Beating the Born Oppenheimer approximation with lasers

Organic chemists love to push electrons to describe reaction mechanisms. Chemists love potential energy surfaces — even protein chemists love them, although they can’t really calculate them. Both depend on the reality of the Born Oppenheimer approximation which says that electrons move first and nuclei follow much more slowly — which makes sense as even in hydrogen they are almost 2000 times as heavy.

A recent paper [ Proc. Natl. Acad. Sci. vol. 111 pp. 912 – 917 ’14 ] was able to use an extremely short laser burst (in 10^-18 seconds– an attoSecond) to move nuclei around in the D2 molecule — the energy had to be in the ultraviolet range, unlike vibratory motion which is in the infraRed range.

Interferences between electronic wave packets (evolving on attosecond timescales) controlled the population of different electronic states of the excited neutral molecule, which can be switched on attosecond timescales. They could control whether D2 ionized to D2+ or flew apart by the type of pulse.

They conclude with the following: “State-of-the-art quantum calculations, which have only recently become feasible, allowed us to interpret this very rich set of quantum dynamics, including both the nuclear motion and the coherently excited electronic state interferences. Thus, we succeed in both observing and rigorously modeling multiscale coherent quantum control in the time domain. The observed richness and complexity of the dynamics, even in this very simplest of molecules, is both remarkable and daunting.”

For more about how complicated even the simplest chemical reaction is please see —

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