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Ion capture in helium droplets: formation of cold ion-neutral clusters

Travis M. Falconer

Published Aug 15, 2019• 0 citations
Publisher

Abstract

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.

DOI
10.17615/a3xk-yt96

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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 an experimental investigation of the effect of kinetic energy on the ion doping efficiency of superfluid helium droplets using cesium cations from a thermionic emission source. The kinetic energy of Cs(+) is controlled by the bias voltage of a collection grid collinearly arranged with the droplet beam. Efficient doping from ions with kinetic energies from 20 eV up to 480 V has been observed in different sized helium droplets. The relative ion doping efficiency is determined by both the kinetic energy of the ions and the average size of the droplet beam. At a fixed source temperature, the number of doped droplets increases with increasing grid voltage, while the relative ion doping efficiency decreases. This result implies that not all ions are captured upon encountering with a sufficiently large droplet, a deviation from the near unity doping efficiency for closed shell neutral molecules. We propose that this drop in ion doping efficiency with kinetic energy is related to the limited deceleration rate inside a helium droplet. When the source temperature changes from 14 K to 17 K, the relative ion doping efficiency decreases rapidly, perhaps due to the lack of viable sized droplets. The size distribution of the Cs(+)-doped droplet beam can be measured by deflection and by energy filtering. The observed doped droplet size is about 5 × 10(6) helium atoms when the source temperature is between 14 K and 17 K.
Abstract Alkali atoms and small clusters are known to reside on the surface of a helium droplet rather than its inside as most other dopant species. A theoretical investigation suggested that alkali clusters (Li–Rb) exceeding a certain critical size can become submerged in the droplet, which was experimentally confirmed for sodium and potassium. Here, we report an analogous experimental study of rubidium cluster submersion by means of electron impact mass spectrometry. We recorded size distributions of Rb cluster ions at various electron energies between 8 and 160 eV. Our data suggest that Rb clusters attached to helium droplets undergo a gradual submersion transition similar to potassium, ultimately leading to the full submersion of clusters larger than ∼100 Rb\sim 100~\hbox {Rb}∼100 Rb ∼ 100 Rb atoms. Our findings are consistent with previous theoretical and experimental studies. Graphic abstract
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.
In this thesis, we investigate the formation process of snowballs (solid-like ordered helium atoms) when embedding ions within helium droplets. This is achieved through a semi-classical methodology, namely the Gaussian-Imaginary Time-dependent Hartree approach. Subsequently, we propose a modified Density Functional, built upon the well-established Orsay-Trento functional, to better capture the primary effects of the snowball on the surrounding helium droplet. In such systems, a local supersolid layer emerges in the transition region between the snowball (near the ion) and the superfluid (far from the ion). We also investigated Rydberg atoms embedded in helium droplets. These atoms were modeled as an ionic core with a weakly bound electron. In this case, the helium density profile arranges similarly to that observed for ionic impurities, as the perturbation induced by the highly excited Rydberg electron on the droplet is negligible. At the same time, the energy spectrum associated with the Rydberg electron undergoes significant modifications. To better capture the perturbation of the Rydberg electron's states induced by the helium droplet, we introduce the droplet-dressed hydrogenic states. These states incorporate the average effect of the droplet on the highly excited electronic states, significantly reducing the computational effort. We propose a standard methodology that employs these states as a basis for efficiently describing Rydberg electrons in both discrete- and continuous-density environments. Furthermore, we demonstrate the existence of droplet-dressed Rydberg molecules, in which a Rydberg atom located at the center of the droplet binds to a ground-state atom located beyond the droplet's surface via the Rydberg electron. In this context, the helium droplet itself acts as an environment that promotes the stability of such molecular structures and enhances some of its properties like bond length and electric dipole moment.
For the past 30 years, helium nano-droplets have been of particular interest to experimentalists.Indeed, this is a cold dense finite liquid environment which is an excellent matrix forspectroscopy applications, in atomic and molecular collision physics as well as in ultra coldchemistry with ability of isolating interesting reactions intermediates like HOCO in OH + CO→ CO2 + H or to synthesize new materials as metallic nano-wires. These phenomena areconsequences of the quantum collective behaviour of helium atoms at temperature T < 1 Kwhich classify helium nano-droplets as quantum liquids. In this thesis work we propose tomodel photo-dissociation [1] of immersed diatomic as molecular iodine, doping and rotationalexcitation of these droplets by using an home-made molecular dynamics developed by Pr.Marius Lewerenz, the CLUSTER code. To take the quantum nature of helium atoms intoaccount the methodology exploits the quantum effective potential technique [2] at experimentalwork temperature 0.37 K.With the photo-dissociation, an excitation above a molecular fragmentation limit leads to twopossible scenarios depending one the amortised energy by the helium droplets. The first one isthe fast recombination so-called ’cage effect’ [3] and the second is where photo-fragmentsreside in the helium bath. But with nano-meter scale of droplets a third scenario is possible withfragments leaving the cluster surrounded by a helium solvating shell. Results are shown forhelium nano-droplets containing thousands atoms (2500 to 9000 He) and statistics on size andvelocity of I@Hen photo-fragments have been computed and are similar to experimentaloutcomes for analogous system CH3I [4].According experiments using 1 fs X-ray flashes [5], the helium nano-droplets are geoids whichis a clue indicating a rotation on itself as the Earth. Our code can set precise angular momentumin order to study the pattern of distortion from fission to multiple splitting. To evaluate thenumbers of sub-cluster a Complete Clustering algorithm based on graph theory has beenprogrammed exhibiting a linear complexity.To conclude a study of xenon doping is presented where the recombination dynamics unveils ametastable state called Atkins’ snowball [6] and different patterns of aggregation as nano-crystals and nano-wires well known by experimentalists
Synopsis The capability of helium droplets to isolate cryogenic molecular ions at a temperature of 0.4 K is exploited to observe ultracold molecular processes in the RIKEN Cryogenic Electrostatic (RICE) storage ring. The development of a pulsed droplet beam consisting of extremely large helium droplets up to 1011 atoms is a promising step to apply this unique and robust cooling method to a wide variety of molecular ions. Production of ions by photoionization of neutral species inside helium droplets will provide internally cold ions below the ring temperature of 4 K.