The dimensionality of ammonium reorientation in (NH4)2S2O8: the view from neutron spectroscopy

The dynamics of the ammonium ion in ammonium persulfate is studied by inelastic neutron scattering at energy transfers between 0.7 μeV and about 100 meV. NH4+ librational modes are clearly split into four energetically different bands, in agreement with the absence of symmetry at the ammonium site. One strong, one medium and two weak hydrogen bonds from the NH4 ion to neighboring oxygens lead to this unusual potential shape with one dominating overlap matrix elements hi. Therefore the ammonium dynamics seen by neutron tunneling spectroscopy looks like a rotation about a single axis, that of the strongest hydrogen bond. Quasi-elastic scattering up to T = 100 K is dominated by jumps about this specific axis, too. Jumps about the second weak axis modify the quasi-elastic scattering at higher temperature. Results of a previous nuclear magnetic resonance (NMR) quadrupole resonance experiment are consistently interpreted to show the dynamics around the remaining two strong axes. The reduced isotope effect of the lowest librational band is due to a special shape of the potential and not to changes of the hydrogen bond with deuteration