Research · 40 min

Pump–probe: watching a molecule react in femtoseconds

Read transient-absorption kinetics at three probe wavelengths: separate the excited-state absorption, the ground-state bleach and the product band, and extract lifetimes.

Goal

Assign each ΔA sign to a spectroscopic origin, extract the decay constant of the excited state (τ ≈ 2.2 ps) and the product rise time (τ ≈ 1.4 ps), and argue what the kinetics prove about intermediates.

Apparatus and reagents

Virtual ultrafast bench: a femtosecond pump beam, a delayed white-light probe, and the computed ΔA traces at 450, 530 and 640 nm.

Procedure

Select 450 nm: the positive signal jumps at t = 0 then decays — excited-state absorption: the S₁ population absorbing probe photons.
Switch to 530 nm: the trace is negative — ground-state bleach, because pumped molecules no longer absorb there. Watch it recover as S₀ refills.
At 640 nm the signal grows in with τ ≈ 1.4 ps — a new species absorbing where neither S₀ nor S₁ does: the product.
Hover on each trace at t = τ: verify the signal has fallen (or risen) to about 63 % of the way between initial and final values.
Compare the ESA decay (2.2 ps) with the product rise (1.4 ps): the mismatch hints the product comes from a hot intermediate, not directly from S₁.

What to observe

All three signals appear exactly at t = 0 — the pump and probe are only femtoseconds long, so the instrument resolves the instant of excitation.
The bleach recovers slower than the ESA decays — part of the population is stored in a dark state or product channel.
No signal exists before t = 0 — ΔA is a difference measurement (pumped minus unpumped), so it is blind to the resting molecule.

Explanation

Transient absorption measures $\Delta A(\lambda, t) = A_\mathrm{pumped} - A_\mathrm{unpumped}$ : positive ΔA means new absorbers (excited states, products), negative ΔA means lost ground-state absorbers (bleach) or stimulated emission. Fitting multi-wavelength data globally decomposes it into species-associated difference spectra — turning raw decays into a movie of intermediates. Zewail’s group used this to watch bonds break in real time (ICN, NaI photodissociation), founding femtochemistry (Nobel 1999); attosecond variants now resolve electron motion itself.

History of the experiment

In 1987 Zewail’s Caltech group resolved the femtosecond dissociation of ICN — the first direct “movie” of a bond breaking. The Nobel Prize followed in 1999; attosecond science was honoured in 2023 (Agostini, Krausz, L’Huillier).

Chemists behind it

Ahmed Zewail Rudolph A. Marcus