and Z rot and Z vib are its rotational and vibrational partition functions, respectively. By increasing the temperature, the hydrogen chemical potential is lowered, i.e. less energy is gained by taking a H atom out of the reservoir and attaching it to the surface. A variation of the hydrogen chemical potential with respect to its molecular value at zero temperature of ∆µ H ∼ -0.6 eV corresponds to the growth conditions in the experiment described below.