The FELion cryogenic ion trap beam line at the FELIX free-electron laser laboratory: infrared signatures of primary alcohol cations

The combination of a 4 K 22-pole ion trap instrument, FELion, with the widely tunable free electron lasers at the FELIX Laboratory is described in detail. It allows for wide-range infrared vibrational spectroscopy of molecular ions. In this study, the apparatus is used for infrared vibrational predissociation (IR-PD) measurements of the simple alcohol cations of methanol and ethanol as well as their protonated forms. Spectra are taken by tagging the cold molecular ions with He atoms. The infrared spectrum of protonated methanol is recorded for the first time, and the wavelength coverage for all other species is substantially extended. The bands of all spectra are analysed by comparison to ab initio calculation results at different levels of theory. Vibrational bands of different isomers and conformers (rotamers) are discussed and identified in the experimental spectra. Besides the measurement of IR-PD spectra, the method of infrared multiple photon dissociation IR-MPD is applied for some cases. Spectral narrowing due to the cold environment is observed and rotational band contours are simulated. This will help in identifying more complex species using the IR-MPD method in future measurements. Overall the IR-PD spectra reveal more bands than are observed for the IR-MPD spectra. In particular, many new bands are observed in the fingerprint region. Depletion saturation of the finite number of trapped ions is observed for the IR-PD spectra of the ethanol cation and the presence of only one isomeric species is concluded. This special feature of ion trapping spectroscopy may be used in future studies for addressing specific isomers or cleaning the ion cloud from specific isomers or conformers. In addition, the results of this study can be used as a basis to obtain high-resolution infrared vibrational and THz rotational spectra of alcohol ions in order to detect them in space.