  We report observation of two-photon emission from coherently excited
vibrational state of solid parahydrogen, known as one of quantum solids. The
coherent state between the ground and the excited states is prepared by
stimulated Raman scattering using two visible laser pulses. The two-photon
emission is triggered by another mid-infrared laser pulse. The observed
two-photon emission persists even when the trigger pulse is injected long after
the excitation. The observed phenomenon is due to a long decoherence time of
the vibrational states of solid parahydrogen, which is attributed to special
nature as quantum solid. Dependence of the decoherence on target temperature
and residual orthohydrogen concentration are studied along with its time
evolution. It is found that the emission intensity increases even after the
excitation pulses pass through the target completely. The coherence development
is highly suppressed at high target temperature and high residual orthohydrogen
concentration. Effects of target annealing and laser-induced damage on the
target are also observed.

Chemical Physics;Atomic Physics