  As per the Franck-Condon principle, absorption spectroscopy reveals changes
in nuclear geometry in molecules or solids upon electronic excitation. It is
often assumed these changes cannot be resolved beyond the ground vibrational
wavefunction width ($\sqrt{\hbar/m\omega}$). Here, we show this resolution
dramatically improves with highly-excited vibrational initial states (with
occupation number $\langle n\rangle$). These states magnify changes in geometry
by $2\langle n\rangle +1$, a possibly counterintuitive result given the spatial
uncertainty of Fock states grows with $n$. We also discuss generalizations of
this result to multimode systems. Our result is relevant to optical
spectroscopy, polariton condensates, and quantum simulators ($\textit{e.g.}$,
boson samplers).
