Charge separation and isolation in strong water droplet impacts

Abstract

Charge separation in condensed matter after strong impacts is a general and intriguing phenomenon in nature, which is often identified and described but not necessarily well understood in terms of a quantitative mechanistic picture. Here we show that charge separation naturally occurs if water droplets/clusters or ice particles with embedded charge carriers, e.g., ions, encounter a high energy impact with subsequent dispersion - even if the involved kinetic energy is significantly below the molecular ionization energy. We find that for low charge carrier concentrations (c < 0.01 mol L(-1)) a simple statistical Poisson model describes the charge distribution in the resulting molecular "fragments" or aggregates. At higher concentrations Coulomb interactions between the charge carriers become relevant, which we describe by a Monte Carlo approach. Our models are compared to experimental data for strong (laser) impacts on liquid micro beams and discussed for the charge generation in cluster-impact mass spectrometry on cosmic dust detectors where particle kinetic energies are below the plasma threshold. Taken together, a simple and intuitive but quantitative microscopic model is obtained, which may contribute to the understanding of a larger range of phenomena related to charge generation and separation in nature.

Analogue spectra for impact ionization mass spectra of water ice grains obtained at different impact speeds in space

“…In the laser‐based analogue experiment, a liquid water beam and dissolved substances therein are primarily ionized by dispersion, a technique known as Laser Induced Liquid Beam Ion Desorption (LILBID) (Figure B). In this process, the energy transferred to the atoms and molecules by the laser is below their nominal ionization potentials, with the formation of charged species accomplished by mechanically breaking up the liquid matrix into charged fragments by laser excitation . A similar mechanism may dominate the abundant cation formation from water ice grains at impact speeds as low as 3 km/s observed by Cassini's CDA.…”

A Complex Catalytic Reaction Caught in the Act: Intermediates and Products Sampling Online by Liquid μ‐Beam Mass Spectrometry and Theoretical Modeling

“…The IR laser excites the OH-stretch vibration of water molecules and thus heats up and disperses the liquid m-water beam containing the sample from the reaction vessel. When the liquid beam absorbs the laser energy it is rapidlyh eated and dispersed into charged [26] nanodroplets.F rom the exploding microbeam, ions and protonated/deprotonated molecules and intermediates, as well as solvent clusters are desorbed and largely desolvated. Since the single-photon energy is below any ionization level and the laser absorption is selective to the water molecules, the dispersion and pseudo "ionization process" can be characterized as very soft.…”

“…It should be emphasized here that IR-FL-MALDI predominantly generates charged aggregates with al ow overall net chargec lose to unity (AE 1). [26,9] Highly charged ions are often stabilized and neutralized in adducts and larger complexes. Thisc an be seen best for Yb 3 + .T he triply chargedi on forms av ariety of clusters with (OH) À ,( OTf) À (OTf = trifluoromethanesulfonate), and solvent adducts with an et charge of + 1; no other charges tates have been detectedu nder the present conditions.…”

IR-MALDI ion mobility spectrometry

“…The curve consists of a linear range spanning two orders of magnitude, followed by a plateau starting at 5 μM. This curve shape was also qualitatively obtained in simulations by Wiederschein et al [25], where the ion yield of sodium ions produced by the dispersion of a NaCl solution was described with a Poisson model. In the simulation, a droplet volume of (5.6 ± 3.5) · 10 3 nm 3 was assumed.…”

“…Simulations have demonstrated the shockwave formation mechanism through the phase explosion of superheated water after excitation by an IR-laser pulse in the ns range [24]. The charge distribution is a result of the dispersion of the liquid and can be described by a Poisson statistical model [25].…”