text: A particular brand of gas-surface chemisorption is the dissociation of diatomic gas molecules, such as hydrogen, oxygen, and nitrogen.  One model used to describe the process is precursor-mediation.  The absorbed molecule is adsorbed onto a surface into a precursor state.  The molecule then diffuses across the surface to the chemisorption sites.  They break the molecular bond in favor of new bonds to the surface.  The energy to overcome the activation potential of dissociation usually comes from translational energy and vibrational energy.
  An example is the hydrogen and copper system, one that has been studied many times over.  It has a large activation energy of 0.35 – 0.85 eV.  The vibrational excitation of the hydrogen molecule promotes dissociation on low index surfaces of copper.