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Understanding collective electronic states such as superconductivity andcharge density waves is pivotal for fundamental science and applications. Thelayered transition metal dichalcogenide 1T-TiSe2 hosts a unique charge densitywave (CDW) phase transition whose origins are still not fully understood. Here,we present ultrafast time- and angle-resolved photoemission spectroscopy(TR-ARPES) measurements complemented by time-resolved reflectivity (TRR) whichallows us to establish the contribution of excitonic and electron-phononinteractions to the CDW. We monitor the energy shift of the valence band (VB)and coupling to coherent phonons as a function of laser fluence. The VB shift,directly related to the CDW gap closure, exhibits a markedly slower recoverydynamics at fluences above Fth = 60 microJ cm-2. This observation coincideswith a shift in the relative weight of coherently coupled phonons to higherfrequency modes in time-resolved reflectivity (TRR), suggesting a phononbottleneck. Using a rate equation model, the emergence of a high-fluencebottleneck is attributed to an abrupt reduction in coupled phonon damping andan increase in exciton dissociation rate linked to the loss of CDW superlatticephonons. Thus, our work establishes the important role of both excitonic andphononic interactions in the CDW phase transition and the advantage ofcombining complementary femtosecond techniques to understand the complexinteractions in quantum materials.

Excitonic and lattice contributions to the charge density wave in 1T-TiSe$_2$ revealed by a phonon bottleneck