An official website of the United States government
Magnesium atoms in liquid helium have been hypothesized to form a metastable foam structure, in which a layer of helium atoms surrounds each magnesium atom, inhibiting their coalescence into a compact cluster. This conjecture is based on the weak interaction between the magnesium atoms themselves and with the helium atoms and was used to explain observations in femtosecond two-photon ionization experiments by different groups. However, this theory is incongruent with previous infrared spectroscopic observations, indicating the formation of tightly bound clusters when different atoms and molecules combine inside liquid helium. In this paper, we report the spectra (from 210 to 2210 nm) of magnesium-doped superfluid helium nanodroplets at different averaged droplet sizes and number of dopants. The measured spectra in this study are consistent with the formation of compact magnesium clusters rather than the metastable foam structure.
more »
« less
Experiments with Large Superfluid Helium Nanodroplets
This chapter aims to look at the properties of large helium nanodroplets from two different perspectives: a.) helium droplets as hosts for assembling and studying clusters at low temperatures; and b.) helium droplets as systems to be studied on their own. First, the thermodynamics and excitations in large droplets are presented, followed by a primer on the droplets’ rate of cooling in vacuum. The chapter then proceeds with the description on producing and characterising the droplets. This subject is followed by a discussion on the kinetics for different regimes of cluster aggregation, such as that for single- and multiple-centre aggregation. Then, experiments involving the spectroscopy of the foreign particles and the deposition of metallic clusters for electron microscopy studies are described. Finally, results from recent x-ray coherent diffractive imaging experiments with pure and doped helium nanodroplets are summarised.
more »
« less
- PAR ID:
- 10057780
- Date Published:
- Journal Name:
- RSC theoretical and computational chemistry series
- Volume:
- 11
- ISSN:
- 2041-3181
- Page Range / eLocation ID:
- 389-443
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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. Ultrafast X-ray and extreme ultraviolet scattering techniques enabled by X-ray free-electron lasers and high-order harmonic generation in particular have facilitated the in situ detection of droplet shapes and the imaging of vortex structures inside individual, isolated droplets. New applications of helium droplets ranging from studies of quantum phase separations to mechanisms of low-temperature aggregation are discussed.more » « less
-
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. Vibrational excitation of the embedded cations leads to the evaporation of the helium atoms in the droplets and the release of the free ions, which are detected by the quadrupole mass spectrometer. In this work, we upgraded the experimental setup by introducing an octupole RF collision cell downstream from the electron impact ionizer. The implementation of the RF ion guide increases the transmission efficiency of the ions. Filling the collision cell with additional He gas leads to a decrease in the droplet size, enhancing sensitivity to the laser excitation. We show that the spectroscopic signal depends linearly on the laser pulse energy, and the number of ions generated per laser pulse is about 100 times greater than in our previous experiments. These improvements facilitate faster and more reproducible measurements of the spectra, yielding a handy laboratory technique for the spectroscopic study of diverse molecular ions and ionic clusters at low temperature (0.4 K) in He droplets.more » « less
-
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.more » « less
-
Direct detection of polar structure formation in helium nanodroplets by beam deflection measurementsLong-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.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- The DOI is not currently available.
