Vacuum ultraviolet photoelectron spectroscopy of transient species

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Abstract
The ground state (X 3 A″) of the HO2 + ion has been characterized by vacuum ultraviolet photoelectron spectroscopy by using the photoionization process The spectra show vibrational structure which is analysed in terms of a progression in the v 2 (essentially v(O-O)) stretching mode of HO2 +. The vibrational frequency is 1560 ± 50 cm-1 and the adiabatic ionization energy associated with reaction (1) is 11·35 ± 0·01 eV. Support for these results is derived from the corresponding spectrum of DO2 and from ΔSCF ab initio molecular orbital calculations with allowance for correlation energy changes on ionization. Use of the adiabatic ionization energy of HO2 allows a considerably improved estimate for the heat of formation of HO2 +, ΔH f(298) ø(HO2 +), of 1104 ± 4 kJ mol-1 to be made. Values for the dissociation energies in HO2 + are calculated as 278·0 ± 4·5 kJ mol-1 and 438·2 ± 5·1 kJ mol-1 for the (H-O2)+ and (HO-O)+ bonds respectively.