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Old 11-26-2014, 06:18 AM
Cycle Cycle is offline
Join Date: Apr 2014
Posts: 35
One interesting thing I took away from that "Dynamics of ultrafast dissociation" paper is that they achieve higher hydrogen yields by shifting the frequency toward a shorter wavelength (higher frequency) than resonance... and I have an inkling of an idea.

What if what is happening is, as the researchers above state, the water molecule dissociation starts by the two hydrogens symmetrically extending away from the oxygen, then one of the hydrogens breaks off, and the other snaps back toward the oxygen. And by "blue-shifting", they just happened to hit a point where they were adding that last little push of energy to the weaker hydrogen bond just as it approached the oxygen, which helped it to rebound and go flying. Or they added just enough energy to one hydrogen as it approached the oxygen, and the molecule, in trying to maintain dipolar moment, transferred just enough of that energy to the other hydrogen just as it was rebounding, causing it to go flying.

The "symmetrical stretching" part isn't a usual vibration mode of the water molecule without excitation. Usually, in the absence of excitation, it undergoes asymmetrical stretching, rocking, scissoring, wagging, and twisting. All of these DOF movements preserve the underlying symmetry and net cumulated O-H-O distances. But symmetrical stretching is different... it stretches both springs at once.

In the parlance of springs and weights, it's a two-spring system (the hydrogens are connected via one spring each, with the two springs connected to each other through the oxygen). So if one hydrogen moves away from the oxygen, the other has to move closer.

But, as energy builds up in the molecule due to our exciting it at resonant frequency, we stretch *both* springs at once (symmetrical stretching), until finally the ever-so-slightly weaker spring snaps, and the ever-so-slightly stronger spring recoils.

The increased distance between the hydrogen and oxygen changes the resonant frequency a bit... so the researchers were using as a baseline the resonant frequency of a water molecule undergoing *no* excitation... but by blue-shifting to a higher frequency when under excitation, they actually hit the *new* resonant frequency of one of the hydrogen's 'springs' as it was rebounding.

If the researchers had red-shifted their frequency, they might have been able to add energy to *both* hydrogens just as they were symmetrically stretched the most, and caused both hydrogens to go flying. *Really* ultrafast dissociation.

Comments? Or am I completely off base here?
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