▁van ▁der ▁Wa als ▁mole cules , ▁electron ically - ex c ited ▁mole cules ▁with ▁the ▁same ▁formula ▁exist . ▁As ▁a ▁formula ▁these ▁can ▁be ▁written ▁Ar X *, ▁with ▁the ▁" *" ▁indicating ▁an ▁excited ▁state . ▁The ▁atoms ▁are ▁much ▁more ▁strongly ▁bound ▁with ▁a ▁c oval ent ▁bond . ▁They ▁can ▁be ▁mode led ▁as ▁an ▁Ar X + ▁surrounded ▁by ▁a ▁higher ▁energy ▁shell ▁with ▁one ▁electron . ▁This ▁outer ▁electron ▁can ▁change ▁energy ▁by ▁ex ch anging ▁phot ons ▁and ▁so ▁can ▁flu ores ce . ▁The ▁widely ▁used ▁arg on ▁flu or ide ▁las er ▁makes ▁use ▁of ▁the ▁Ar F * ▁exc imer ▁to ▁produce ▁strong ▁ult rav io let ▁radiation ▁at ▁ 1 9 2   nm . ▁The ▁arg on ▁ch lor ide ▁las er ▁using ▁Ar Cl * ▁produces ▁even ▁shorter ▁ult rav io let ▁at ▁ 1 7 5   nm , ▁but ▁is ▁too ▁fee ble ▁for ▁application . ▁The ▁arg on ▁ch lor ide ▁in ▁this ▁las er ▁comes ▁from ▁arg on ▁and ▁ch lor ine ▁mole cules . ▁ ▁Arg on ▁clusters ▁Co o led ▁arg on ▁gas ▁can ▁form ▁clusters ▁of ▁atoms . ▁Di arg on , ▁also ▁known ▁as ▁the ▁arg on ▁dim er , ▁has ▁a ▁binding ▁energy ▁of ▁ 0 . 0 1 2   e V , ▁but ▁the ▁Ar 1 3 ▁and ▁Ar 1 9 ▁clusters ▁have ▁a ▁sub lim ation ▁energy ▁( per ▁atom ) ▁of ▁ 0 . 0