Direct comparisons of rates for low temperature diffusion of hydrogen and deuterium on Cu(001) from quantum mechanical calculations and scanning tunneling microscopy experiments

Recent experiments by Lauhon and Ho using scanning tunneling microscopy (ST M) observed the direct hopping of H and D on Cu(001) as a function of tempe rature, They found nearly temperature independent tunneling for H below 60 K, but could not detect the tunneling threshold for D (it is at least 1000 times lower than for H). The availability of such direct and accurate measu rement provides the opportunity for validating the level of theory required to predict the diffusion of adsorbates on surfaces. Thus, we carried out d ensity functional theory (DFT) using the generalized gradient approximation (GGA-II) on periodic slabs. The calculated tunneling rate of 4.74 X 10(-4) s(-1) for H is in close agreement with the experimental value of 4.4 X 10( -4) s(-1). We predict 4.66 X 10(-9) s(-1) for the tunneling rate of D (one hop every 83 months!). Between 60 and 80 K, the calculated thermally activa ted diffusion rate of H is 10(12.88) exp(-0.181 eV/kT) s(-1) in close agree ment with the STM value: 10(12.9+/-0.3) exp(-0.197 eV/kT). For deuterium, b etween 50 and 80 K, the calculated rate is 10(12.70) exp(-0.175eV/kT) s(-1) in close agreement with the STM value: 10(12.7+/-0.2) exp(-0.194 eV/k-T) s (-1). These results validate that such first principle theory can be used t o predict the diffusion (including tunneling) for adsorbates on surfaces, p roviding important data needed to unravel surface processes in catalysis an d crystal growth. (C) 2001 American Institute of Physics.