Comment on “Interference in the Collective Electron Momentum in Double Photoionization of H2”

We give supporting evidence for and elaborate further on the fact [see Kreidi et al, Phys. Rev. Lett. 100, 133005 (2008)] that ionic states are essential for covalent dihydrogen H2. Kreidi et al [1] proved that interference in double photoionization of H2 results from a non-HeitlerLondon fraction of the H2 ground state, where both electrons are at the same atomic center, i.e. photoionization reveals ionic states for covalent H2 [1]. We give supporting experimental evidence for [1] and report on bond theories of ionic type, justified by [1] but not fully elaborated therein. (i) Whenever 2 electrons are at 1 nucleon [1], the HL bonding region in between the 2 nucleons is deprived of electron-density. These remarkable details on the internal mechanics of H2 [1] are supported by Dunitz and Seiler [2], who found earlier that, for covalent bonds, the HL bonding region between the nuclei is void of electron-density. Hence, both [1] and [2] allow for an ionic view on H2. At the time, results [2] faced fierce opposition from theorists [3]. With [1,2], such opposition must be moderated and a search for refined, if not alternative bond theories is justified [4]. (ii) Although Kreidi et al. [1] argue that non-HL ionic states [5] are indeed needed, they do not say how important these states really are. Knowing that HL-theory uses 0% of ionic states, they refer [1] to VB-theory [5] with a small % of ionic states but not to MO-theory with 50 % or to ionic theory with 100% of ionic contributions to the H2 ground state. For instance, 19 century ionic models [4,6] applied to covalent H2 give H2=[HH+HH], where 2 electrons are at 1 nucleon, either at the right +1⁄2r0 or at the left -1⁄2r0 of the center of mass (or vice versa), as in Fig. 2 of [1]. It is obvious that any ionic contribution for covalent H2, however large, can only make sense a) if its 2 valence electrons were centered at one atomic center instead of the two [1] b) if electrons are absent in the HL-bonding region [2]. For a 100% ionic bond however, c) the driving force must be Coulomb attraction –e/r [4,6], where r is (close to) the internucleon separation. While qualitative constraints a and b are met with [1,2], even rather severe quantitative constraint c for a 100 % ionic bond is obeyed, since ionic bond energy Dion=-e/r0 rationalizes lower order spectroscopic constants of diatomic ionic and covalent bonds between all monovalent atoms in the periodic table, including covalent bond H2 [4,7,8]. Despite this, ionic models give problems, although equilibrium value –e/r0= 19,5 eV for H2 is largely encompassed by photon energies varying from 130 to 240 eV used in [1]. QM bond theories use the Born-Oppenheimer approximation (BOA) [7], where bond stability depends on internucleon repulsion +e/r, which is mutually exclusive with attraction –e/r. Hence, only