Size and Velocity Distribution of Negatively Charged Helium Nanodroplets

Abstract

Precharged helium nanodroplets can be used in doping experiments with the advantage that they are amenable to size selection with electrostatic fields, therefore adding a useful tuning parameter for dopant growth. For all these applications, the knowledge of the size distribution of charged droplets is an essential parameter, which we have so far assumed would be equivalent to that of their neutral precursors. Here, this assumption is experimentally investigated for negatively charged clusters for temperatures between 4 and 9 K at a stagnation pressure of 2 MPa. We observe a dependency of the velocity of the droplets on mass per charge, especially at the lowest temperatures of the investigated range, and values 20% lower than those known from the literature. Below 6 K, a large deviation from the literature is also found for the average droplet sizes. This information has to be taken into consideration in future experiments where large, charged droplets are sought to produce large dopant clusters. Possible origins for this deviation are discussed in the text.

Efficient Formation of Size-Selected Clusters upon Pickup of Dopants into Multiply Charged Helium Droplets

“…We performed these measurements with two different configurations of the same instrument, i.e., electron ionization of neutral He droplets doped with C 60 (red circles, Figure 5 ) [ 57 ] and surface impact of charged helium droplets subsequently doped with C 60 (blue squares, Figure 5 ) [ 58 ]. The average size of the He droplets initially formed were in all cases about 10 6 He atoms [ 59 ]. He droplets of this size can easily accommodate more than 10 positive charges, however only one anionic charge center [ 60 ].…”

“…This results in a kinetic energy release due to the Coulomb repulsion of the remaining charged droplet in the order of 50 meV, with most of it going into the ejected dopant cluster ion. This energy is negligible in comparison to the kinetic energy of Au 10 + of 410 meV due to the velocity of the droplet (typically 200 m/s [ 59 ]). However, in the case of light dopants, such as sodium, this kinetic energy release becomes comparable to the initial forward energy.…”

“…This enabled source temperatures between 4.2 K and 25 K with ±0.1 K precision. For the present study source temperatures were used between 7 K and 9.7 K. At a stagnation pressure of 2 MPa, this resulted in average droplet sizes between 20 and 1 million He atoms [ 59 ]. After passing a 0.5 mm skimmer (Beam Dynamics, Inc., Dallas, TX, USA) positioned about 5 mm from the nozzle, the helium droplets were ionized via electron bombardment.…”

Electron Ionization of Size-Selected Positively and Negatively Charged Helium Droplets

“…A schematic diagram of the apparatus is shown in Figure 1. The velocity distributions at different nozzle temperatures of the droplet beam were measured recently by Laimer et al via a time-of-flight method by pulsing the electron energy [35]. The velocities of the singly charged He droplets of the selected precursor mass-per charge-values at 7 K and 9 K ranged from v = 155 to 169 m/s and 196 m/s, respectively.…”

“…Positively charged He droplets were formed upon electron bombardment of neutral He droplets (expansion conditions of 7 K and 2 MPa, average neutral droplet size of 1.1 × 10 7 [38]) with an electron energy of 40 eV and an electron current of 300 µA. This results in a log-normal-shaped m/z distribution with an average value of about 7 million He atoms per charge [35]. The first energy filter (Analyzer 1 in Figure 1) selected a narrow slice of this distribution at a relative m/z of 2.7 × 10 6 He atoms per charge (corresponding to a relative m/z value of 1 in Figure 3).…”