Catalysis in Extreme Field Environments: A Case Study of Strongly Ionized SiO2 Nanoparticle Surfaces.

High electric fields can significantly alter catalytic environments and the resultant chemical processes. Such fields arise naturally in biological systems but can also be artificially induced through localized nanoscale excitations. Recently, strong field excitation of dielectric nanoparticles has emerged as an avenue for studying catalysis in highly ionized environments, producing extreme electric fields. While the dynamics of laser-driven surface ion emission has been extensively explored, understanding the molecular dynamics leading to fragmentation has remained elusive. Here, we employ a multiscale approach performing nonadiabatic quantum molecular dynamics (NAQMD) simulations on hydrogenated silica surfaces in both bare and wetted environments under field conditions mimicking those of an ionized nanoparticle. Our findings indicate that hole localization drives fragmentation dynamics, leading to surface silanol dissociation within 50 fs and charge transfer-induced water splitting in wetted environments within 150 fs. Further insight into such ultrafast mechanisms is critical for the advancement of catalysis on the surface of charged nanosystems.

Infrared spectroscopy of isomers of C3H4+ in superfluid helium droplets.

Superfluid helium nanodroplets are unique nanomatrices for the isolation and study of transient molecular species, such as radicals, carbenes, and ions. In this work, isomers of C3H4+ were produced upon electron ionization of propyne and allene molecules and interrogated via infrared spectroscopy inside He nanodroplet matrices. It was found that the spectrum of C3H4+ has at least three distinct groups of bands. The relative intensities of the bands depend on the precursor employed and its pickup pressure, which indicates the presence of at least three different isomers. Two isomers were identified as allene and propyne radical cations. The third isomer, which has several new bands in the range of 3100-3200 cm-1, may be the elusive vinylmethylene H2C=CH-CH+ radical cation. The observed bands for the allene and propyne cations are in good agreement with the results of density functional theory calculations. However, there is only moderate agreement between the new bands and the theoretically calculated vinylmethylene spectrum, which indicates more work is necessary to unambiguously assign it.

Generation of Large Vortex-Free Superfluid Helium Nanodroplets.

Superfluid helium nanodroplets are an ideal environment for the formation of metastable, self-organized dopant nanostructures. However, the presence of vortices often hinders their formation. Here, we demonstrate the generation of vortex-free helium nanodroplets and explore the size range in which they can be produced. From x-ray diffraction images of xenon-doped droplets, we identify that single compact structures, assigned to vortex-free aggregation, prevail up to 10^{8} atoms per droplet. This finding builds the basis for exploring the assembly of far-from-equilibrium nanostructures at low temperatures.

Formation of the C4Hn+ (n = 2-5) ions upon ionization of acetylene clusters in helium droplets.

Infrared (IR) spectroscopy using ultracold helium nanodroplet matrices has proven to be a powerful method to interrogate encapsulated ions, molecules, and clusters. Due to the helium droplets' high ionization potential, optical transparency, and ability to pick up dopant molecules, the droplets offer a unique modality to probe transient chemical species produced via photo- or electron impact ionization. In this work, helium droplets were doped with acetylene molecules and ionized via electron impact. Ion-molecule reactions within the droplet volume yield larger carbo-cations that were studied via IR laser spectroscopy. This work is focused on cations containing four carbon atoms. The spectra of C4H2+, C4H3+, and C4H5+ are dominated by diacetylene, vinylacetylene, and methylcyclopropene cations, respectively, which are the lowest energy isomers. On the other hand, the spectrum of C4H4+ ions hints at the presence of several co-existing isomers, the identity of which remains to be elucidated.

Infrared spectroscopy of ions and ionic clusters upon ionization of ethane in helium droplets.

Helium droplets are unique hosts for isolating diverse molecular ions for infrared spectroscopic experiments. Recently, it was found that electron impact ionization of ethylene clusters embedded in helium droplets produces diverse carbocations containing three and four carbon atoms, indicating effective ion-molecule reactions. In this work, similar experiments are reported but with the saturated hydrocarbon precursor of ethane. In distinction to ethylene, no characteristic bands of larger covalently bound carbocations were found, indicating inefficient ion-molecule reactions. Instead, the ionization in helium droplets leads to formation of weaker bound dimers, such as (C2H6)(C2H4)+, (C2H6)(C2H5)+, and (C2H6)(C2H6)+, as well as larger clusters containing several ethane molecules attached to C2H4 +, C2H5 +, and C2H6 + ionic cores. The spectra of larger clusters resemble those for neutral, neat ethane clusters. This work shows the utility of the helium droplets to study small ionic clusters at ultra-low temperatures.

Infrared spectroscopy of carbocations upon electron ionization of ethylene in helium nanodroplets.

The electron impact ionization of helium droplets doped with ethylene molecules and clusters yields diverse CXHY + cations embedded in the droplets. The ionization primarily produces C2H2 +, C2H3 +, C2H4 +, and CH2 +, whereas larger carbocations are produced upon the reactions of the primary ions with ethylene molecules. The vibrational excitation of the cations leads to the release of bare cations and cations with a few helium atoms attached. The laser excitation spectra of the embedded cations show well resolved vibrational bands with a few wavenumber widths-an order of magnitude less than those previously obtained in solid matrices or molecular beams by tagging techniques. Comparison with the previous studies of free and tagged CH2 +, CH3 +, C2H2 +, C2H3 +, and C2H4 + cations shows that the helium matrix typically introduces a shift in the vibrational frequencies of less than about 20 cm-1, enabling direct comparisons with the results of quantum chemical calculations for structure determination. This work demonstrates a facile technique for the production and spectroscopic study of diverse carbocations, which act as important intermediates in gas and condensed phases.

Disintegration of diminutive liquid helium jets in vacuum.

The phenomenon of liquid jets disintegrating into droplets has attracted the attention of researchers for more than 200 years. An overwhelming fraction of these studies considered classical viscous liquid jets issuing into ambient atmospheric gases, such as air. Here, we present an optical shadowgraphy study of the disintegration of a cryogenic liquid helium jet produced with a 5 µm diameter nozzle into vacuum. The physical properties of liquid helium, such as its density, surface tension, and viscosity, change dramatically as the jet flows through the nozzle and evaporatively cools in vacuum, eventually reaching the superfluid state. In this study, we demonstrate that, at different stagnation pressures and temperatures, droplet formation may involve spraying, capillary breakup, jet branching, and/or flashing and cavitation. The average droplet sizes produced in this work range from 3.4 × 1012 to 6.5 × 1012 helium atoms or 6.7-8.3 µm in diameter. This paper also reports on the distributions of sizes and shapes of the resulting droplets.

Tracking surface charge dynamics on single nanoparticles.

Surface charges play a fundamental role in physics and chemistry, in particular in shaping the catalytic properties of nanomaterials. However, tracking nanoscale surface charge dynamics remains challenging due to the involved length and time scales. Here, we demonstrate time-resolved access to the nanoscale charge dynamics on dielectric nanoparticles using reaction nanoscopy. We present a four-dimensional visualization of the spatiotemporal evolution of the charge density on individual SiO2 nanoparticles under strong-field irradiation with femtosecond-nanometer resolution. The initially localized surface charges exhibit a biexponential redistribution over time. Our findings reveal the influence of surface charges on surface molecular bonding through quantum dynamical simulations. We performed semi-classical simulations to uncover the roles of diffusion and charge loss in the surface charge redistribution process. Understanding nanoscale surface charge dynamics and its influence on chemical bonding on a single-nanoparticle level unlocks an increased ability to address global needs in renewable energy and advanced health care.

Aggregation of solutes in bosonic versus fermionic quantum fluids.

Quantum fluid droplets made of helium-3 (3He) or helium-4 (4He) isotopes have long been considered as ideal cryogenic nanolabs, enabling unique ultracold chemistry and spectroscopy applications. The droplets were believed to provide a homogeneous environment in which dopant atoms and molecules could move and react almost as in free space but at temperatures close to absolute zero. Here, we report ultrafast x-ray diffraction experiments on xenon-doped 3He and 4He nanodroplets, demonstrating that the unavoidable rotational excitation of isolated droplets leads to highly anisotropic and inhomogeneous interactions between the host matrix and enclosed dopants. Superfluid 4He droplets are laced with quantum vortices that trap the embedded particles, leading to the formation of filament-shaped clusters. In comparison, dopants in 3He droplets gather in diffuse, ring-shaped structures along the equator. The shapes of droplets carrying filaments or rings are direct evidence that rotational excitation is the root cause for the inhomogeneous dopant distributions.