  We report a comprehensive Raman study of the phonon behaviour in PdSe$_2$ in
the temperature range of 5 K to 300 K. A remarkable change in the Raman
spectrum is observed at 120 K: a significant enhancement of the out-of-plane
phonon A$^{1}_{g}$ mode, accompanied by a suppression of the in-plane
A$^{2}_{g}$ and B$^{2}_{1g}$ modes. This intriguing behavior is attributed to a
temperature-dependent resonant excitation effect. The results are supported by
density functional theory (DFT) calculations, which demonstrate that the
electron-phonon coupling for the phonon modes varies and is strongly associated
with the relevant electronic states. Furthermore, nonlinear frequency shifts
are identified in all modes, indicating the decay of an optical phonon into
multiple acoustic phonons. The study of Raman emission reported here,
complemented by linear optical spectroscopy, reveals an unexpected scenario for
the vibrational properties of PdSe$_2$ that holds substantial promise for
future applications in PdSe$_2$-based optoelectronics.
