  We report a neutron scattering study of the spin-1/2 alternating bond
antiferromagnet Cu(NO_3)_2. 2.5D_2O for 0.06<k_BT/J_1<1.5. For k_BT/J_1 << 1
the excitation spectrum is dominated by a coherent singlet-triplet mode
centered at J_1=0.442(2) meV with sinusoidal dispersion and a bandwidth of
J_2=0.106(2) meV. A complete description of the zero temperature contribution
to the scattering function from this mode is provided by the Single Mode
Approximation. At finite temperatures we observe exponentially activated band
narrowing and damping. The relaxation rate is thermally activated and wave
vector dependent with the period icity of the reciprocal lattice.
