  Understanding collective electronic states such as superconductivity and
charge density waves is pivotal for fundamental science and applications. The
layered transition metal dichalcogenide 1T-TiSe2 hosts a unique charge density
wave (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) which
allows us to establish the contribution of excitonic and electron-phonon
interactions 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 recovery
dynamics at fluences above Fth = 60 microJ cm-2. This observation coincides
with a shift in the relative weight of coherently coupled phonons to higher
frequency modes in time-resolved reflectivity (TRR), suggesting a phonon
bottleneck. Using a rate equation model, the emergence of a high-fluence
bottleneck is attributed to an abrupt reduction in coupled phonon damping and
an increase in exciton dissociation rate linked to the loss of CDW superlattice
phonons. Thus, our work establishes the important role of both excitonic and
phononic interactions in the CDW phase transition and the advantage of
combining complementary femtosecond techniques to understand the complex
interactions in quantum materials.
