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 Announcements »

Subject Areas

OpticsChemical Physics

Previous Synopsis

Next Synopsis

Biological Physics

The Hairs Rustling in Your Ears

Read More »

Related Articles

Viewpoint: Deciphering Water’s Dielectric Constant
Chemical Physics

Viewpoint: Deciphering Water’s Dielectric Constant

The combination of two spectroscopic techniques reveals the microscopic mechanisms that control the behavior of water’s dielectric constant. Read More »

Synopsis: Hydrogen  Bonding Comes to the Rescue
Chemical Physics

Synopsis: Hydrogen Bonding Comes to the Rescue

Hydrogen bonding may safeguard biomolecules against the damaging effects of UV light. Read More »

Viewpoint: Cold Results from Fast Lasers

Viewpoint: Cold Results from Fast Lasers

Ultrafast lasers show promise to cool down and trap atomic species inaccessible to more traditional methods. Read More »

More Articles