Schematic of the He droplet beam vacuum apparatus. NZ: cold nozzle, SK: skimmer, PC1 and PC2: pickup cells, BG: Baratron vacuum gauge, IG1 and IG2: ion gauges, SH1 and SH2: beam shutters, A1 and A2: 6 mm diameter apertures, GV1 and GV2: gate valves, EI: electron impact ionizer, IB: ion bender, QMS: quadrupole mass spectrometer.
Illustration of measurement of He droplet sizes by titration of droplet beam with collisional helium atoms at room temperature. Decrease of He flux into the detection chamber (far right) is measured as a pressure drop, ΔP.
Intensity of the He droplet beam, produced by continuous expansion of He gas at 20 bar through a 5 μm nozzle, as measured by the partial pressure rise of He in the detection chamber. Nominal reading of the ion gauge (Leybold-Heraeus IM 510 gauge, IE 514 sensor) has been corrected for the sensitivity of the gauge to He gas. Partial pressure rise was obtained as a pressure difference between the beam unblocked and blocked using shutter SH1. Scale on the right-hand side shows flux of He transported by the beam into the detection chamber. Solid line shows dependence normalized to experimental data point at T 0 = 100 K.
Normalized He flux versus collisional helium gas pressure in the main experimental chamber for droplets obtained at T 0 = 5.7, 6, 7, 9, and 12 K. Filled symbols represent values obtained by measuring the He partial pressure rise in the detection chamber with the mass spectrometer set to m = 4. Open symbols correspond to the measurements of the droplet beam intensity at m = 8. Solid and dashed lines are fits according to Eq. (1) for measurements with m = 4 and m = 8, respectively.
Typical mass spectra of He droplet beams obtained at P 0 = 20 bars and T 0 = 7 (blue trace), 9 (red), and 14.5 K (black). The spectra have been normalized to the same intensity at m = 8. Electron beam energy 100 eV, emission current 4 mA. Contributions from effusive helium gas and rest gas in the detection chamber have been subtracted out.
〈N He〉 obtained at various nozzle temperatures from continuous expansion of He, at P 0 = 20 bar, through a 5 μm nozzle. Results obtained via measurements of α 4 with collisional helium and argon gases are shown by filled squares and circles, respectively. Results of previous deflection measurements are shown by open triangles17,30 and stars.16,28,29 Right-hand scale shows corresponding liquid droplet diameters obtained using n LHe.
Pressure-temperature phase diagram of 4He. Isentropes (dashed lines) are for 4He at a stagnation pressure of P 0 = 20 bar from Ref. 23. Locus of sonic points specifies the states at the orifice for the isentropic expansions. Reprinted with permission from J. Chem. Phys. 92, 6875 (1990). © 1990 American Institute of Physics.
Intensity ratios 0.1 × I 16/I 12 (circles) and I 16/I 8 (triangles) obtained from the mass spectra of the continuous He droplet beams comprised of different droplet sizes. Corresponding nozzle temperatures from Table I are shown in the upper part of the plot for reference for some of the data points. Average sizes from Ref. 28 are used for N He < 104 and sizes obtained in this work for larger droplets.
I 16/I 8 ratio of He droplet beam obtained from a 1 mm diameter pulsed-valve. Expansion obtained at P 0 = 15 bar for different nominal nozzles temperatures and two repetition rates of 1 Hz and 10 Hz, solid circles and open squares, respectively. For comparison, the results obtained with the 5 μm continuous-nozzle at P 0 = 20 bar from Table I are shown by triangles. Average droplet sizes are indicated for some of the data points.
Nozzle temperature, T 0; attenuation coefficients α 4 and α 8 (in parenthesis) obtained with helium titration; droplet beam velocity, v D; and average number of He atoms in the droplets, 〈N He〉, according to Eq. (2). Table I also contains intensity ratios of peak m = 16 to m = 8 and m = 12. Values of 〈N He〉 obtained previously, using deflection techniques of Refs. 17 and 28–30, are shown in the penultimate column. Italicized values of 〈N He〉 in the last column are beyond the range of the applied technique as described in the text. All measurements are at nozzle stagnation pressure of P 0 = 20 bars.
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