Tag Archives: Born Oppenheimer approximation

Watching electrons being pushed

Would any organic chemist like to watch electrons moving around in a molecule? Is the Pope Catholic? Attosecond laser pulses permit this [ Science vol. 346 pp. 336 – 339 ’14 ]. An attosecond is 10^-18 seconds. The characteristic vibrational motion of atoms in chemical bonds occurs at the femtosecond scale (10^-15 seconds). An electron takes 150 attoseconds to orbit a hydrogen atom [ Nature vol. 449 p. 997 ’07 ]. Of course this is macroscopic thinking at the quantum level, a particular type of doublethink indulged in by chemists all the time — https://luysii.wordpress.com/2009/12/10/doublethink-and-angular-momentum-why-chemists-must-be-adept-at-it/.

The technique involves something called pump probe spectroscopy. Here was the state of play 15 years ago — [ Science vol. 283 pp. 1467 – 1468 ’99 ] Using lasers it is possible to blast in a short duration (picoseconds 10^-12 to femtoseconds 10^-15) pulse of energy (pump pulse ) at one frequency (usually ultraviolet so one type of bond can be excited) and then to measure absorption at another frequency (usually infrared) a short duration later (to measure vibrational energy). This allows you to monitor the formation and decay of reactive intermediates produced by the pump (as the time between pump and probe is varied systematically).

Time has marched on and we now have lasers capable of producing attosecond pulses of electromagnetic energy (e.g. light).

A single optical cycle of visible light of 6000 Angstrom wavelength lasts 2 femtoseconds. To see this just multiply the reciprocal of the speed of light (3 * 10^8 meters/second) by the wavelength (6 * 10^3 *10^-10). To get down to the attosecond range you must use light of a shorter wavelength (e.g. the ultraviolet or vacuum ultraviolet).

The paper didn’t play around with toy molecules like hydrogen. They blasted phenylalanine with UV light. Here’s what they said “Here, we present experimental evidence of ultrafast charge dynamics in the amino acid phenylalanine after prompt ionization induced by isolated attosecond pulses. A probe pulse then produced a doubly charged molecular fragment by ejection of a second electron, and charge migration manifested itself as a sub-4.5-fs oscillation in the yield of this fragment as a function of pump-probe delay. Numerical simulations of the temporal evolution of the electronic wave packet created by the attosecond pulse strongly support the interpretation of the experimental data in terms of charge migration resulting from ultrafast electron dynamics preceding nuclear rearrangement.”

OK, they didn’t actually see the electron dynamics but calculated it to explain their results. It’s the Born Oppenheimer approximation writ large.

You are unlikely to be able to try this at home. It’s more physics than I know, but here’s the experimental setup. ” In our experiments, we used a two-color, pump-probe technique. Charge dynamics were initiated by isolated XUV sub-300-as pulses, with photon energy in the spectral range between 15 and 35 eV and probed by 4-fs, waveform-controlled visible/near infrared (VIS/NIR, central photon energy of 1.77 eV) pulses (see supplementary materials).”

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 — https://luysii.wordpress.com/2014/01/08/well-never-be-able-to-really-understand-chemistry/