▁Arg on - tag ging ▁is ▁a ▁technique ▁where by ▁arg on ▁atoms ▁are ▁weak ly ▁bound ▁to ▁a ▁mole c ule ▁under ▁study . ▁ ▁It ▁results ▁in ▁a ▁much ▁lower ▁temperature ▁of ▁the ▁tag ged ▁mole cules , ▁with ▁sh ar per ▁inf ra - red ▁absor ption ▁lines . ▁ ▁The ▁arg on - tag ged ▁mole cules ▁can ▁be ▁dis rupted ▁by ▁phot ons ▁of ▁a ▁particular ▁w avel ength . ▁ ▁Lith ium ▁ ions ▁add ▁arg on ▁atoms ▁to ▁form ▁clusters ▁with ▁more ▁than ▁a ▁hundred ▁arg on ▁atoms . ▁The ▁clusters ▁Li + Ar 4 , ▁and ▁Li + Ar 4 ▁are ▁particularly ▁stable ▁and ▁common . ▁Calcul ations ▁show ▁that ▁the ▁small ▁clusters ▁are ▁all ▁quite ▁symmet rical . ▁Li + Ar 2 ▁is ▁linear , ▁Li + Ar 3 ▁is ▁flat ▁and ▁tri angular ▁sh aped ▁with ▁D 3 h ▁symmetry , ▁Li + Ar 4 ▁is ▁t etra hed ral , ▁Li + Ar 5 ▁could ▁be ▁a ▁square ▁py ram id ▁or ▁trig onal ▁b ip y ram id ▁shape . ▁Li + Ar 6 ▁is ▁an ▁oct ah ed ron ▁shape ▁with ▁Li ▁at ▁the ▁centre . ▁Li + Ar 7 ▁or ▁slightly ▁larger ▁clusters ▁have ▁a ▁core ▁oct ah ed ron ▁of ▁arg on ▁atoms ▁with ▁one ▁or ▁more ▁tri angular ▁faces ▁c apped ▁by ▁other ▁arg on ▁atoms . ▁ ▁The ▁bond ing ▁is ▁much ▁we aker , ▁which ▁explains ▁their ▁greater ▁scarc ity . ▁ ▁S od ium