Lattice coupling in direct molecular dissociation

The importance of lattice coupling in direct molecular dissociation is at present poorly understood. However, there are at least two ways in which inclusion of the lattice can affect direct dissociative adsorption. First, conversion of Et to Eq competes with translational activation in dissociation. Second, thermal distortion of lattice atoms from their equilibrium positions may affect the PES, e.g., the barriers to dissociation V ( ). These two effects can be most simply thought of as a phonon induced modulation of the barrier along the translational coordinate and in amplitude, respectively. 

The other lattice coupling V is also likely to be a general phenomenon in activated direct dissociation since barriers are generally lower for less coordinated sites, i.e., distortion of the lattice can give lower/higher barriers. However, this barrier 

The presence of lattice coupling in direct dissociation complicates somewhat the definition of V and how V extracted from measurement should be compared to DFT calculations. There are two well defined limits for DFT calculations of V, for a frozen lattice (Ts = 0) and for the case where the lattice is fully relaxed at the transition state (Ts = oo). Since experiments are performed at finite Ts, the best V description is probably intermediate between these two extremes. 

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