  In ionic Raman scattering, infrared-active phonons mediate a scattering
process that results in the creation or destruction of a Raman-active phonon.
This mechanism relies on nonlinear interactions between phonons and has in
recent years been associated with a variety of emergent lattice-driven
phenomena in complex transition-metal oxides, but the underlying mechanism is
often obscured by the presence of multiple coupled order parameters in play.
Here, we use time-resolved spectroscopy to compare coherent phonons generated
by ionic Raman scattering with those created by more conventional electronic
Raman scattering on the nonmagnetic and non-strongly-correlated wide band-gap
insulator LaAlO$_3$. We find that the oscillatory amplitude of the
low-frequency Raman-active $E_g$ mode exhibits a sharp peak when we tune our
pump frequency into resonance with the high-frequency infrared-active $E_u$
mode, consistent with first-principles calculations. Our results suggest that
ionic Raman scattering can strongly dominate electronic Raman scattering in
wide band-gap insulating materials. We also see evidence of competing
scattering channels at fluences above 28~mJ/cm$^2$ that alter the measured
amplitude of the coherent phonon response.
