6   e V . ▁For ▁liquid ▁arg on , ▁which ▁could ▁be ▁written ▁as ▁Ar ∞ , ▁the ▁energy ▁increases ▁to ▁ 0 . 0 8   e V . ▁Cl ust ers ▁of ▁up ▁to ▁several ▁hundred ▁arg on ▁atoms ▁have ▁been ▁detected . ▁These ▁arg on ▁clusters ▁are ▁ icos ah ed ral ▁in ▁shape , ▁consisting ▁of ▁shell s ▁of ▁atoms ▁arranged ▁around ▁a ▁central ▁atom . ▁The ▁structure ▁changes ▁for ▁clusters ▁with ▁more ▁than ▁ 8 0 0 ▁atoms ▁to ▁res emble ▁a ▁tiny ▁cry stal ▁with ▁a ▁face - center ed ▁cub ic ▁( f cc ) ▁structure , ▁as ▁in ▁solid ▁arg on . ▁It ▁is ▁the ▁surface ▁energy ▁that ▁maintain s ▁an ▁ icos ah ed ral ▁shape , ▁but ▁for ▁larger ▁clusters ▁internal ▁pressure ▁will ▁attract ▁the ▁atoms ▁into ▁an ▁f cc ▁arrangement . ▁Ne ut ral ▁arg on ▁clusters ▁are ▁transparent ▁to ▁visible ▁light . ▁ ▁Di atomic ▁van ▁der ▁Wa als ▁mole cules ▁{ | class =" wik itable " ▁! M ole c ule ▁! Binding ▁energy ground ▁ Σ ▁state ( cm − 1 ) ▁! Binding ▁energy ex c ited ▁ Π ▁state ( cm − 1 ) ▁! Gr ound ▁state b ond ▁length ( Å ) ▁! Ex c ited ▁state b ond ▁length ( Å ) ▁! C AS ▁number ▁| - ▁| Ar H ▁| ▁| ▁| ▁| ▁| 3 0 7 3 6 - 0 4 - 0 ▁| - ▁| Ar He ▁|