Synopsis: Following the Footsteps of a Chemical Reaction

Femtosecond laser spectroscopy can identify otherwise inaccessible precursors in photoinduced chemical reactions.
Synopsis figure
S. Ruetzel et al., Phys. Rev. Lett. (2013)

Prying open the secrets of chemical reactions on an atomic scale requires speed and cleverness. Speed comes from ultrafast laser spectroscopy in which short light pulses induce reactions—for instance, by exciting a molecule to a higher electronic state—and then probe the unfolding of chemical events in real time. Yet in these so-called pump-probe methods, it is difficult to trace the specific “precursor” state that triggers the reaction because the pump pulses excite a multitude of states. But clever schemes may overcome this limitation. As reported in Physical Review Letters, Stefan Ruetzel and colleagues at the University of Würzburg, Germany, have developed pump-probe techniques capable of unambiguously identifying reaction precursors.

To follow a chemical reaction from start to finish, researchers measure molecular spectra as a function of time. In conventional schemes, a pump pulse prepares the reactant in a desired state, and a second pulse probes intermediate states as product states are reached. The authors add a third pulse to enable an intriguing trick: by measuring correlations of pulses at different frequencies as a function of time, the scheme can determine whether certain electronic transitions in the initial and final states are quantum mechanically connected. In other words, whether a certain electronic state is the precursor of another one.

Similar techniques have been demonstrated for vibrational spectroscopy, but Ruetzel et al. extend them to electronic spectroscopy by using visible wavelength pulses instead of infrared. As a demonstration, they have studied merocyanine, which occurs in two conformations (isomers), and have shown that only one of the isomers becomes a radical cation following photoexcitation. For that isomer, the authors identified, among a multitude of excited states, the specific state that needs to be excited for the reaction to occur. Such detailed tracing of reaction pathways through electronic states may be applicable to study chemical processes underlying photovoltaics and reversible optical data storage. – David Voss


More Features »


More Announcements »

Subject Areas

OpticsChemical Physics

Previous Synopsis

Next Synopsis

Biological Physics

The Hairs Rustling in Your Ears

Read More »

Related Articles

Focus: 3D Images 10 Times Faster
Interdisciplinary Physics

Focus: 3D Images 10 Times Faster

3D x-ray phase-contrast images take as little as one-tenth the usual time to acquire using a technique that halves the number of required “photos.” Read More »

Viewpoint: Photonic Hat Trick

Viewpoint: Photonic Hat Trick

Two independent groups have provided the first experimental demonstration of genuine three-photon interference. Read More »

Synopsis: A Neat Way to Slow Down Light

Synopsis: A Neat Way to Slow Down Light

A new technique slows down light in a crystal by simply shining a laser on it and varying an applied voltage. Read More »

More Articles