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 the 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 redshift of 44 cm-1 for the OH stretching vibration, while the stepwise attachment of additional atoms up to N = 7 causes small and progressive blueshifts, 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.
Highlights
The spectroscopic study of neutral molecules and clusters embedded in liquid helium nanodroplets is well established.[1,2,3] In a typical experiment, the dopant is added by flowing a beam of helium nanodroplets through a region of low pressure gas or vapor
We demonstrate that IR spectra of ions tagged with multiple helium atoms can be obtained using electron ionization of doped helium nanodroplets combined with vibrational predissociation spectroscopy
The most intense peaks correspond to cation fragments CH3CO+ (m/z 43) and (CH3COOH)H+ (m/z 61), which are primarily formed by dissociative ionization of metastable neutral acetic acid dimers, whilst the HOCO+ peak at m/z 45 principally derives from the neutral trans-monomer.[36]
Summary
The spectroscopic study of neutral molecules and clusters embedded in liquid helium nanodroplets is well established.[1,2,3] In a typical experiment, the dopant is added by flowing a beam of helium nanodroplets through a region of low pressure gas or vapor. Absorption spectra can be recorded using a form of action spectroscopy, commonly referred to as depletion spectroscopy, which utilizes mass-selective ion detection.[4] In contrast to the extensive work on neutral species, far fewer spectroscopic studies have been carried out on charged species formed from doped helium nanodroplets. Scheier and co-workers used electron ionization of doped helium nanodroplets to generate He-tagged C60+ cations, which were subsequently probed using electronic spectroscopy.[12,13,14] In the electron ionization process, the released energy strips helium from the cation but some molecular ions emerge with helium atoms still attached. Photodissociation spectra were measured mass-selectively, enabling the electronic spectrum of the ion to be recorded as a function of the number of helium atom tags. Since the ions must be cold in order to retain the weakly-bound tags, the result is a form of cold ion electronic predissociation spectroscopy
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