Ab initio explanation of the apparent violation of detailed balance for H₂ adsorption/desorption from Si(100)

The experimentally observed apparent violation of detailed balance in the adsorption and desorption of H₂ on Si(100)-2 × 1 is explained via a dihydride intermediate that can be accessed via two different transition states, as found by complete active space self-consistent field/multireference single and double excitation configuration interaction (CASSCF/MRSDCI) calculations. Our calculations explain the low sticking probability by predicting an early symmetric transition state to adsorption with a high barrier. We predict that desorption, on the other hand, occurs through an asymmetric transition state and find excellent agreement between our calculated barrier to desorption and experimental values. For the asymmetric transition state we calculate a small barrier to adsorption, however, we suggest that impinging H₂ seldom sees this orientationally constrained late transition state to adsorption, explaining the apparent violation of detailed balance.