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Ta₂NiSe₅ is a leading candidate material for hosting an excitonic insulating ground state. While above the transition temperature it is orthorhombic and semimetallic, it undergoes a monoclinic distortion and semimetal-semiconductor transition below 328 K. The contributions to the symmetry-breaking bandgap opening are highly debated because of the complicated interplay between the structural and electronic degrees of freedom.

We employ ultrafast near-infrared laser pulses to excite Ta₂NiSe₅ and generate coherent lattice oscillations of A_1g and B_2g symmetry. By using time- and angle-resolved photoemission spectroscopy we precisely map the coupling of those different optical phonon modes to the electronic band structure. Such coupling strengths are heavily momentum dependent and highlight the orbital characters of each band. Moreover, our experimental results are in good agreement with first principles predictions of the coupling strengths. Detailed studies of the momentum-dependent coherent phonon dynamics yield significant insights into the nature of the low-temperature broken-symmetry phase.