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Chemistry - A European Journal

Multiply Charged Helium Droplet Anions

Felix Laimer, Fábio Zappa, P. Scheier, Michael Gatchell

Published Jan 1, 2021• 24 citations
Publisher

Abstract

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.

DOI
10.1002/chem.202005004

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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.
We report on the successful trapping of micrometer-sized, multiply-charged helium droplets over periods of several seconds up to a minute-the first observation of trapped charged helium droplets. Within a meter-long multireflection mass spectrometer, the droplets complete thousands of revolutions, opening up new possibilities for precision measurements of these unique systems. Their confinement is limited by collisions with residual gas, which gradually reduce their kinetic energy through evaporation of helium atoms from the droplet and prevent extraction from the trap below a critical value. The rate of energy loss is strongly influenced not only by the residual gas density, but also by the type of species picked up by the droplets. In particular, infrared absorption by water cluster ions inside the droplets enhances energy transfer to the helium matrix, leading to accelerated evaporation of helium atoms.
A beam of size-selected charged helium droplets was crossed with an electron beam, and the ion efficiency curves for the product droplets in all different charge states were recorded. We estimate that the selected helium droplets on their passage through the electron beam are hit by several hundred electrons which can interact with the individual He atoms of the droplets. Reaction channels corresponding to the removal or capture of up to eight electrons were identified, and in all cases, inelastic scattering and the formation of metastable helium played a significant role.
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.
Superfluid helium nanodroplets are used to cool ions and form ion-neutral clusters at a temperature of 0.37 K. A desolvation technique was developed that allowed for the study of captured ions by mass spectrometry. From the mass spectrometry results it was determined that helium droplets may successfully capture sodium cations with kinetic energy of ~200 eV. Clusters of the neutral molecules H2O, N2, and HCN with Na+ were observed. Based on binding strength considerations, it is argued that the desolvation process imparts little energy into the ion-neutral clusters, avoiding dissociation. This result leads to the conclusion that ion-neutral clusters are formed within the droplet prior to desolvation, indicating that the helium snowball that is assumed to form around Na+ does not prevent ion-neutral cluster formation. This conclusion is supported by ab initio calculations, the results of which indicate the presence of barrierless pathways for neutral molecule insertion into the helium snowball surrounding Na+. The process of ion capture by helium droplets was studied by comparison of measured ion-doped droplet size distributions to known pre-ion capture droplet size distributions. The measured ion-doped droplet size distributions were affected by nozzle temperature, ion kinetic energy, and ion mass. These factors primarily affect two parameters of the ion-doped droplet size distribution, the minimum droplet size threshold, Nthr, and the droplet size at maximum signal intensity, Nmax. The effects of the studied factors on the measured distributions cannot be explained in terms of currently accepted droplet cooling mechanisms. Analysis of the results suggests that droplet doping efficiency can be improved in several ways, including a higher flux of lower-energy ions. An apparatus for the production and focusing of alkali cations at high fluxes and low energies is described. This apparatus was able to produce higher ion currents at lower kinetic energies than previous ion sources.
We present two facile time-of-flight (TOF) methods of detecting superfluid helium droplets and droplets with neutral dopants. Without an electron gun and with only a heated filament and pulsed electrodes, the electron impact ionization TOF mass spectrometer can resolve ionized helium clusters such as He2(+) and He4(+), which are signatures of superfluid helium droplets. Without ionizing any helium atoms, multiphoton non-resonant laser ionization of CCl4 doped in superfluid helium droplets at 266 nm generates complex cluster ions of dopant fragments with helium atoms, including (He)(n)C(+), (He)(n)Cl(+), and (He)(n)CCl(+). Using both methods, we have characterized our cryogenic pulsed valve—the Even-Lavie valve. We have observed a primary pulse with larger helium droplets traveling at a slower speed and a rebound pulse with smaller droplets at a faster speed. In addition, the pickup efficiency of dopant is higher for the primary pulse when the nozzle temperature is higher than 13 K, and the total time duration of the doped droplet pulse is only on the order of 20 μs. These results stress the importance of fast and easy characterization of the droplet beam for sensitive measurements such as electron diffraction of doped droplets.
Many microscopic properties of the only natural superfluid element helium have become available within the last 18 years through a number of novel experiments. The diffraction of matter-waves of small helium clusters has established the unusually large bond distance (5.2 nm) and extremely weak binding energy (10−7 eV) of the dimer and other exotic properties of small helium clusters. The high-resolution spectroscopy of closed-shell chromophore molecules attached to small clusters or embedded inside large superfluid droplets have revealed that as a result of superfluidity, molecules rotate virtually without friction and that only a handful of He atoms are required for microscopic superfluidity. The frictionless translational motion of atoms and molecules inside the droplets has also been observed. These and a number of other experiments demonstrate that helium droplets are the most gentle and coldest of all cryomatrices with enormous potential for a wide range of future applications.