  We demonstrate that metastable helium molecules can be used as tracers to
visualize the flow of the normal fluid in superfluid $^{4}$He using a
laser-induced-fluorescence technique. The flow pattern of a normal-fluid jet
impinging on the center of a copper disc is imaged. A ring-shaped circulation
structure of the normal fluid is observed as the jet passes across the disc
surface. The fluorescence signal for the molecules trapped in the circulation
structure is measured as a function of time after we turn off the molecule
source. The radiative lifetime and density of the molecules can be determined
by fitting the measured data using a simple analytic model. We also discuss a
proposed experiment on using a previously developed molecule tagging-imaging
technique to visualize the normal-fluid velocity profile during the transition
of quantum turbulence in a thermal counterflow channel.
