. ▁ ▁Arg on ▁poly h yd rides ▁The ▁arg on ▁di h ydro gen ▁c ation ▁ ▁has ▁been ▁predicted ▁to ▁exist ▁and ▁to ▁be ▁detect able ▁in ▁the ▁inter stell ar ▁medium . ▁However ▁it ▁has ▁not ▁been ▁detected ▁. ▁ ▁is ▁predicted ▁to ▁be ▁linear ▁in ▁the ▁form ▁Ar − H − H . ▁The ▁H − H ▁distance ▁is ▁ 0 . 9 4   Å . ▁The ▁dis soci ation ▁bar rier ▁is ▁only ▁ 2   k cal / m ol ▁( 8   k J / m ol ), ▁and ▁ ▁readily ▁los es ▁a ▁hydro gen ▁atom ▁to ▁yield ▁Ar H + . ▁The ▁force ▁constant ▁of ▁the ▁Ar H ▁bond ▁in ▁this ▁is ▁ 1 . 8 9 5   md y ne / Å 2 ▁( ). ▁ ▁The ▁arg on ▁tri h ydro gen ▁c ation ▁ ▁has ▁been ▁observed ▁in ▁the ▁labor atory . ▁Ar H 2 D + , ▁ ▁and ▁ ▁have ▁also ▁been ▁observed . ▁The ▁arg on ▁tri h ydro gen ▁c ation ▁is ▁plan ar ▁in ▁shape , ▁with ▁an ▁arg on ▁atom ▁off ▁the ▁vertex ▁of ▁a ▁triangle ▁of ▁hydro gen ▁atoms . ▁ ▁Ar go x on ium ▁The ▁ar go x on ium ▁ion ▁Ar OH + ▁is ▁predicted ▁to ▁be ▁bent ▁mole cular ▁geometry ▁in ▁the ▁ 1 1 A ′ ▁state . ▁ 3 Σ − ▁is ▁a ▁triple t ▁state ▁ 0 . 1 2   e V ▁higher ▁in