  We observed two-photon emission signals from the first vibrationally excited
state of parahydrogen gas excited coherently by counter-propagating laser
pulses. A single narrow-linewidth laser source was used to excite the
parahydrogen molecules and induce the two-photon emission process. We measured
how the signal energy depended on the detuning, target gas pressure, and input
pulse energies. These results are qualitatively consistent with those obtained
by numerical simulations based on the Maxwell-Bloch equations with one spatial
dimension and one temporal dimension. This study of two-photon emission with
counter-propagating injection is an important step toward neutrino mass
spectroscopy.
