Movies of Inelastic H+Li Collisions

These movies try to visualise what happens during a H+Li atom collision, using a very crude model (brief outline of details is given at the end for those interested). It should be noted, as always, reality is much more complex. But this gives a simpified physical picture of what happens.

The Li atom is shown at the centre of the left panel, and the H atom moves past it. The surface shown visualises the electronic wavefunction. The right panel shows the (1Sigma) potential energy surfaces, where the size of the dots represent the probability that the system is in that state.

Mpeg movie of adiabatic collision for H+Li(2s)->H+Li(2s)
Mpeg movie of adiabatic collision for H+Li(2p)->H+Li(2p)
Mpeg movie of adiabatic collision for H-+Li+->H-+Li+
Mpeg movie of inelastic collision for H+Li(2s)->H+Li(mixture)

The first three movies visualise the wavefunctions starting from different initial Li states assuming no transitions, that is, the purely adiabatic case which would correspond to an infinitely slow collision.

In the first movie, the H+Li(2s)->H+Li(2s) case, the H atom approaches the Li atom, and at around 7.3 Bohr separation the Li atom 2s electron tunnels to the H atom to form an ionic H-+Li+ configuration at small separations. This tunnelling may occur since at this separation the H+Li(2s) and H-+Li+ states have the same energy. If the collision is very slow, i.e. adiabatic, then the electron has plenty of time to tunnel over to the H atom.

In the second movie, the H+Li(2p)->H+Li(2p) case, the H atom approaches the Li atom, and at around 11.3 Bohr separation the Li atom 2p electron tunnels to the H atom to form an ionic H-+Li+ configuration. At 7.3 Bohr separation the electron tunnels back from the H atom, but now into a 2s state.

In the third movie, the H-+Li+->H-+Li+ case, the H- ion approaches the Li+ ion, and at around 11.3 Bohr separation one of the electrons from the H- ion tunnels to the Li nucleus to form a covalent H+Li(2p) configuration.

The final simulation shows an inelastic collision starting from the 2s state, where the transition probabilities have been set to give the maximum possible (unrealistic) effect. Here we see that at each crossing, the system has a probability to traverse the crossing adiabatically (staying on the same curve, in the same adiabatic state, thus changing configuration), or diabatically (changing curve, changing adiabatic state, thus staying in the same configuration). For example, the H+Li(2s) system at the crossing around 7.3 Bohr may either change to H-+Li+ (adiabatic case) or remain as H+Li(2s) (diabatic case). This reflects the fact that because the particles are moving past each other, there is only a finite time for the tunnelling to occur, and hence a probability that it will or won't occur. In the case that the atoms are moving very fast (diabatic case) there is basically no chance of the tunnelling occuring and the transition probability between adiabatic states is zero (i.e. they remain in the same configurations).

Brief calculation details for the interested

In this simple simulation I have considered three adiabatic states, and made the assumption of a classical straight line path with impact parameter of 5 Bohr radii for the nuclei. A simple Landau-Zener type picture has been used for the electronic wavefunctions, that is, the adiabatic states fully swap characteristics at avoided crossings of the potential curves (seen at 7.3 and 11.3 Bohr in this case), and are assumed to be written as simple linear combinations of the atomic states. The electronic wavefunctions are modelled using Hartree-Fock orbitals. The calculations are done in the molecular frame considering only 1Sigma states and radial coupling. The surface where the square of the electronic wavefunction reaches a low value is rendered, to visualise the electronic wavefunction.