The Breaking Point

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Science  31 Jul 2009:
Vol. 325, Issue 5940, pp. 519
DOI: 10.1126/science.325_519c

A chemical reaction fundamentally involves the cleavage and formation of bonds between atoms. In general, the electrons and nuclei don't move at precisely the same time; more often than not, a rapid electronic rearrangement precedes a slower nuclear rearrangement, after which the electron distribution may adapt once again. In this context, the question arises of when exactly a bond can be considered broken. Should the criterion be electronic arrangement? Nuclear separation? Some combination of the two?

Wernet et al. explore this question in a study of the photoinduced dissociation of gaseous diatomic bromine. Specifically, they excite the molecule to a state that leads rapidly to dissociation and then use ultrafast laser pulses to track changes in the electronic distribution along the way. The probe pulses eject electrons from the valence shell of the dissociating molecule, and the measured kinetic energy variations of these photoelectrons reflect the evolving bonding framework. By shortening the probe pulses below their duration in prior studies of this system, the authors successfully map out the transition from a molecular (Br2) electronic arrangement to an atomic (2 Br) arrangement, defining the bond-breaking point (∼85 fs after excitation) based on the appearance of the atomic signature.

Phys. Rev. Lett. 103, 13001 (2009).

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