Infrared Spectroscopy of CH Cations in Helium Nanodroplets.

The methanium CH is a prototypical fluxional ion whose infrared spectra remain unassigned. Here we report on the infrared spectra of CH cations and its deuterated isotopomer, CHD, in helium droplets at a low temperature of 0.38 K.

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.

Phase Separation in Cold Para-H_{2} D_{2} Clusters.

Low temperature phase separation in mixtures of ^{3}He and ^{4}He isotopes is a unique property of quantum fluids. Hydrogen has long been considered as another potential quantum liquid and has been predicted to be superfluid at T≤1  K, well below freezing temperature of ≈14  K. Phase separation has also been predicted in mixtures of para-H_{2} and D_{2} at temperatures ≤3  K.

Infrared spectroscopy of cations in helium nanodroplets.

Here, we describe our pulsed helium droplet apparatus for spectroscopy of molecular ions. Our approach involves the doping of the droplets of about 10 nm in diameter with precursor molecules, such as ethylene, followed by electron impact ionization. Droplets containing ions are irradiated by the pulsed infrared laser beam.

Isolation and Infrared Spectroscopic Characterization of Hemibonded Water Dimer Cation in Superfluid Helium Nanodroplets.

The structure of the minimum unit of the radical cationic water clusters, the (HO) dimer, has attracted much attention because of its importance for the radiation chemistry of water. Previous spectroscopic studies indicated that the dimers have a proton-transferred structure (HO·OH), though the alternate metastable hemibonded structure (HO·OH) was also predicted based on theoretical calculations. Here, we produce (HO) dimers in superfluid helium nanodroplets and study their infrared spectra in the range of OH stretching vibrations.

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.

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.

Anisotropic Surface Broadening and Core Depletion during the Evolution of a Strong-Field Induced Nanoplasma.

Strong-field ionization of nanoscale clusters provides excellent opportunities to study the complex correlated electronic and nuclear dynamics of near-solid density plasmas. Yet, monitoring ultrafast, nanoscopic dynamics in real-time is challenging, which often complicates a direct comparison between theory and experiment. Here, near-infrared laser-induced plasma dynamics in ∼600  nm diameter helium droplets are studied by femtosecond time-resolved x-ray coherent diffractive imaging.

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.

Sizes of pure and doped helium droplets from single shot x-ray imaging.

Advancements in x-ray free-electron lasers on producing ultrashort, ultrabright, and coherent x-ray pulses enable single-shot imaging of fragile nanostructures, such as superfluid helium droplets. This imaging technique gives unique access to the sizes and shapes of individual droplets. In the past, such droplet characteristics have only been indirectly inferred by ensemble averaging techniques.

Aggregation of solutes in bosonic versus fermionic quantum fluids.

Quantum fluid droplets made of helium-3 (He) or helium-4 (He) 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 He and He nanodroplets, demonstrating that the unavoidable rotational excitation of isolated droplets leads to highly anisotropic and inhomogeneous interactions between the host matrix and enclosed dopants.

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 CH cations embedded in the droplets. The ionization primarily produces CH , CH , CH , and CH , 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.

Rotation of CH Cations in Helium Droplets.

Observation of the free rotation of molecules in helium droplets enabled microscopic study of interaction of quantum rotors with a superfluid environment at = 0.4 K. This work extends studies of rotation in helium to molecular cations, such as methenium, CH.

Doxorubicin delivery systems based on doped CaCO cores and polyanion drug conjugates.

In order to prolong the release and reduce the toxicity of anticancer drug - doxorubicin (DOX), delivery systems (DS) using different polyanions have been developed. Structural (size, morphological stability) and functional (encapsulation efficiency, DOX release) characteristics of three types of DS are compared: CaCO porous vaterites doped with polyanions by co-precipitation and coating techniques, and DOX-polyanion conjugates. Using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), it was shown that the doping enhances the morphological stability of CaCO-based DS during the DOC loading.

Infrared Spectroscopy of Water and Zundel Cations in Helium Nanodroplets.

Here, we show that electron impact ionization of helium (He) droplets doped with water molecules and clusters yield water and Zundel cations embedded in the droplets consisting of a few thousand helium atoms. Infrared spectra in the OH-stretching range were obtained using the release of the cations from the droplets upon laser excitation. The spectra in He droplets appear to have about a factor of 10 narrower bands and similar matrix shifts as compared to those obtained via tagging with He and Ar atoms.

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.

Angular Momentum in Rotating Superfluid Droplets.

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated submicrometer superfluid ^{4}He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host.

Evaporation Dynamics from Ag-Doped He Droplets upon Laser Excitation.

Silver clusters were assembled in helium droplets of different sizes ranging from 10 to 10 atoms. The clusters were heated upon laser irradiation at 355 nm, and evaporation dynamics of He atoms were studied by quadrupole mass spectroscopy using signals from He, He, and He splitter ions. We found that for droplets containing less than 10 atoms the laser irradiation leads to evaporation of He atoms.

Triplet Excitons in Small Helium Clusters.

An electron traveling through liquid helium with sufficient kinetic energy can create a low-lying triplet exciton via inelastic scattering. Accompanying repulsion between the exciton and nearby atoms results in bubble formation. That is not all, however.

Imaging Quantum Vortices in Superfluid Helium Droplets.

Free superfluid helium droplets constitute a versatile medium for a diverse range of experiments in physics and chemistry that extend from studies of the fundamental laws of superfluid motion to the synthesis of novel nanomaterials. In particular, the emergence of quantum vortices in rotating helium droplets is one of the most dramatic hallmarks of superfluidity and gives detailed access to the wave function describing the quantum liquid. This review provides an introduction to quantum vorticity in helium droplets, followed by a historical account of experiments on vortex visualization in bulk superfluid helium and a more detailed discussion of recent advances in the study of the rotational motion of isolated, nano- to micrometer-scale superfluid helium droplets.

Two-level delivery systems based on CaCO cores for oral administration of therapeutic peptides.

Two-level systems for oral delivery of therapeutic peptides were developed; the carriers consist of CaCO cores included into alginate granules. Such systems were first used for the delivery of low molecular weight drugs. It was shown that efficiency of encapsulation of peptides depends on their pI value, hydrophobicity, characteristics of the compounds used for doping CaCO cores, their surface potential and the techniques employed for loading peptides into the first-level carriers.

Conversion of He(2 S) to He( aΣ ) in Liquid Helium.

The report of an anomalously intense He peak in electron impact mass spectra of large helium droplets created a stir 3 decades ago that continues to this day. When the electron kinetic energy exceeds 41 eV, an additional pathway opens that yields He predominantly in an electronically excited metastable state. A pair of He*(2 S) atoms has been implicated based on the isolated He* energy of 19.

Formation of He via electron impact of helium droplets.

Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of He ions that peaks at n = 2 and decreases monotonically toward larger n. In larger droplets (say 10 or more atoms), however, the He signal intensity is anomalously large. We have studied the mechanism for the formation of He ions in large helium droplets by varying the duration of the electron impact excitation pulse.

Two-level delivery systems for oral administration of peptides and proteins based on spore capsules of Lycopodium clavatum.

Two-level delivery systems (DSs) for oral administration of therapeutic proteins and peptides were developed. The first level consists of outer walls of Lycopodium clavatum spores (sporopollenin exine capsules, SECs) with included target objects; the alginate microgranules serve as the second (outer) level. Alginate (a pH-dependent natural polymer) protects peptides from gastric acidity and enzyme exposure and provides slow release of target objects in an alkaline intestinal medium.