## 2 Experiments and Results  
Our femtosecond photoemission experiments were carried out in a pump-probe scheme in normal emission geometry on in situ cleaved single crystals of \(1T\)-TaS\({}_{2}\) that were grown by a standard chemical vapor transport method (see [13] for details). The samples were cleaved in ultrahigh vacuum at room temperature. The obtained surfaces were excited with 50 fs laser pulses from a commercial regenerative Ti:Sa laser amplifier (Coherent Rega 9040, \(\hbar\omega_{1}=\)1.55 eV) and subsequently probed by direct photoemission using UV pulses (\(\hbar\omega_{2}=\)6.2 eV), which were generated by two consecutive frequency-doubling processes of the fundamental pulses using \(\beta\)-barium borate crystals. The overall temporal and spectral resolutions were 95 fs and 55 meV, respectively. Excitation fluences (given here as incident fluences \(F\)) increased up to the regime where switching of the samples into the hidden state could be expected (\(F>1\) mJ cm\({}^{-2}\)) [25; 27]. Time zero was determined with an accuracy better than 5 fs, similar to the method used in Ref. [13]. A typical dataset obtained in the c-CDW/Mott phase (\(T=80\) K, \(F=\)730 \(\upmu\)) cm\({}^{-2}\) is shown in Figure 2a as a false-color representation, which suitably visualizes the variety of effects observed upon optical excitation. Such data resemble previous studies under similar conditions [19; 20; 23; 24] that revealed (i) a transient quench and subsequent recovery of the LHB signature at \(E-E_{F}\approx-0.17\) eV, (ii) a partial filling of the formerly gapped spectral region around \(E_{F}\), (iii) the generation of a hot carrier population continuum and (iv) oscillations of the spectral weight due to the coherent excitation of the CDW amplitude mode phonons with frequencies of 2.4 and 2.5 THz. A similar behavior is found for all excitation fluences used. Our discussion here will focus on the dynamics of the LHB and the signature of the amplitude mode of the commensurate CDW phase. A particular aspect we focus on is detailed in Figure 2b, which shows photoemission spectra before and after (\(\Delta t=+100\) fs) laser excitation for the c-CDW/Mott phase in comparison with an equilibrium room-temperature spectrum. Upon optical excitation, a quench of the LHB signature is observed that  
Figure 2: (a) False-color representation of data obtained for an incident pump fluence \(F=730\)\(\upmu\) cm\({}^{-2}\) at sample base temperature \(T=80\) K. An instantaneous collapse and subsequent recovery of the LHB signature is observed along with the generation of a hot carrier continuum and oscillations of the spectral weight due to the coherent excitation of the CDW amplitude mode; (b) Photoemission spectra of the room-temperature and insulating phase (red and black solid lines, respectively). The shaded area indicates the photo-induced change of the low-temperature spectrum upon strong optical pumping (\(F=1700\)\(\upmu\)/cm\({}^{2}\)); (c) Illustration of the laser-excited charge ordered state, as concluded from the femtosecond photoemission studies. The sample is excited by a femtosecond IR pulse to partly turn single polaron clusters (blue stars) to become metallic (red stars). Subsequent probing through photoemission by the UV pulse maps the coexistence of both phases at delay \(\Delta t\). is accompanied by a partial filling of the formerly gapped spectral region (\(E-E_{F}\approx-0.2\)... + 0.2 eV), indicating that the Mott phase is (partially) molten and that the corresponding short-range correlations are quenched. The shown spectra are recorded under similar conditions, which allows for a direct comparison of the excited c-CDW phase spectrum with the room-temperature equilibrium spectrum. Comparing the spectra, we find for the maximum achievable quench of the LHB that the spectra of the excited c-CDW phase and the equilibrium metallic phase become comparable in the energetic window of the LHB signature.