  We study nearly degenerate four-wave mixing using a two-photon-allowed
vibrational transition of parahydrogen. A signal photon is generated by a
trigger photon and coherence among parahydrogen, which is prepared by two
counterpropagating pump pulses. The dependence of the signal pulse energy on
the trigger frequency are investigated. The measured spectra vary depending on
the geometry. They shift depending on the direction of the signal pulse and on
the small angle formed by the counterpropagating pump pulses. Furthermore, the
dependence of signal pulse energy on the incident time of the trigger pulse is
investigated. The measured signal pulse energy is high if the trigger pulse is
slightly delayed with respect to the pump pulses. We demonstrate that these
geometry-dependent spectra and coherent-transient response can be explained by
using simple models.
