of low-lying excitations associated to changes of the
relative-motion wave function -the analogues of the vibration modes of a
conventional molecule- do not depend on the rotational state represented by the
total angular momentum. Theoretical simulations via the
configuration-interaction (CI) method are in agreement with the observed
roto-vibrational modes and indicate that such molecular excitations develop at
the onset of short-range correlation.
