The ultrafast optical pulse induced CDW phase transition monitored by time-resolved x-ray photoemission spectroscopy (TR-XPS). (a) Scheme of CCDW 1T-TaS₂ consisting of David-star clusters with inequivalent a, b, and c Ta atoms. (b) Ta 4f XPS spectra at different delay times. The color represents the electron density, which increases towards the center Ta atom. The Ta 4f photoemission is split into two peaks, corresponding to sites b and c, separated by 2 Δ (i.e., Δ CDW ). (c) Two-dimensional false color map of Ta 4f photoemission spectra at different delays. (d) Time-resolved photoemission intensity at E − E F = − 22.26 eV . (e) Dependence of 2 Δ CDW on the delay time. (f) Delay-time-dependent electron temperature T_e. This figure was reproduced from Ref. [23] with permission. Copyright 2010, American Physical Society.