ree – F ock ▁theory ▁and ▁a ▁long - range ▁van ▁der ▁Wa als ▁att raction — the ▁equilibrium ▁position ▁of ▁phys is or ption ▁for ▁rare ▁g ases ▁ad s or bed ▁on ▁j elli um ▁substr ate ▁can ▁be ▁determined . ▁ ▁Fig . ▁ 2 ▁shows ▁the ▁phys is or ption ▁potential ▁energy ▁of ▁He ▁ad s or bed ▁on ▁Ag , ▁Cu , ▁and ▁Au ▁substr ates ▁which ▁are ▁described ▁by ▁the ▁j elli um ▁model ▁with ▁different ▁dens ities ▁of ▁sm ear - out ▁background ▁positive ▁charges . ▁ ▁It ▁can ▁be ▁found ▁that ▁the ▁weak ▁van ▁der ▁Wa als ▁interaction ▁leads ▁to ▁shall ow ▁attract ive ▁energy ▁well s ▁( < 1 0   me V ). ▁ ▁One ▁of ▁the ▁experimental ▁methods ▁for ▁expl oring ▁phys is or ption ▁potential ▁energy ▁is ▁the ▁scatter ing ▁process , ▁for ▁instance , ▁in ert ▁gas ▁atoms ▁scattered ▁from ▁metal ▁surfaces . ▁ ▁C ertain ▁specific ▁features ▁of ▁the ▁interaction ▁potential ▁between ▁scattered ▁atoms ▁and ▁surface ▁can ▁be ▁extracted ▁by ▁analyz ing ▁the ▁experiment ally ▁determined ▁angular ▁distribution ▁and ▁cross ▁sections ▁of ▁the ▁scattered ▁particles . ▁ ▁Com parison ▁with ▁chem is or ption ▁▁ ▁Ph ysis or ption ▁is ▁a ▁general ▁phenomen on ▁and ▁occurs ▁in ▁any ▁solid / fl uid ▁or ▁solid / gas ▁system . ▁Chem is or ption ▁is ▁character ized ▁by ▁chemical ▁specific ity . ▁ ▁In ▁phys is or ption , ▁perturb ation ▁of ▁the ▁electronic ▁states ▁of ▁ad s orb ent ▁and