An official website of the United States government
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
This content will become publicly available on March 7, 2026
Formation of magnesium clusters in superfluid helium nanodroplets
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
- Award ID(s):
- 2404883
- PAR ID:
- 10592860
- Publisher / Repository:
- APS
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 162
- Issue:
- 9
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We studied luminescence accompanied an injection of the nitrogen-helium gas mixture after passing discharge into dense cold helium gas. Initially, when the experimental beaker was filled with superfluid helium and the nitrogen-helium gas was injected into bulk superfluid helium at T ≈ 1.5 K, the dominant band in the emission spectra was the α-group of nitrogen atoms. At these conditions, the nanoclusters of molecular nitrogen with high concentrations of stabilized nitrogen atoms were formed. When superfluid helium was evaporated from the beaker and the temperature at the bottom of the beaker was increased to T ≈ 20 K, we observed a drastic change in the luminescence spectra. The β-group of oxygen atoms was dominated in the luminescence spectra, and the emission of the α-group became small. At high temperatures (T ≈ 20 K), most of the nitrogen atoms recombine on the surface of N2 nanoclusters with the formation of excited nitrogen molecules. We explained the effect of the enhancement of β-group emission by effective energy transfer from excited nitrogen molecules to the stabilized impurity oxygen atom inside N2 nanoclusters.more » « less
-
Impurity-helium condensates (IHCs) formed by injecting the discharge products of gaseous mixtures of helium atoms and nitrogen molecules into bulk superfluid 4He at temperature 1.5 K, were studied by X-band electron spin resonance. IHCs consists of collections of N2 nanoclusters which form aerogel-like structure inside bulk HeII. It was found that N2 nanoclusters have a two shell structure, an outer shell which contains high concentration of stabilized N atoms and an interior shell with lower concentrations of N atoms. In this paper, we have studied the dependence of the shell structure of the N2 nanoclusters which compose the IHCs by varying the ratio of nitrogen to helium in the prepared gas mixture from 0.06 to 1%. The highest local concentration of N atoms in nanoclusters (1.2 ⋅ 1021 cm−3 ) was observed in the sample prepared from the gas mixture containing the lowest nitrogen admixture (0.06%). Additionally, the evolution of nanocluster structure was studied as the samples were drained of liquid helium (T ≤ 3.5 K) and warmed beyond the point of explosive recombination (3.5 K ≤ T ≤ 6.5 K).more » « less
-
We studied the electron spin resonance (ESR) spectra of nitrogen atoms stabilized in nitrogen-neon nanoclusters immersed in superfluid 4He. The nanoclusters were formed during the condensation of the products of the discharge in N2–Ne–He gas mixtures into bulk superfluid 4He at temperature 1.5 K. We studied nanoclusters formed by injection of gas mixtures with different ratios of heavy impurities in the helium N2/(Ne + N2 ) ranging from 2% to 90%. Analysis of the ESR spectra of nitrogen atoms stabilized in nitrogen-neon nanoclusters provides important information about the environment of the stabilized atoms and a shell structure of the nanoclusters was revealed. For all samples studied, preferential stabilization of N atoms on the surfaces of the nanoclusters was observed. Annealing of the collection of the nanoclusters in the temperature range 1.1–10 K resulted in substantial changes in the structure of the nanoclusters.more » « less
-
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.more » « less
