Classification of collective modes in a charge density wave by momentum-dependent modulation of the electronic band structure

We present time- and angle-resolved photoemission spectroscopy (trARPES) measurements on the charge density wave system CeTe3. Optical excitation transiently populates the unoccupied band structure and reveals a gap size of 2Δ=0.59eV. The occupied Te-5p band dispersion is coherently modified by three modes at Ω1=2.2,Ω2=2.7, and Ω3=3THz. All three modes lead to small rigid energy shifts whereas Δ is only affected by Ω1 and Ω2. Their spatial polarization is analyzed by fits of a transient model dispersion and density-functional theory frozen phonon calculations. We conclude that modes Ω1 and Ω2 result from in-plane ionic lattice motions, which modulate the charge order and that Ω3 originates from a generic out-of-plane A1g phonon. We thereby demonstrate how the rich information from trARPES allows identification of collective modes and their spatial polarization, which explains the mode-dependent coupling to charge order.