The aim of the present work is to perform Attenuated Total Reflectance-Fourier Transform Infra-Red spectroscopy (ATR-FTIR) investigations during N-2 plasma treatment of GaAs surfaces at 500 degrees C inside an UHV chamber using the high intensity light of SOLEIL synchrotron at SMIS beamline. Moreover this work uses the complementarities of Density Functional Theory (DFT) normal-mode frequency calculations with ATR-FTIR measurements to assist in the assignment of vibrations involving bonds to nitrogen atoms, especially by rendering the GaN layer partially amorphous (by removing the crystal symmetry constraint, using Car Parrinello dynamics to simulate annealing in geometry optimization). s and p polarizations were used to study the evolution of the different GaN vibrations during the nitridation of GaAs for two surface orientations (0 0 1) and (1 1 1)B. Results show that normal stretch vibrations are always detected at the same frequency matching the DFT calculated values very well, whereas other vibration modes (bridge symmetric and asymmetric stretches for (0 0 1) GaAs and tripod symmetric and asymmetric stretches for (1 1 1)B GaAs) induced by the non-crystallinity of the GaN layer exhibit variable frequencies. Furthermore evolutions of these frequencies with the nitridation time provide much information on bond creations, especially at the very beginning of the process.