  We report coherent enhancement of two-photon emission from the excited
vibrational state of molecular hydrogen triggered by irradiating mid-infrared
pulses externally. We previously observed the two-photon emission triggered by
the internally generated fourth Stokes photons. By injecting independent
mid-infrared pulses externally, it is possible to control experimental
parameters and investigate the mechanism in more detail. In this article, we
describe the two-photon emission using the external trigger pulses. Its
spectrum and dependence on the energy and timing of the trigger pulse are
presented along with numerical simulations based on the Maxwell-Bloch
equations. The measured number of emitted photons is 6 10^11 photons/pulse and
the resulting enhancement factor from the spontaneous emission is more than
10^18. This value is three orders of magnitude higher than that of the previous
experiment. External control of emission process is expected to be essential
for observation of weaker process of radiative emission of neutrino pair.
