The spontaneous condensation of excitons in the excitonic insulating phase has been reported in \(\hbox {Ta}_2\hbox {NiSe}_5\) (TNSe) below 325 K. In this context, we present the temperature-dependent optical pump–optical probe spectroscopy of \(\hbox {Ta}_2\hbox {NiSe}_5\) , with the focus on coherent phonon (CP) dynamics. In addition to the fast relaxation process involving excitonic recombination, we observe a systematic behaviour for the slow relaxation process associated with the relaxation of hot phonons. The asymmetry parameter and cubic anharmonicity of the 3 THz mode demonstrate the structural transition across \(T_C=325\) K, whereas the nature of the order parameter and asymmetry of 2 THz modes reveal its coupling with the excitonic phase of TNSe. Coherent phonon modes display less anharmonicity than the corresponding Raman modes. Continuous wavelet transform (CWT) reveals that the peak time \(t_{\textrm{peak}}\) of the phonons is similar for all modes except the 3 THz mode. The temperature dependence of \(t_{\textrm{peak}}\) for the M3 mode exhibits a possible role of excitonic condensate below \(T_C\) in the formation of quasiparticle (phonon). CWT analysis supports the time-dependent asymmetry of the M3 mode caused by photoexcited carriers. This study illustrates the role of photoexcited carriers in depicting a structural transition and dressing of coherent phonons, hence, demonstrating many-body effects.
