Length Of Time For Hydrogen Ion To Localise Electron

Of all the molecular ions that exist, by far the least complicated is the dihydrogen cation (H2+) which is just the hydrogen molecule with an electron removed. The H2 only has two electrons in it normally leaving only one when it is turned into an ion. This means that solving the Schrödinger equation can be solved with quite a bit more ease than in most cases with only the quantum mechanics of one electron to consider. Another feature is that the covalent bond that holds the hydrogens together is only composed of one electron.

This makes it about half as strong as the original and, considering hydrogen wasn’t the most stable molecule before, it means that H2is very willing to break apart and disassociate into an atom and an ion. The electron has to go one way or another:

 

One of the ideal things we would like to be able to achieve with lasers is to be able to control the breaking of chemical bonds, and as this is very similar to a chemical bond breaking, but requires significantly less energy, it is perfect to study. The action the electron can be seen as a result of distance between the two nuclei. When they are close together, about two atomic units, (also known as the Bohr radii, 5.29177×1011 m) the electron can easily transfer between the nuclei holding them together. But when the H2+ is excited the individual hydrogens will start moving farther apart. Eventually the potential barrier, the energy the electron must have to bridge the gap and transport between nuclei, gets too big and the electron simply can’t manage it. It is trapped in its current hydrogen and the bond is officially broken.

In order to get more details on this process an experimented was recently designed where a pulsing laser was used to input energy for only 5 femtoseconds (fs, 10−15 s) at a time. By recording the direction of the proton (H+) given off it was found that the probing laser pulse, not the one pumps energy into the system, could actually control which hydrogen the electron ended on to within 15 fs after the H2+ ion was made. It can therefore be concluded that the electron localisation is complete in these 15 fs and the separation of nuclei was 8 atomic units.

Paper links: Observing electron localization in a dissociating H2+ molecule in real time

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