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. 2021 May 3;27(25):7283-7287.
doi: 10.1002/chem.202005004. Epub 2021 Mar 12.

Multiply Charged Helium Droplet Anions

Felix Laimer  1 , Fabio Zappa  1   2 , Paul Scheier  1 , Michael Gatchell  1   3
Affiliations

Multiply Charged Helium Droplet Anions

Felix Laimer et al. Chemistry. .

Abstract

The detection of multiply charged helium droplet anions is reported for the first time. By ionizing droplets of superfluid helium with low energy electrons (up to 25 eV), it was possible to produce droplets containing up to five negative charges, which remain intact on the timescale of the experiment. The appearance sizes for different charge states are determined and are found to be orders of magnitude larger than for the equivalent cationic droplets, starting at 4 million He atoms for dianions. Droplets with He*- as charge carriers show signs of being metastable, but this effect is quenched by the pickup of water molecules.

Keywords: charge carriers; helium droplets; multiple charges; superfluid.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Mass‐per‐charge distributions of He droplets, produced with a nozzle temperature of 8 K and backing pressure of 20 bar, ionized with 36 eV (cationic droplets, dashed blue) and 22 eV (anionic droplets, solid red) electrons.
Figure 2
Figure 2
Negatively charged droplets containing 2.49×107 He per charge were selected in the first stage after being ionized by 25.9 eV electrons. Product distributions were measured for three different settings on the second ion source: 0 eV (second source off, in blue), anionic products with 23 eV electrons (green), and cationic products with 38 eV electrons (red). Peak positions correspond to the ratios z1/z2 .
Figure 3
Figure 3
Measured critical sizes of Hen z droplets for charge states up to z=5± (cation data from ref. [13]). The appearance sizes for anions are significantly larger than for cations and the negative charge that can be held by a droplet apparently scales with √n.
Figure 4
Figure 4
Scans of selected droplet with 4.5×106 He atoms per negative charge ionized a second time with 1.8 eV or 25.3 eV electrons for two different chamber pressures. The residual gas consists mainly of water and the higher pressure increases the yield of doped droplets, which has a stabilizing effect on the negative charges, most notable for the He*− ions (green curves). Features indicated with asterisks (*) originate from photon emission as charged droplets strike the walls of the analyzer, see the Supporting Information for more details.
See this image and copyright information in PMC

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