Next: 6 Implementation Issues Up: 5 Algorithmic Issues Previous: 5.1 Particle Placement Contents
Depending on the hydrogen's position there are several possible moves that it can make. Each of these moves has a required activation energy associated with it. The activation energy requirement simply means that the hydrogen atom must have at least the required amount of energy to make the move. If the hydrogen's energy is less than the activation energy then the the hydrogen cannot move. To discuss the possible moves for the hydrogen please refer to the Figure 5. Also the required activation energies for each move are listed in Figure 6.
For Case 1: This is an example of terrace movements. (i) The black hydrogen atom is not paired with another hydrogen atom. It has four possible moves shown in red. a) an intra-dimer move within the dimer pair b) a intra-row move within the dimer row and c) a dimer row jump. Of these the intra-dimer move is the most probable and the dimer row jump is the least likely. (ii) In this case there is a pair of hydrogen atoms. For an atom to move it must complete a pair break. The three possible moves are shown in green. We've seen c) before above but the new type of move is d) an intra-dimer move with a pair break.
For Case 2: In this example we examine the moves onto, off of, and around a B-step. For simplicity the terrace moves are not shown. Also note that the bonds from the top of the lattice down the B-step have been included in the illustration. (i) The black hydrogen atom is not paired with another atom and its possible moves are shown in red e) a jump off of the B-step down onto the lower terrace and g) intra-row move off of the B-step back onto the terrace. (ii) The hydrogen atom is on the lower terrace. The possible moves are shown in green f) a jump off of the lower terrace onto the B-step above. and j) a dimer row jump away from the B-step. (iii) The hydrogen is on the upper terrace. A pair break is required for movement and the possible moves are shown in yellow h) a intra-row move off the B-step with a pair break.
Other moves not shown in the pictures are listed with their activation energies in Figure 6. In general these include, jumps onto and off of the A-step, intra-row move onto a B-step (with and without a pair break), and the activation energies required for desorption at the different sites.
Chris Siefert and Molly Moore 2002
