  Low temperature optical spectroscopy in applied magnetic fields provides
clear evidence of magnetoelastic coupling in the spin ice material Ho2Ti2O7. In
IR measurements, we observe field dependent features around 61, 72 and 78 meV,
energies corresponding to crystal electronic field doublets. Calculating the
electronic band structure based on the crystal field Hamiltonian allows
determination of crystal field energies, values for the crystal field
parameters, and confirmation that the observed features in IR are consistent
with magnetic-dipole-allowed transitions between 5I8 CEF levels. Additionally,
we identify a weak field-dependent feature near one of the CEF doublets, which
we associate with a vibronic bound state that was previously observed by others
in inelastic neutron measurements.
