Fig. 2. Vibrational wave-packet dynamics.

a Schematic of the ultrafast pump–probe experiment on FAPbBr₃ PNCs. b, c Chirp-corrected transient differential absorption (ΔΑ) spectra of the FAPbBr₃ PNCs depicted at specific time delays of 0.2, 1, and 10 ps, and in a contour diagram, respectively. The color bar in c represents ΔΑ (mOD). The linear absorption spectrum is also shown on the top panel of b for comparison. The PNCs are excited with a ~100 fs laser pulse at 400 nm (excitation photon density ~3.5 × 10¹³ cm⁻²). d The time trace obtained at 510 nm is shown as a blue dotted line. The residual time trace, obtained after subtraction of the exponential fit, is also shown as a blue line. e Illustration of a vibrational wave packet motion along a particular phonon displacement coordinate of PbBr₆⁴⁻ octahedral, generated by the interaction of the pump pulse with the PNCs. The vertical and horizontal axes denote the potential energy and phonon displacement coordinate Q_i, respectively. The interaction of the above bandgap pump pulse with the PNCs leads to the generation of wave packets in the ground state (G.S.), as well as the excited state (E.S., not shown). Subsequently, the wave packet starts oscillating at the ground state from its origin, far from its equilibrium position, which can be detected by the time–dependent probe pulse at the bleach position.