Femtochemistry

Femtochemistry is the study of chemical reactions on the femtosecond timescale, about 10^-15 seconds, exposing the motions of atoms during bond formation and breaking. The field was established in the 1980s through the work of Ahmed H. Zewail, whose ultrafast laser experiments to observe reaction dynamics earned him the Nobel Prize in Chemistry in 1999.

The core approach is pump-probe spectroscopy. A first ultrafast laser pulse (the pump) excites a molecule or material, initiating a reaction. A second delayed pulse (the probe) interrogates the evolving system, recording signals such as transient absorption, fluorescence, photoelectron emission, or diffraction patterns as a function of delay time. By varying the time delay, researchers reconstruct the reaction trajectory on femtosecond to picosecond scales. As techniques have advanced, time-resolved X-ray and electron diffraction, along with multidimensional spectroscopies, have expanded the kinds of dynamics that can be tracked.

Femtochemistry reveals how bonds form and break, how molecules traverse potential energy surfaces, and how transient intermediates and vibrational wave packets evolve on excited-state landscapes. It helps visualize processes such as photoisomerization, electron transfer, energy flow within excited molecules, and nonadiabatic dynamics near conical intersections. The insights connect fundamental chemical dynamics to broader fields, including photochemistry, catalysis, photovoltaics, and biological photoresponses.

Today, femtochemistry remains an active area, driving improvements in time resolution and complementary techniques such as ultrafast electron diffraction and time-resolved X-ray methods. Challenges include interpreting complex, overlapping signals from multidimensional dynamics and mitigating sample damage from intense laser fields.