We use time-domain terahertz spectroscopy to measure the position and linewidth of single photon transitions in Mn12-acetate. This linewidth is compared to the linewidth measured in tunneling experiments. We conclude that local magnetic fields (due to dipole or hyperfine interactions) cannot be responsible for the observed linewidth, and suggest that the linewidth is due to variations in the anisotropy constants for different clusters. We also calculate a lower limit on the dipole field distribution that would be expected due to random orientations of clusters and find that collective effects must narrow this distribution in tunneling measurements.

Single photon transitions in molecular magnets provide valuable probes of the widths of energy levels. We use time-domain terahertz spectroscopy to measure the width of the transition from the ground state to the first excited state in Mn12 acetate in zero applied field. The width of this transition, approximately 5.5 GHz full width at half maximum, is too large to be caused by local magnetic fields. Experiments by Mukhin et al. rule out homogeneous broadening as the source of the linewidth [Phys. Rev. B 63, 214411 (2001)]. The linewidth can be explained if the anisotropy constant in the spin Hamiltonian is not uniform due to crystal defects such as dislocations. Since similar linewidths are observed in several other materials, we conclude that whatever disorder causes this broadening is not limited to Mn12 acetate.