  We calculate the eigenstates of a diatomic molecule in a range of model
mean-field potentials, and evaluate the evolution of their associated Raman
spectra with field strength. We demonstrate that dramatic changes in the
appearance of the Raman spectrum for a diatomic molecule occur without any
associated change in the symmetry of the surrounding potential. The limiting
cases of the quantum eigenstates correspond, in the classical sense, to free
rotation, and libration of well-oriented molecules. However, there are also
many mixed modes which are neither rotons nor librons. The consequence for
Raman spectroscopy is a series of complications - the non-harmonic potential
splits the Raman active modes, and breaks the selection rules on forbidden
modes. The mass-dependence of the various states is different - rotors,
oscillators and reorientations have $1/m$, $1/\sqrt{m}$ and weaker mass
dependence respectively. This may allow one to identify the character of the
mode with isotope spectroscopy. However it is complicated by mixed modes and
transitions between two different eigenstates with different character. We
conclude that significant changes in the Raman spectrum of molecular systems
are insufficient to demonstrate a phase transition since such changes can also
occur in a fixed symmetry potential upon increasing field strength.
