Nonempiric calculation (MP2/UHF/4-31++G**) shows the presence of inherent H3O and OH fragments in small water cluster cations. According to the arrangement of these fragments, the structures of cations are divided into two groups: either OH fragment acts exclusively as a proton acceptor in all its hydrogen bonds, or it is directly bonded to H3O and acts also as a proton donor in the H-bond with a water molecule. At the external effect of about 0.4 eV, the former cations can dissociate into free or quasifree OH radical and a protonated water cluster of the corresponding size. An extrapolation of the adiabatic ionization potentials of water clusters to an infinite cluster size provides the value of 8.5 eV close to the experimental photoelectric threshold of amorphous ice. When the adiabatic hydration of the electron knocked out is taken into account, the energy of 6.8 eV should be sufficient for the ionization of an ice specimen.