Site occupancy of chlorine on Cu(111) using normal-incidence x-ray standing waves: the energy difference between fcc and hcp hollow sites

It has previously been established that the lowest energy site for chlorine atoms on Cus111d is the “fcc” hollow. However, substantial population of the “hcp” hollow at room temperature indicates that there is a relatively small difference in energy between the two sites. We show that this energy difference must be less than 10 meV by measuring the relative populations using normal-incidence x-ray standing waves and comparing the results to Monte Carlo simulations. This result is consistent with recent density functional theory calculations which indicate an energy difference of approximately 5 meV.