  Inelastic neutron scattering was used to study a quantum S=1/2
antiferromagnetic Heisenberg system-Bis(2-amino-5-fluoropyridinium)
Tetrachlorocuprate(II). The magnetic excitation spectrum was shown to be
dominated by long-lived excitations with an energy gap as 1.07(3) meV. The
measured dispersion relation is consistent with a simple two-dimensional square
lattice of weakly-coupled spin dimers. Comparing the data to a random phase
approximation treatment of this model gives the intra-dimer and inter-dimer
exchange constants J=1.45(2) meV and J'=0.31(3) meV, respectively.
