Infrared spectroscopy of a small ion solvated by helium: OH stretching region of HeN−HOCO+

Abstract

Messenger spectroscopy is a well-established method for recording infrared (IR) spectra of molecular ions. It relies upon the tagging of weakly bound atoms or molecules, known as the 'messenger', to the ion of interest. The ideal tag species is helium since it has the weakest possible interaction with any molecular ion and is consequently the least likely to alter their structure and function. However the attachment of a helium tag is challenging because of the exceptionally cold conditions that are inherently required. In this work, electron ionization of doped liquid helium nanodroplets has been used to create cations tagged with a variable number (N) of helium atoms. Mass-selective ion detection has made it possible to record IR spectra as a function of N, thus revealing the effect on the structure and charge distribution within the ionic core as solvation becomes more extensive. We illustrate this capability for protonated carbon dioxide tagged with up to 14 helium atoms, HeN-HOCO + . The first atom preferentially binds to the proton and results in a substantial red-shift of 44 cm -1 for the OH stretching vibration, whilst the stepwise attachment of additional atoms up to N = 7 causes small and progressive blue-shifts, which are attributed to the gradual formation of a ring of helium around the carbon atom. The methodology described herein offers a new route to obtain IR spectra of He-tagged ions and provides an insight into ion-solvent interactions at the molecular level.

Infrared Spectroscopy of Water and Zundel Cations in Helium Nanodroplets

“…Laser-induced dissociation of the He n C 60 + was used to study the solvation of C 60 + with atomic resolution. 24 Ellis et al recently produced complexes of protonated acetic acid tagged with one or two He atoms 25 and protonated carbon dioxide with up to 14 He atoms 26 upon EI of the doped He droplets containing about 5000 He atoms. The infrared spectra were obtained via depletion infrared laser spectroscopy of the complexes.…”

Helium Nanodroplet Infrared Action Spectroscopy of the Proton-Bound Dimer of Hydrogen Sulfate and Formate: Examining Nuclear Quantum Effects

“…The analysis presented herein has not addressed the contribution of anion interactions with the surrounding helium to the observed line positions. Although such interactions are generally expected to cause minimal perturbation, ,− recent spectroscopic interrogations of helium-solvated cations have observed shifts of up to 50 cm –1 with respect to bare cations. , The situation for anionic species remains much more poorly known. However, theoretical investigations suggest that the solvation shell around anions remains liquid-like, , potentially decreasing spectral perturbation.…”

IR Spectroscopy of the Cesium Iodide–Water Complex

“…Experiments were performed using a helium nanodroplet instrument that has been described previously. − The nanodroplets were prepared by expanding helium gas at high pressure (32 bar) through the 5 μm diameter aperture of a cooled nozzle (16 K), resulting in a mean droplet size of ∼5000 helium atoms. The continuous beam of droplets then passed through a skimmer and through the center of a cylindrical oven containing a powdered sample of CsI.…”