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

Abstract

The dynamics of the ammonium ion in ammonium persulfate isstudied by inelastic neutron scattering at energy transfers between0.7 µeV and about100 meV. NH4+ librational modes are clearly split into four energetically differentbands, in agreement with the absence of symmetry at the ammoniumsite. One strong, one medium and two weak hydrogen bonds from theNH4 ion toneighboring oxygens lead to this unusual potential shape with one dominating overlap matrix elementshi. Therefore the ammonium dynamics seen by neutron tunneling spectroscopy looks like arotation about a single axis, that of the strongest hydrogen bond. Quasi-elastic scattering up toT = 100 K is dominated by jumps about this specific axis, too. Jumps about the secondweak axis modify the quasi-elastic scattering at higher temperature. Results of aprevious nuclear magnetic resonance (NMR) quadrupole resonance experiment areconsistently interpreted to show the dynamics around the remaining two strongaxes. The reduced isotope effect of the lowest librational band is due to a specialshape of the potential and not to changes of the hydrogen bond with deuteration.