Anisotropic emission of neutral atoms: evidence of an anisotropic Rydberg sheath in nanoplasma

Intense laser-produced plasma is a complex amalgam of ions, electrons and atoms both in ground and excited states. Little is known about the spatial composition of the excited states that are an integral part of most gaseous or cluster plasma. In cluster-plasma, Rydberg excitations change the charge composition of the ions through charge transfer reactions and shape the angular distributions. Here, we demonstrate a non-invasive technique that reveals the anisotropic Rydberg excited cluster sheath by measuring anisotropy in fast neutral atoms. The sheath is stronger in the direction of light polarization and the enhanced charge transfer by the excited clusters results in larger neutralization

Generation of energetic negative ions from clusters using intense laser fields

Intense laser fields are known to induce strong ionization in atoms. In nanoclusters, ionization is only stronger, resulting in very high charge densities that lead to Coulomb explosion and emission of accelerated highly charged ions. In such a strongly ionized system, it is neither conceivable nor intuitive that energetic negative ions can originate. Here we demonstrate that in a dense cluster ensemble, where atomic species of positive electron affinity are used, it is indeed possible to generate negative ions with energy and ion yield approaching that of positive ions. It is shown that the process behind such a strong charge reduction is extraneous to the ionization dynamics of single clusters within the focal volume. Normal and well-known charge transfer reactions are insufficient to explain the observations. Our analysis reveals the formation of a manifold of Rydberg excited clusters around the focal volume that facilitate orders of magnitudes more efficient electron transfer. This phenomenon, which involves an active role of laser-heated electrons, comprehensively explains the formation of copious accelerated negative ions from the nano-cluster plasma

Call for emergency action to restore dietary diversity and protect global food systems in times of COVID-19 and beyond: Results from a cross-sectional study in 38 countries

Background: The COVID-19 pandemic has revealed the fragility of the global food system, sending shockwaves across countries\u27 societies and economy. This has presented formidable challenges to sustaining a healthy and resilient lifestyle. The objective of this study is to examine the food consumption patterns and assess diet diversity indicators, primarily focusing on the food consumption score (FCS), among households in 38 countries both before and during the first wave of the COVID-19 pandemic. Methods: A cross-sectional study with 37 207 participants (mean age: 36.70 ± 14.79, with 77 % women) was conducted in 38 countries through an online survey administered between April and June 2020. The study utilized a pre-tested food frequency questionnaire to explore food consumption patterns both before and during the COVID-19 periods. Additionally, the study computed Food Consumption Score (FCS) as a proxy indicator for assessing the dietary diversity of households. Findings: This quantification of global, regional and national dietary diversity across 38 countries showed an increment in the consumption of all food groups but a drop in the intake of vegetables and in the dietary diversity. The household\u27s food consumption scores indicating dietary diversity varied across regions. It decreased in the Middle East and North Africa (MENA) countries, including Lebanon (p \u3c 0.001) and increased in the Gulf Cooperation Council countries including Bahrain (p = 0.003), Egypt (p \u3c 0.001) and United Arab Emirates (p = 0.013). A decline in the household\u27s dietary diversity was observed in Australia (p \u3c 0.001), in South Africa including Uganda (p \u3c 0.001), in Europe including Belgium (p \u3c 0.001), Denmark (p = 0.002), Finland (p \u3c 0.001) and Netherland (p = 0.027) and in South America including Ecuador (p \u3c 0.001), Brazil (p \u3c 0.001), Mexico (p \u3c 0.0001) and Peru (p \u3c 0.001). Middle and older ages [OR = 1.2; 95 % CI = [1.125–1.426] [OR = 2.5; 95 % CI = [1.951–3.064], being a woman [OR = 1.2; 95 % CI = [1.117–1.367], having a high education (p \u3c 0.001), and showing amelioration in food-related behaviors [OR = 1.4; 95 % CI = [1.292–1.709] were all linked to having a higher dietary diversity. Conclusion: The minor to moderate changes in food consumption patterns observed across the 38 countries within relatively short time frames could become lasting, leading to a significant and prolonged reduction in dietary diversity, as demonstrated by our findings

Non-Maxwellian electron-energy distribution from cluster nanoplasmas

Studies on ionization and electron emission from cluster nanoplasmas in intense laser fields are largely limited to experiments on single isolated clusters. We measure electron emission from ionization of clusters at higher cluster ensemble densities and find that the electron-energy spectra is highly non-Maxwellian. A comprehensive model is developed to account for the change in electron-energy spectrum and the depletion of the low-energy electron yield. It is shown that elastic and inelastic collisions account for the observed non-Maxwellian spectral shape and the computed spectrum compares well with the measurements. Furthermore, it is deciphered that the electrons that stream out of the focal volume, collisionally ionize every atom in the clusters at the periphery (< 100 μm) of laser focus

A quasi-directional emission of MeV neutrals from a dense cluster nano plasma

Nanoclusters are strongly ionised at 1016 Wcm-2 to generate even MeV ions. In a dense cluster ensemble a near 100% charge reduction of the ions to form fast neutrals is demonstrated. Neutrals atom emission is quasi-directional and neutralisation is more effective along the laser polarization

Electronic excitation as a mode of heat dissipation in laser-driven cluster plasmas

Electrons streaming out of laser plasma are known for non-local heat transport and energy deposition by the ionization wave. At 100 eV electron temperature, since the electronic excitation cross section is comparable to that of ionization for Ar and CO₂, a non-local excitation wave akin to the ionization wave is envisaged where energy deposition in excitations forms a excited cluster sheath beyond the laser focus. Here, we show that nano-cluster systems have the right parameters to form such an exciton sheath and experimentally demonstrate this via charge transfer reactions

A Thomson parabola ion imaging spectrometer designed to probe relativistic intensity ionization dynamics of nanoclusters

Conventional techniques of probing ionization dynamics at relativistic intensities for extended target systems such as clusters are difficult both due to problems of achieving good charge resolution and signal integration over the focal volume. Simultaneous measurement of arrival time, necessary for these systems, has normally involved complicated methods. We designed and developed a Thomson parabola imaging spectrometer that overcomes these problems. Intensity sampling method evolved in this report is proved to be mandatory for probing ionization dynamics of clusters at relativistic intensities. We use this method to measure charge resolved kinetic energy spectra of argon nanoclusters at intensities of 4 × 1018 W cm−2

Anisotropic negative-ion emission from cluster nanoplasmas

Recent experiments have shown that the Enhanced Charge Transfer by Rydberg Excited Clusters (ECTREC) reduces the highly charged ions very efficiently to neutral atoms and negative ions with little loss of momentum. Neutral-atom emission is anisotropic with respect to the laser polarization and the anisotropy is larger than that of the ion emission from Coulomb explosion of isolated single clusters. In such a scenario, it is expected that the negative-ion emission (like neutrals) should be anisotropic and have larger propensity along the laser polarization than in the perpendicular direction. Further, it may be anticipated that negative-ion emission is more anisotropic than neutral-atom emission if ECTREC is taken in to account. We demonstrate that the negative-ion emission is anisotropic. Contrary to expectations, the negative-ion emission anisotropy is not more than that of the neutral-atom emission. We show that this can be rationalized if low-energy (about 10 eV) electron collisional detachment of the negative ions is taken into account. Electron collisional detachment depletes the negative-ion yield preferentially along the laser polarization direction and reduces the negative-ion emission anisotropy