Femtosecond lasers provide control of chemical bonding of molecules

Coherent control is when a molecule is dropped into a light field, the electrons begin to move with the light’s electric field. This happens even when the light does not have the right frequency to drive the electron from one state to another. As the electrons rock out to the beat of an intense laser pulse, their movement distorts the shape of the electron cloud around the molecule. If this distortion becomes too large, then the electrons will leave their current state and jump to a new quantum state. In this process, even though the color of the light is wrong, the electrons can jump from a bonding to an anti-bonding state, destroying the molecule.

The typical laser used for coherent control has a pulse duration of just 100fs (10^-15s). Typically, the pulses are spaced by about a microsecond. That means that the light is doing its thing for only 1ns (10^-9s) out of every second of experimental time. So, let’s imagine that we choose a reaction that, on its own, proceeds very slowly at one reaction per second. Let’s also imagine that an optimized pulse shape enhances that rate by one million (an unreasonably high number). Given those numbers, every 20 minutes, we expect one additional new molecule. And that is for the optimized pulse.

Faster is better, though. Lets take a reaction that proceeds at a rate of 10^12 reactions per second. Now, for the optimized pulse, we get 10^9 additional molecules. Or, in other words, we have to detect a 0.1 percent increase in molecular product.

Coherent Control of Bond Making

Researchers demonstrate coherent control of bond making, a milestone on the way to coherent control of photoinduced bimolecular chemical reactions. In strong-field multiphoton femtosecond photoassociation experiments, we find the yield of detected magnesium dimer molecules to be enhanced for positively chirped pulses and suppressed for negatively chirped pulses. Our ab initio model shows that control is achieved by purification combined with chirp-dependent Raman transitions. Experimental closed-loop phase optimization using a learning algorithm yields an improved pulse that utilizes vibrational coherent dynamics in addition to chirp-dependent Raman transitions. Our results show that coherent control of binary photoreactions is feasible even under thermal conditions.

SOURCES- Ars Technica, Physical Review Letters

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