The pump pulse arrives at Δt = 0 and instantly promotes the A–B molecule from its ground-state equilibrium onto an excited-state potential curve whose minimum lies further out. Because the nuclei can't move during the pulse itself (Franck–Condon principle), the molecule starts this new curve at the old bond length but at rest — so it swings back and forth around the new equilibrium, a coherent vibrational wavepacket. Each outward swing carries it toward a curve-crossing point where a small probability exists of hopping onto a purely repulsive surface; once that happens the bond breaks and the fragments fly apart.
r(t) = r0 + A·(1 − cos ωt) t < t_cross (bound wavepacket)
r(t) = r_cross + Δr·(1 − e^-(t−t_cross)/τ) t ≥ t_cross (dissociating)
- Probe delay Δt — the second, weaker laser pulse arrives this many femtoseconds after the pump and "photographs" the instantaneous bond length; scrubbing it replays the reaction frame by frame, exactly like Zewail's original pump-probe experiments.
- Pump energy — higher energy launches the wavepacket with more vibrational amplitude and reaches the crossing region sooner, so the molecule dissociates after fewer oscillation periods.
- Transient signal plot — plotting the sampled bond length against delay reconstructs the reaction "movie" as a curve: clean oscillations while the molecule vibrates, then a monotonic rise once it dissociates.
- Molecular state — bound while oscillating inside the well, transition state during the final approach to the crossing point, dissociated once the fragments separate past the well radius.
Real timescale: one vibrational period here is 180 fs — comparable to real diatomics such as I₂ or NaI. The animation is slowed by roughly 10¹³× so the motion is visible; the femtosecond labels on the axes are the true physical timescale.