Highly Charged Droplets of Superfluid Helium

We report on the production and study of stable, highly charged droplets of superfluid helium. Using a novel experimental setup we produce neutral beams of liquid helium nanodroplets containing millions of atoms or more that can be ionized by electron impact, mass-per-charge selected, and ionized a second time before being analyzed. Droplets containing up to 55 net positive charges are identified and the appearance sizes of multiply charge droplets are determined as a function of the charge state. We show that the droplets are stable on the millisecond timescale of the experiment and decay through the loss of small charged clusters, not through symmetric Coulomb explosions.

Figure 1

(a) Mass per charge distributions of cationic He droplets (150 eV electrons at $105.4\mu \mathrm{A}$, blue curve) and anionic droplets (22.6 eV electrons at $0.3\mu \mathrm{A}$, red curve) by electron bombardment. The droplets were produced under identical conditions with a 8.5 K nozzle temperature. The lower energy gives a distribution of essentially purely singly charged droplets that peaks near $3\times {10}^6$ atoms and closely matches the neutral distribution. The distribution of positively charged droplets is pushed to lower mass per charge ratios. (b)–(d) Distributions of He droplets that are $m/z$-selected slices from the blue distribution in panel (a) and ionized a second time. The parent ions have sizes of $8.5\times {10}^5$, $1.275\times {10}^6$, and $2.125\times {10}^6$ He atoms per charge (indicated by arrows), respectively, and the products all have rational fractions of the parent mass per charge ratio.

Figure 2

(a) Mass-per-charge-selected ($3.8\times {10}^6$ He atoms per charge from 7 K nozzle) droplets ionized a second time with 80 (blue) and 22 eV (red) electrons. The higher energy leads mainly to an increase in charge state and narrow peaks with mass per charge ratios at distinct rational fractions of the parent. At 22 eV, the droplets are for the most part partially neutralized by helium anions, causing their mass per charge ratios to increase. (b) Wider range spectrum that shows that parent droplets containing up to 12 charges have had their net charge state reduced to $+1$. Peaks at half-integer positions show that droplets containing up to at least 17 charges have had their net charge reduced to $+2$.

Figure 3

Plot of the droplet charge state versus the square of the minimum droplet radius needed to form that particular charge state. The red curve is a fit to the experimental data. The horizontal error bars originate from the statistical uncertainties in determining the sizes of the droplets and the vertical bars from the uncertainties in the appearance of specific charge numbers amongst the series of higher charge states.