Cs⁺ Solvated in Hydrogen—Evidence for Several Distinct Solvation Shells

Abstract

Helium nanodroplets are doped with cesium and molecular hydrogen and subsequently ionized by electrons. Mass spectra reveal HxCs + 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 H2Cs + . The dominance of even-numbered species is in stark contrast to previous studies of pure or doped hydrogen cluster ions. Second, the abundance of (H2)nCs + 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  14 are reported as well.

A combined experimental and theoretical investigation of Cs+ ions solvated in HeN clusters

“…In the inset of Fig. 2, the present ion mass abundance is compared for N ≤ 28 with that reported in the recent study by Kranabetter et al 30 of helium nanodroplets doped not only with Cs + but also with H 2 . The comparison between the curves shown in Fig.…”

“…Integrated counts from the HeNCs + complexes have been obtained as a function of the number of He atoms, N, as in previous investigations for similar systems. 10,30,31 The ion intensities, shown in Fig In the inset, present results (black squares) are compared for N ≤ 28 with previous measurements (red circles) obtained in an experiment performed with helium nanodroplets doped with cesium and molecular hydrogen. 30 relate both anomalies to a stable structure for He 12 Cs + , perhaps an icosahedral arrangement as suggested for He 12 Kr + , 27 and to possible indications of complete filling of the first shell, respectively.…”

“…The feature observed in Ref. 30 has its origin in a double peak at m/z ∼ 193 assigned both to He 15 Cs + and a fragment of a polydimethylsiloxane, S 3 C 3 H 9 O + 4 , used as a lubricant for turbo pumps. Once this additional contribution is removed after extended baking, no intensity anomaly for He 15 Cs + is found and the cluster abundance does not exhibit a suggested peak at N = 15 (see Fig.…”

“…In a recent experiment, 30 helium nanodroplets doped with cesium in the presence of molecular hydrogen were subsequently ionized with a crossed electron beam. The obtained mass spectra were analyzed in order to study mainly the abundance of (H 2 )NCs + .…”

“…The corresponding ion abundance curve as a function of the number of He atoms was shown for sizes smaller than N = 30, revealing just a suggested feature around N = 15. 30 In this work, we present results for a similar experiment performed only with cesium atoms in the pickup chamber in an attempt to obtain much clearer conditions for a proper inspection of the relative abundances of the different Cs + ions solvated with helium atoms. In addition to this and in order to search for the most stable configurations, we have carried out extensive theoretical calculations by means of both classical and quantum mechanical (QM) approaches.…”