  We report on far- and mid-infrared reflectivity of NdMnO3 from 4 K to 300K.
Two main features are distinguished in the infrared spectra: active phonons in
agreement with the expected for orthorhombic D2h 16-Pbnm (Z=4) space group
remaining constant down to 4 K and a well-defined collective excitation in the
THz region due to eg electrons in a d-orbital fluctuating environment. We trace
its origin to the NdMnO3 high temperature orbital disordered intermediate phase
not being totally dynamically quenched at lower temperatures. This results in
minute orbital misalignments that translate in randomize non-static eg
electrons within orbitals yielding a room temperature collective excitation.
Below TN~78 K, electrons gradually localize inducing long-range magnetic order
as the THz band condenses into two modes that emerge pinned to the A-type
antiferromagmetic order. They harden simultaneously down to 4 K obeying power
laws with TN as the critical temperature and exponents {\beta}~0.25 and
{\beta}~0.53, as for a tri-critical point and Landau magnetic ordering,
respectively. At 4K they match known zone center spin wave modes. The power law
dependence is concomitant with a second order transition in which spin modes
modulate orbital instabilities in a magnetoelectric hybridized
orbital/charge/spin/lattice scenario. We also found that phonon profiles also
undergo strong changes at TN~78 K due to magnetoelasticity.
