Metastable He2* molecules can be easily created in helium via ionization or excitation of ground-state helium atoms:
where “*” denotes excited electronic states . These excited molecules are created in both electron-spin singlet (A1Σu+) state and triplet (a3Σu+) state. The singlet state molecules radiatively decay in a few nanoseconds , but the triplet state molecules have a lifetime of about 13 s due to a strongly forbidden spin flip during the decay . These triplet molecules form little bubbles (~ 6 Å in radius) in liquid helium  and they can serve as tracer particles. Due to their small size and hence small binding energy on vortex cores , trapping of the He2* tracers by quantized vortices can occur only below 0.6 K when the normal-fluid fraction is nearly zero . At above 1 K where most of the He II based applications take place, He2* tracers are solely entrained by the normal fluid since the viscous drag force dominates other forces. Therefore, He2* molecules are ideal tracers of the normal-fluid flow above 1 K, and they also grant us the opportunity to imaging vortex lines in pure superfluid helium below 0.6 K.
To image the He2* tracers, a cycling-transition laser-induced fluorescence (LIF) technique has been developed and advanced [7-8]. A schematic diagram of the optical transitions of the He2* molecules is shown in Fig. 1 (a). These molecules can be excited by two infra-red photons at 905 nm from their triplet ground state a3Σu+ to the excited electronic state d 3Σu+. Over 90% of the molecules in the d state decay to the b3Πg state in about 10 ns, emitting red photons at 640 nm, which can be detected by an intensified CCD camera. A filter can be used to block unwanted laser light to minimize background noise. From the b3Πg state, molecules quench back to the a3Σu+ state, and the process can be repeated. To enhance the cycling transition efficiency, re-pumping lasers at 1073 nm and 1099 nm can be used to recover the molecules that fall to the long-lived excited vibrational levels. We have successfully applied this LIF method to study various flows in helium. The fluorescence images in Fig. 1 (b) are examples showing the motion of a normal-fluid jet impinging on a plate in He-II.
So far, the LIF method has not yet been pushed to the limit for imaging individual He2* molecules. Instead, our strategy for quantitative velocity-field measurement is to create and track special patterns formed by large amounts of He2* tracers.
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