Metastable Helium Molecules as Tracers in Superfluid ${}^4\mathrm{He}$

Metastable helium molecules generated in a discharge near a sharp tungsten tip immersed in superfluid ${}^4\mathrm{He}$ are imaged using a laser-induced-fluorescence technique. By pulsing the tip, a small cloud of ${\mathrm{He}}_2^{*}$ molecules is produced. We can determine the normal-fluid velocity in a heat-induced counterflow by tracing the position of a single molecule cloud. As we run the tip in continuous field-emission mode, a normal-fluid jet from the tip is generated and molecules are entrained in the jet. A focused 910 nm pump laser pulse is used to drive a small group of molecules to the first excited vibrational level of the triplet ground state. Subsequent imaging of the tagged molecules with an expanded 925 nm probe laser pulse allows us to measure the flow velocity of the jet. The techniques we developed provide new tools in quantitatively studying the normal fluid flow in superfluid helium.

Figure 1

Schematic diagram showing the cycling transitions for imaging the ${\mathrm{He}}_2^{*}$ triplet molecules.

Figure 2

(a) Schematic diagram showing the setup for the molecule cloud experiment. (b) Fluorescence images for ${\mathrm{He}}_2^{*}$ molecule clouds created with (1) 10 ms, (2) 30 ms and (3) 90 ms pulse on the tip, respectively. The grey bars indicate the tip. The images are a sum of 50 camera exposures.

Figure 3

The motion of a molecule cloud with 15 V on the heater. The images were taken at 0 s, 0.2 s and 0.4 s, respectively, after the cloud was created. The duration of the pulse on the tungsten tip was 5 ms. The images are a sum of 25 camera exposures.

Figure 4

(a) The average vertical positions of a molecule cloud as a function of its drift time for different heat powers. (b) The obtained normal-fluid velocity as a function of the heat power. The solid line and the dashed line are the theoretical curves as discussed in the text.

Figure 5

(a) Schematic diagram showing the lasers used in the molecule tagging experiment. (b) and (c) show the molecule fluorescence images taken with pump laser alone and probe laser alone, respectively. Both the pump and the probe lasers were tuned to 905 nm in order to show the beam sizes and positions of the lasers.

Figure 6

Fluorescence images showing the positions of a small group of $a(1)$ molecules at different delay times after they were created. The delay times between the pump and probe laser pulses are (a) 0 ms, (b) 10 ms, (c) 40 ms and (d) 70 ms. The dc voltage on the tungsten tip was 805 V.

Figure 7

(a) The vertical position of the $a(1)$ molecule cloud as a function of the pump-probe delay time at several different dc voltages on the tip. (b) Obtained normal-fluid velocity as a function of the square root of the current measured on the nickel mesh plate.