Fig. 2.

Temperature series at low fluence. Normalized tr-RXS intensity ($I(\mathrm{\Delta }t)/I_0$) at $Q=(001)$ as a function of pump–probe delay in LESCO ($x=0.15$) with $T_{\text{CDW}}\approx$ 75 K (44, 59). Pump photon energy 1.55 eV and fluence 50 $\mathrm{\mu }$J/cm². (A) The tr-RXS intensity at the apical O $K$ resonance, sensitive only to structural distortions, is unperturbed by the optical pump at this low fluence. The planar O $K$ resonance (B) and the Cu $L_3$ resonance (C) are sensitive to pump-induced nematicity dynamics in the CDW phase.