▁ ▁or ▁ ▁is ▁called ▁a ▁chrom oph ore . ▁Its ▁spectrum ▁is ▁modified ▁by ▁the ▁first ▁shell ▁of ▁neutral ▁atoms ▁attached . ▁Lar ger ▁clusters ▁have ▁the ▁same ▁spectrum ▁as ▁the ▁smaller ▁ones . ▁When ▁phot ons ▁are ▁absor bed ▁in ▁the ▁chrom oph ore , ▁it ▁is ▁initially ▁electron ically ▁excited , ▁but ▁then ▁energy ▁is ▁transferred ▁to ▁the ▁whole ▁cluster ▁in ▁the ▁form ▁of ▁v ibration . ▁Ex cess ▁energy ▁is ▁removed ▁by ▁outer ▁atoms ▁ev apor ating ▁from ▁the ▁cluster ▁one ▁at ▁a ▁time . ▁The ▁process ▁of ▁destroy ing ▁a ▁cluster ▁by ▁light ▁is ▁called ▁phot of ragment ation . ▁ ▁Neg atively - charg ed ▁arg on ▁clusters ▁are ▁therm od ynam ically ▁un stable , ▁and ▁therefore ▁cannot ▁exist . ▁Arg on ▁has ▁a ▁negative ▁electron ▁aff inity . ▁ ▁Arg on ▁mon oh yd ride ▁Ne ut ral ▁arg on ▁hyd ride , ▁also ▁known ▁as ▁arg on ▁mon oh yd ride ▁( Ar H ), ▁was ▁the ▁first ▁discovered ▁noble ▁gas ▁hyd ride . ▁J .   W .   C . ▁Joh ns ▁discovered ▁an ▁emission ▁line ▁of ▁Ar H ▁at ▁ 7 6 7   nm ▁and ▁announced ▁the ▁find ▁in ▁ 1 9 7 0 . ▁The ▁mole c ule ▁was ▁synth es ized ▁using ▁X - ray ▁ir rad iation ▁of ▁mi xt ures ▁of ▁arg on ▁with ▁hydro gen - rich ▁mole cules ▁such ▁as ▁H 2 , ▁H 2 O , ▁CH 4 ▁and ▁CH 3 OH .