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Bond breaking dance

August 28, 2011

Picture from Wikipedia - Carbon disulphide orbital

Chemical reactions alter the arrangement of atoms and their constituent electrons in reactant molecules to form new product molecules. Almost every change occuring in our world is at some level the result of such a reaction. Visualizing a chemical reaction at the single-molecule level and ‘seeing’ how this rearrangement takes place in real time would give us deeper insight and greater understanding of the fundamental dynamics of the process.

Atoms consist of negatively charged electrons orbiting around a positively charged nucleus. Molecules consist of two or more atoms, each with its own nucleus and set of electrons, and in addition ‘valence’ electrons which have an orbit encompassing all the constituent atomic nuclei. These ‘outermost’  electrons of the molecule are responsible for the chemical ‘bond’ between the constituent atoms, which keeps the molecule tied together. It is these valence electrons which participate in a chemical reaction, breaking old bonds and forming new ones.

Quantum mechanics forbids the assignment of exact positions to electrons in an atom or molecule. Instead, there are fuzzy clouds called ‘orbitals’ surrounding the nuclei which represent probability distributions in space for finding the electrons. The valence electron orbital determines the size and shape of a molecule, since it is the ‘outermost’ cloud. Researchers at the National Research Council in Ontario, Canada have now been able to image the dynamical evolution of the valence electron orbital in carbon disulphide (CS2) molecules, during a chemical reaction where CS2 dissociates into CS and S.

CS2 is a linear molecule where the sulphur (S) atoms are at the ends, bonded to the carbon (C) atom in the center. Gaseous CS2 molecules in a chamber are first all aligned in a particular direction using an intense laser pulse i.e. a short burst of monochromatic light. An ultraviolet laser pulse then excites the aligned CS2 molecules to a state where the CS bonds are stretched and bent, from where they can dissociate into CS and S. Then, another ultraviolet ‘probe’ laser pulse is incident on the excited CS2 molecules after a controllable delay, which ionizes the molecules to break the valence electrons free and release them as energetic ‘photoelectrons’ that are detected.

The velocity distribution of these emitted photoelectrons i.e. the directions in which they travel outward from the aligned CS2 molecules, is completely determined by the ‘shape’ of the valence electron orbital at the exact moment of ionization. One can thus reconstruct the valence electron orbital at different times by measuring the photoelectron spatial distribution for different delays between the excitation and ionization laser pulses.

The CS2 molecule remains in the excited state for about a picosecond (a millionth of a millionth of a second) on average, before dissociating into the products CS and S. Even light, the fastest thing in the universe, only travels a distance roughly equal to the width of a human hair in that time. The alignment, excitation and ionization laser pulses used in the experiment all had a duration of about a tenth of a picosecond and the delay between the excitation and ionization pulses was varied from zero to one picosecond.

The reconstructed valence orbital had an hourglass-like shape in the initial linear form of the CS2 molecule, right after excitation. Thereafter, the orbital evolved over half a picosecond into a distorted dumbbell-like shape due to the stretching and bending of the CS bonds. The orbital shape oscillated back and forth between these two configurations for 1-2 picoseconds before the CS2 molecule decayed into CS and S. The experimental results matched very well with extensive computer simulations and theoretical calculations based on quantum mechanics.

The authors have essentially developed a camera that allows one to zoom into a molecule and watch the dance of the electrons choreographed by nature during a chemical reaction.

 

Hockett, P., Bisgaard, C., Clarkin, O., & Stolow, A. (2011). Time-resolved imaging of purely valence-electron dynamics during a chemical reaction Nature Physics, 7 (8), 612-615 DOI: 10.1038/nphys1980

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From → Chemistry, Physics

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