  Molecular vibrations play a key role in magnetic relaxation processes of
molecular spin qubits as they couple to spin states, leading to the loss of
quantum information. Direct experimental determination of vibronic coupling is
crucial to understand and control the spin dynamics of these nano-objects,
which represent the limit of miniaturization for quantum devices. Herein, we
measure the vibrational properties of the molecular spin qubit
$[$Ho(W$_5$O$_{18}$)$_2]^{9-}$ by means of magneto-infrared spectroscopy. Our
results allow us to unravel the vibrational decoherence pathways in combination
with $ab$ $initio$ calculations including vibronic coupling. We observe
field-induced spectral changes near 63 and 370 cm$^{-1}$ that are modeled in
terms of $f$-manifold crystal field excitations activated by odd-symmetry
vibrations. The overall extent of vibronic coupling in this system is limited
by a transparency window in the phonon density of states that acts to keep the
intramolecular vibrations and $M_J$ levels apart. These findings advance the
understanding of vibronic coupling in molecular magnets, place significant
constraints on the pattern of crystal field levels in these systems, and
provide a strategy for designing molecular spin qubits with improved coherence
lifetimes.
