  Selectively exciting target molecules to high vibrational states is
inefficient in the liquid phase, which restricts the use of IR pumping to
catalyze ground-state chemical reactions. Here, we demonstrate that this
inefficiency can sometimes be solved by confining the liquid to an optical
cavity under vibrational strong coupling conditions. For a liquid solution of
13CO2 solute in a 12CO2 solvent, cavity molecular dynamics simulations show
that exciting a polariton (hybrid light-matter state) of the solvent with an
intense laser pulse, under suitable resonant conditions, may lead to a very
strong (> 3 quanta) and ultrafast (< 1 ps) excitation of the solute, even
though the solvent ends up being barely excited. By contrast, outside a cavity
the same input pulse fluence can excite the solute by only half a vibrational
quantum and the selectivity of excitation is low. Our finding is robust under
different cavity volumes, which may lead to observable cavity enhancement on IR
photochemical reactions in Fabry-P\'erot cavities.
