O ▁corresponds ▁to ▁the ▁ion ization ▁energy ▁to ▁form ▁the ▁H 2 O + ▁ion ▁in ▁its ▁ground ▁state ▁( 1 a 1 ) 2 ▁( 2 a 1 ) 2 ▁( 1 b 2 ) 2 ▁( 3 a 1 ) 2 ▁( 1 b 1 ) 1 . ▁The ▁energy ▁of ▁the ▁second - high est ▁MO ▁ 3 a 1 ▁refers ▁to ▁the ▁ion ▁in ▁the ▁excited ▁state ▁( 1 a 1 ) 2 ▁( 2 a 1 ) 2 ▁( 1 b 2 ) 2 ▁( 3 a 1 ) 1 ▁( 1 b 1 ) 2 , ▁and ▁so ▁on . ▁In ▁this ▁case ▁the ▁order ▁of ▁the ▁ion ▁electronic ▁states ▁corresponds ▁to ▁the ▁order ▁of ▁the ▁orb ital ▁energ ies . ▁Ex c ited - state ▁ion ization ▁energ ies ▁can ▁be ▁measured ▁by ▁photo elect ron ▁spect ro sc opy . ▁ ▁For ▁H 2 O , ▁the ▁near - H art ree – F ock ▁orb ital ▁energ ies ▁( with ▁sign ▁changed ) ▁of ▁these ▁orbit als ▁are ▁ 1 a 1 ▁ 5 5 9 . 5 , ▁ 2 a 1 ▁ 3 6 . 7 ▁ 1 b 2 ▁ 1 9 . 5 , ▁ 3 a 1 ▁ 1 5 . 9 ▁and ▁ 1 b 1 ▁ 1 3 . 8 ▁e V . ▁The ▁corresponding ▁ion ization ▁energ ies ▁are ▁ 5 3 9 . 7 , ▁ 3 2 . 2 , ▁ 1 8 .