Aggregation-Induced Dissociation of HCl(H ₂ O) ₄ Below 1 K: The Smallest Droplet of Acid

Abstract

Acid dissociation and the subsequent solvation of the charged fragments at ultracold temperatures in nanoenvironments, as distinct from ambient bulk water, are relevant to atmospheric and interstellar chemistry but remain poorly understood. Here we report the experimental observation of a nanoscopic aqueous droplet of acid formed within a superfluid helium cluster at 0.37 kelvin. High-resolution mass-selective infrared laser spectroscopy reveals that successive aggregation of the acid HCl with water molecules, HCl(H2O)n, readily results in the formation of hydronium at n = 4. Accompanying ab initio simulations show that undissociated clusters assemble by stepwise water molecule addition in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ringlike undissociated HCl(H2O)3 then spontaneously yields the compact dissociated H3O+(H2O)3Cl- ion pair. This aggregation mechanism bypasses deep local energy minima on the n = 4 potential energy surface and offers a general paradigm for reactivity at ultracold temperatures.

Cluster cross sections from pickup measurements: Are the established methods consistent?

“…1 Since then it has been developed into a very powerful tool in the cluster science: especially numerous experiments with large helium clusters use this technique [2][3][4] to study the spectroscopy of molecules [5][6][7][8][9][10] and their reactions [11][12][13][14][15] in the cold superfluid environment of the helium nanodroplets. Here we focus on the pickup to argon clusters which was also exploited in many studies, e.g., for cluster isolated chemical reactions on large Ar clusters in the group of Mestdagh 16,17 and others.…”

Communication: Infrared spectroscopy of salt-water complexes

“…Such an approach has recently been used to explore the HCl/H 2 O system by spectroscopic observation of HCl(H 2 O) n complexes. [7][8][9][10][11] With this method, it becomes possible to determine how the ion-pair evolves as the hydration number is changed. Indeed, with sufficient water molecules, the ions should separate into a SIP complex.…”

On subthreshold ionization of helium droplets, ejection of He+, and the role of anions

“…[2][3][4][5][6][7] Still, not only do the droplets provide new ways for synthesis but the products also provide new insight into properties of helium droplets. For example, the shape of silver aggregates grown in very large droplets reflects the presence of quantized vortices in superfluid droplets.…”