Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of Hen+ ions that peaks at n = 2 and decreases monotonically toward larger n. In larger droplets (say 105 or more atoms), however, the He4+ signal intensity is anomalously large. We have studied the mechanism for the formation of He4+ ions in large helium droplets by varying the duration of the electron impact excitation pulse. Droplets of different average sizes were generated using the expansion of helium at 20 bars and 9-20 K through a pulsed valve nozzle. The resulting ions were analyzed by time-of-flight mass spectroscopy (TOFMS) and quadrupole mass spectroscopy (QMS). The intensity distributions obtained with the TOFMS technique initially showed much smaller He4+ signals than those obtained using QMS. However, we discovered that the intensity anomaly is associated with the duration of the electron bombardment pulse in the TOFMS instrument. Measurements with different electron bombardment pulse durations enabled us to discern a characteristic time of ∼10 μs for enhanced He4+ production in large droplets under our experimental conditions. A qualitative model is presented in which metastables interact on droplet surfaces, yielding two He2+ cores that share a Rydberg electron while minimizing repulsion between the cores. This is the He4+(4A2) state suggested by Knowles and Murrell.
We present neutron-scattering measurements of the momentum distribution of liquid $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ mixtures. The experiments were performed at wave vectors
Q, $26\ensuremath{\leqslant}Q\ensuremath{\leqslant}29\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$, on the MARI time-of-flight spectrometer at the ISIS Facility, Rutherford Appleton Laboratory, a spallation neutron source. Mixtures with
3He concentrations
x between 0 and 20% were investigated both in the superfluid and normal phases. From the data, we extract the Bose-Einstein condensate fraction
n0 and the momentum distributions of
3He and
4He atoms. We find that
n0 increases somewhat above the pure
4He value when
3He is added; e.g., from ${n}_{0}=(7.25\ifmmode\pm\else\textpm\fi{}0.75)%$ at
x=0 to $(11\ifmmode\pm\else\textpm\fi{}3)%$ at
x=15−−20. This agrees with predictions but is less than the only previous measurement. We find a
4He kinetic energy
K4 for pure
4He that agrees with previous determinations.
K4 decreases somewhat with increasing
3He concentration, less than observed previously and found in early calculations but in agreement with a more recent Monte Carlo calculation. The
3He response is not well reproduced by a Fermi-gas momentum distribution,
n(k). Rather an
n(k) having a small step height at the Fermi surface and a substantial high-momentum tail characteristic of a strongly interacting Fermi liquid provides a good fit. This
n(k) is consistent with calculated
n(k). Thus agreement between theory and experiment is obtained comparing
n(k) in contrast to earlier findings based on comparing calculated and observed
3He kinetic energies.
Experimental and theoretical results for mixed helium droplets doped with one calcium atom are reported. The absorption spectrum around the $4s4p\ensuremath{\leftarrow}4{s}^{2}$ transition is calculated as a function of the composition of the droplet and compared to the experiment. Our study reveals the distinct feature that for specific
3He concentrations, Ca atoms sit at the
3He-4He interface. This particular property of Ca, likely not shared with any other atomic or molecular species but the heavier alkaline-earth atoms, might offer the possibility of using it to probe the
3He-4He interface of the droplet.
We present neutron scattering measurements of the momentum distribution of liquid
3He-
4He mixtures. The experiments were performed at wavevectors
Q, 26
≤ Q ≤ 29 {Å
−1}, on the MARI time-of-flight spectrometer at the ISIS Facility, Rutherford Appleton Laboratory, a spallation neutron source. Mixtures with
3He concentrations
x between 0 and 20% were investigated both in the superfluid and normal phases. From the data, we extract the Bose-Einstein condensate fraction
n0 and the momentum distributions of
3He and
4He atoms. We find that
n0 increases somewhat above the pure
4He value when
3He is added; e.g from
n0=(7.25±0.75) at
x= 0 to
(11±3) at
x= 15-20%. This agrees with predictions but is less than the only previous measurement. We find a
4He kinetic energy
K4 for pure
4He that agrees with previous determinations.
K4 decreases somewhat with increasing
3He concentration, less than observed previously and found in early calculations but in agreement with a more recent Monte Carlo calculation. The
3He response is not well reproduced by a Fermi gas momentum distribution,
n(k). Rather an
n(k) having a small step height at the Fermi surface and a substantial high momentum tail characteristic of a strongly interacting Fermi liquid provides a good fit. This
n(k) is consistent with calculated
n(k). Thus agreement between theory and experiment is obtained comparing
n(k) in contrast to earlier findings based on comparing calculated and observed
3He kinetic energies.
We show that very low velocity droplets can be used to carry out an experiment to test whether condensate mediated transmission processes can occur in a superfluid droplet of
4He. By appropriately choosing the droplet radius and temperature, we can eliminate the competing roton, phonon, and ripplon mediated elastic transmission events. Then a calculation shows that if a few percent or more of the incident atoms experience anomalous condensate mediated transmission, the effects should be detectable in the droplet trajectories. We consider two forms of the experiment, involving a freely falling droplet in ambient vapor in the first instance and an oscillating droplet in a magnetic trap in the second.