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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 inagreement with the expected for orthorhombic D2h 16-Pbnm (Z=4) space groupremaining constant down to 4 K and a well-defined collective excitation in theTHz region due to eg electrons in a d-orbital fluctuating environment. We traceits origin to the NdMnO3 high temperature orbital disordered intermediate phasenot being totally dynamically quenched at lower temperatures. This results inminute orbital misalignments that translate in randomize non-static egelectrons within orbitals yielding a room temperature collective excitation.Below TN~78 K, electrons gradually localize inducing long-range magnetic orderas the THz band condenses into two modes that emerge pinned to the A-typeantiferromagmetic order. They harden simultaneously down to 4 K obeying powerlaws 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 lawdependence is concomitant with a second order transition in which spin modesmodulate orbital instabilities in a magnetoelectric hybridizedorbital/charge/spin/lattice scenario. We also found that phonon profiles alsoundergo strong changes at TN~78 K due to magnetoelasticity.

Paramagnetic Collective Electronic Mode and Low Temperature Hybrid Modes in the Far Infrared Dynamics of Orthorhombic NdMnO3