Adsorption of Sodium and Cesium on Aggregates of C60

We explore the formation of C60 sodium and C60 cesium complexes in superfluid helium nanodroplets. Anomalies in mass spectra of these doped droplets reveal anomalies in the stability of ions. (C60) m Cs+ n ions ( m ≤ 6) are particularly abundant if they contain n = 6m + 1 cesium atoms; (C60) m Cs2+ n dications ( m ≤ 3 or 5) are abundant if n = 6m + 2. These findings are consistent with the notion that alkali metal atoms (A) transfer their valence electrons into the three-fold degenerate lowest unoccupied orbital of C60, resulting in particularly stable C60A6 building blocks. However, (C60) 4CsCs2+ n dications display an entirely different pattern; instead of an expected anomaly at n = 6 × 4 + 2 = 26 we observe a strong odd-even alternation starting at n = 6. Also surprising is the effect of adding one H2O or CO2 molecule to (C60) m Cs n mono- or dications; anomalies shift by two units as if the impurity were acting as an acceptor for two valence electrons from the alkali metal atoms

Doubly Charged CO2 Clusters Formed by Ionization of Doped Helium Nanodroplets

Helium nanodroplets are doped with carbon dioxide and ionized by electrons. Doubly charged cluster ions are, for the first time, identified based on their characteristic patterns of isotopologues. Thanks to the high mass resolution, large dynamic range, and a novel method to eliminate contributions from singly charged ions from the mass spectra, we are able to observe doubly charged cluster ions that are smaller than the ones reported in the past. The likely mechanism by which doubly charged ions are formed in doped helium droplets is discussed

Fission of Multiply Charged Cesium and Potassium Clusters in Helium Droplets - Approaching the Rayleigh Limit

Electron ionization of helium droplets doped with sodium, potassium or cesium results in doubly and, for cesium, triply charged cluster ions. The smallest observable doubly charged clusters are Na92+, K112+, and Cs92+; they are a factor two to three smaller than reported previously. The size of sodium and potassium dications approaches the Rayleigh limit nRay for which the fission barrier is calculated to vanish, i.e. their fissilities are close to 1. Cesium dications are even smaller than nRay, implying that their fissilities have been significantly overestimated. Triply charged cesium clusters as small as Cs193+ are observed; they are a factor 2.6 smaller than previously reported. Mechanisms that may be responsible for enhanced formation of clusters with high fissilities are discussed

Ordered Phases of Ethylene Adsorbed on Charged

In spite of extensive investigations of ethylene adsorbed on graphite, bundles of nanotubes, and crystals of fullerenes, little is known about the existence of commensurate phases; they have escaped detection in almost all previous work. Here we present a combined experimental and theoretical study of ethylene adsorbed on free C60 and its aggregates. The ion yield of measured by mass spectrometry reveals a propensity to form a structurally ordered phase on monomers, dimers and trimers of C60 in which all sterically accessible hollow sites over carbon rings are occupied. Presumably the enhancement of the corrugation by the curvature of the fullerene surface favors this phase which is akin to a hypothetical 1 × 1 phase on graphite. Experimental data also reveal the number of molecules in groove sites of the C60 dimer through tetramer. The identity of the sites, adsorption energies and orientations of the adsorbed molecules are determined by molecular dynamics calculations based on quantum chemical potentials, as well as density functional theory. The decrease in orientational order with increasing temperature is also explored in the simulations whereas in the experiment it is impossible to vary the temperature

Communication: Dopant-induced solvation of alkalis in liquid helium nanodroplets

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C60, is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C60. Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C60 is added. The sole exception is atomic Cs, which remains at the surface

Cs+^{+} Solvated in Hydrogen - Evidence for Several Distinct Solvation Shells

Helium nanodroplets are doped with cesium and molecular hydrogen and subsequently ionized by electrons. Mass spectra reveal H$_x$Cs$^{+}$ ions that contain as many as 130 hydrogen atoms. Two features in the spectra are striking: First, the abundance of ions with an odd number of hydrogen atoms is very low; the abundance of HCs$^+$ is only 1 % that of H$_2$Cs$^+$. The dominance of even-numbered species is in stark contrast to previous studies of pure or doped hydrogen cluster ions. Second, the abundance of (H$_2$)$_n$Cs$^+$ features anomalies at n = 8, 12, 32, 44, and 52. Guided by previous work on ions solvated in hydrogen and helium we assign the anomalies at n = 12, 32, 44 to the formation of three concentric, solid-like solvation shells of icosahedral symmetry around Cs$^+$. Preliminary density functional theory calculations for n $\le$ 14 are reported as well.Comment: 14 pages, 5 figure

Vasa previa in singleton pregnancies: Diagnosis and clinical management based on an international expert consensus

There are limited data to guide the diagnosis and management of vasa previa. Currently, what is known is largely based on case reports or series and cohort studies. (s): To systematically collect and classify expert opinions and achieve consensus on the diagnosis and clinical management of vasa previa using focus group discussions (FGD) and a Delphi technique. A four-round FGD and a three-round Delphi survey of an international panel of experts on vasa previa were conducted. Experts were selected based on their publication record on vasa previa. First, we convened an FGD panel of 20 experts and agreed on which issues were unresolved in the diagnosis and management of vasa previa. A three-round anonymous electronic survey was then sent to the full expert panel. Survey questions were presented on the diagnosis and management of vasa previa that the experts were asked to rate on a 5-point Likert scale (from strongly disagree = 1 to strongly agree = 5). Consensus was defined as a median score of 5. Following responses to each round, any statements that had median scores of 3 or less were deemed to have had no consensus and excluded. Statements with a median score of 4 were revised and re-presented to the experts in the next round. Consensus and non-consensus statements were then aggregated. Sixty-eight international experts were invited to participate in the study, of which 57 participated. Experts were from 13 countries on five continents and have contributed to over 80% of published cohort studies on vasa previa, as well as national and international society guidelines. Completion rates were 84%, 93%, 91% for the first, second, and third rounds, respectively, and 71% completed all three rounds. The panel reached a consensus on 26 statements regarding the diagnosis and key points of management of vasa previa, including: 1) While there is no agreement on a distance between the fetal vessels and the cervical internal os to define vasa previa, the definition should not be limited to a 2 cm distance; 2) All pregnancies should be screened for vasa previa with routine examination for placental cord insertion and a color Doppler sweep of the region over the cervix at the second-trimester anatomy scan; 3) When a low-lying placenta or placenta previa is found in the second trimester, a transvaginal ultrasound with Doppler should be performed at around 32 weeks to rule out vasa previa; 4) Outpatient management of asymptomatic patients without risk factors for preterm birth is reasonable; 5)Asymptomatic patients with vasa previa should be delivered by scheduled cesarean between 35- and 37-weeks of gestation; and 6) There was no agreement on routine hospitalization, avoidance of intercourse, or use of 3-dimensional ultrasound for diagnosis of vasa previa. Through FGD and a Delphi process, an international expert panel reached consensus on the definition, screening, clinical management, and timing of delivery in vasa previa, which could inform the development of new clinical guidelines. [Abstract copyright: Copyright © 2024. Published by Elsevier Inc.