Electrostatic Deflection of a Molecular Beam of Massive Neutral Particles: Fully Field-Oriented Polar Molecules within Superfluid Nanodroplets

Joint electric and magnetic beam deflection experiments and quantum chemical studies of MSn12 clusters (M = Al, Ga, In): on the interplay of geometric structure and magnetic properties in nanoalloys

MSn₁₂ clusters (M = Al, Ga, In) were studied in electric and magnetic beam deflection experiments at temperatures of 16 K and 30 K. For all three species, the results of the electric beam deflection experiments indicate the presence of two structural isomers of which one is considerably polar. The magnetic beam deflection experiments show atom-like beam splitting (superatomic behavior) with g-factors of 2.6-2.7 for a fraction of the clusters in the molecular beam, indicating significant spin-orbit coupling. On the one hand, we investigate by several experiments combining electric and magnetic deflectors how the superatomic and polar fractions are linked proving the correlation of the Stark and Zeeman effects. On the other hand, the magnetic deflection behavior is examined more thoroughly by performing quantum chemical calculations. By systematic distortion of an artificial icosahedral tin cage towards the global minimum structure, which has a pyritohedral geometry, the shifts in the magnitude of the g-factor are found to be mainly caused by a single dominant electronic excitation. This allows one to develop a semi-quantitative understanding of the magnetic behavior. On the basis of avoided crossings in the rotational Zeeman diagram, simulations of the magnetic beam deflection comprising computed rotational constants, vibrational modes, g-factors and spin-rotation coupling constants are performed which resemble our experimental findings in satisfactory agreement. With this, a better understanding of the magnetic properties of nanoalloy clusters can be achieved. However, the geometric structures of the polar isomers are still unknown.

Probing the presence and absence of metal-fullerene electron transfer reactions in helium nanodroplets by deflection measurements

Metal-fullerene compounds are characterized by significant electron transfer to the fullerene cage, giving rise to an electric dipole moment. We use the method of electrostatic beam deflection to verify whether such reactions take place within superfluid helium nanodroplets between an embedded C₆₀ molecule and either alkali (heliophobic) or rare-earth (heliophilic) atoms. The two cases lead to distinctly different outcomes: C₆₀Na_n (n = 1-4) display no discernable dipole moment, while C₆₀Yb is strongly polar. This suggests that the fullerene and small alkali clusters fail to form a charge-transfer bond in the helium matrix despite their strong van der Waals attraction. The C₆₀Yb dipole moment, on the other hand, is in agreement with the value expected for an ionic complex.

Discriminating the influence of thermal excitation and the presence of structural isomers on the Stark and Zeeman effect of AlSn12 clusters by combined electric and magnetic beam deflection experiments

A combination of electric and magnetic beam deflection experiments shows a connection between non-polar and superatomic species for AlSn₁₂ clusters.

Metal clusters synthesized in helium droplets: structure and dynamics from experiment and theory

Metal clusters have drawn continuous interest because of their high potential for the assembly of matter with special properties that may significantly differ from the corresponding bulk. Controlled combination of particular elements in one nanoparticle can increase the options for the creation of new materials for photonic, catalytic, or electronic applications. Superfluid helium droplets provide confinement and ultralow temperature, i.e. an ideal environment for the atom-by-atom aggregation of a new nanoparticle. This perspective presents a review of the current research progress on the synthesis of tailored metal and metal oxide clusters including core-shell designs, their characterization within the helium droplet beam, deposition on various solid substrates, and analysis via surface diagnostics. Special attention is given to the thermal properties of mixed metal clusters and questions about alloy formation on the nanoscale. Experimental results are accompanied by theoretical approaches employing computational chemistry, molecular dynamics simulations and He density functional theory.

Strong permanent magnet gradient deflector for Stern-Gerlach-type experiments on molecular beams

We describe the design, assembly, and testing of a magnet intended to deflect beams of paramagnetic nanoclusters, molecules, and atoms. It is energized by high-grade permanent neodymium magnets. This offers a convenient option in terms of cost, portability, and scalability of the construction while providing field and gradient values (1.1 T, 330 T/m), which are fully comparable with those of commonly used electromagnet deflectors.

Direct detection of polar structure formation in helium nanodroplets by beam deflection measurements

Long-range intermolecular forces are able to steer polar molecules submerged in superfluid helium nanodroplets into highly polar metastable configurations. We demonstrate that the presence of such special structures can be identified, in a direct and determinative way, by electrostatic deflection of the doped nanodroplet beam. The measurement also establishes the structures' electric dipole moments. In consequence, the introduced approach is complementary to spectroscopic studies of low-temperature molecular assembly reactions. It is enabled by the fact that within the cold superfluid matrix the molecular dipoles become nearly completely oriented by the applied electric field. As a result, the massive (tens of thousands of helium atoms) nanodroplets undergo significant deflections. The method is illustrated here by an application to dimers and trimers of dimethyl sulfoxide (DMSO) molecules. We interpret the experimental results with ab initio theory, mapping the potential energy surface of DMSO complexes and simulating their low temperature aggregation dynamics.

Oriented Polar Molecules Trapped in Cold Helium Nanodropets: Electrostatic Deflection, Size Separation, and Charge Migration

Helium nanodroplets doped with polar molecules are studied by electrostatic deflection. This broadly applicable method allows even polyatomic molecules to attain subkelvin temperatures and nearly full orientation in the field. The resulting intense force from the field gradient strongly deflects even droplets with tens of thousands of atoms, the most massive neutral systems studied by beam "deflectometry." We use the deflections to extract droplet size distributions. Moreover, since each host droplet deflects according to its mass, spatial filtering of the deflected beam translates into size filtering of neutral fragile nanodroplets. As an example, we measure the dopant ionization probability as a function of droplet radius and determine the mean free path for charge hopping through the helium matrix. The technique will enable separation of doped and neat nanodroplets and size-dependent spectroscopic studies.

Thermodynamic Model for Calculation the Position of Quitting Surface of Supersonic Beam Based on Speed Measurements

For a nozzle jet beam, one fascinating fact is where a transition occurs from continuous to discontinuous beam during the expansion, which is typically known as the quitting surface. In our previous work, we applied a pickup method to investigate the fragmentation properties of water cluster during electron ionization, and we roughly estimated the possible quitting surface position (∼1 to 1.5 mm in front of the nozzle) to make sure the dopants were not accelerated by the beam expansion. Thus, the momentum conservation law can be applied to evaluate the possible fragmented water molecules. In this work, we applied a simple thermodynamic method to more accurately calculate the position of quitting surface of supersonic beam based on speed measurements. From our calculation in this paper, the quitting surface should be exactly located 1 mm in front of the nozzle, and this paper can further support our previous work.

Bacterial Outer Membrane Porins as Electrostatic Nanosieves: Exploring Transport Rules of Small Polar Molecules

Transport of molecules through biological membranes is a fundamental process in biology, facilitated by selective channels and general pores. The architecture of some outer membrane pores in Gram-negative bacteria, common to other eukaryotic pores, suggests them as prototypes of electrostatically regulated nanosieve devices. In this study, we sensed the internal electrostatics of the two most abundant outer membrane channels of Escherichia coli, using norfloxacin as a dipolar probe in single molecule electrophysiology. The voltage dependence of the association rate constant of norfloxacin interacting with these nanochannels follows an exponential trend, unexpected for neutral molecules. We combined electrophysiology, channel mutagenesis, and enhanced sampling molecular dynamics simulations to explain this molecular mechanism. Voltage and temperature dependent ion current measurements allowed us to quantify the transversal electric field inside the channel as well as the distance where the applied potential drops. Finally, we proposed a general model for transport of polar molecules through these electrostatic nanosieves. Our model helps to further understand the basis for permeability in Gram-negative pathogens, contributing to fill in the innovation gap that has limited the discovery of effective antibiotics in the last 20 years.