--- title: "Ultrafast Spectroscopy and Watching a Reaction Happen" description: "A pulse short enough to beat a vibration lets one experiment start a reaction and a second one look at it a known delay later. What comes back is a series of spectra against time, from which intermedi" canonical: https://lightmysky.com/learn/science/ultrafast-spectroscopy-and-watching-a-reaction-happen-mt_wxQgetzzW5 source: https://lightmysky.com/learn/science/ultrafast-spectroscopy-and-watching-a-reaction-happen-mt_wxQgetzzW5.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Ultrafast Spectroscopy and Watching a Reaction Happen A pulse short enough to beat a vibration lets one experiment start a reaction and a second one look at it a known delay later. What comes back is a series of spectra against time, from which intermediates and their lifetimes are read. Subject: Science · Area: Chemistry · Ages 23 to 24 Page: https://lightmysky.com/learn/science/ultrafast-spectroscopy-and-watching-a-reaction-happen-mt_wxQgetzzW5 ## Ready when they can - Explains why the pulse duration sets the shortest process the experiment can resolve - Reads a transient absorption map and identifies bleach, excited-state and product features - Extracts a lifetime from decay traces and states the model that fitting assumed - Connects a measured intermediate to a step in a proposed mechanism ## Lesson: Filming a reaction with light You fire a pump pulse to start the reaction and a probe pulse to look at it after a set delay. Nothing briefer than the pulse width can be resolved, so shorter pulses see faster steps. You pick pulses briefer than the motion you chase. **Example.** Suppose you watch signal against time and colour. A negative dip where the starting material absorbed is the bleach. A fresh absorption that fades quickly is the excited state, and one that stays grown is the product. You slice one colour and watch its signal fall with delay. You fit that fall with a model, often a single exponential, and the fit constant is the lifetime. You always name the model, because a different model gives a different number. **Tip.** You match each short lived feature to a step of the proposed path. A feature that grows in over two picoseconds and then decays is an intermediate that is made and then spent. When no feature matches a proposed step, you revise the mechanism. **Recap.** You start and probe with brief pulses, read bleach and fresh absorptions off the map, fit decays to get lifetimes, and tie each intermediate to a mechanism step. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [What an Excited Molecule Does Next: Fluorescence and Phosphorescence](https://lightmysky.com/learn/science/what-an-excited-molecule-does-next-fluorescence-and-phosphorescence-mt_5AVF6uWed_) - [Reaction Dynamics: Trajectories Across a Potential Energy Surface](https://lightmysky.com/learn/science/reaction-dynamics-trajectories-across-a-potential-energy-surface-mt_6ctTUYqYtS) - [Stimulated Emission and How a Laser Works](https://lightmysky.com/learn/science/stimulated-emission-and-how-a-laser-works-mt_xgjD0mzEAV)