Infrared Action Spectroscopy of Analytes

Sample solutions were directly infused
in a 3D quadrupole ion trap (QIT) mass spectrometer (Bruker, Amazon
Speed ETD, Bremen, Germany) coupled to the beamline of the free-electron
laser for Infrared eXperiments (FELIX),^{52 (link)} as explained in the Supporting Information. During an IRMPD “action” spectroscopy experiment,
the absorption of multiple resonant IR photons leads to an increase
in the internal energy of the analyte, causing its unimolecular dissociation
and, consequently, the appearance of fragment ions in the mass spectrum.
To prevent excessive depletion of the parent ions and to minimize
the formation of fragment ions below the low mass cut of the QIT,
the IR spectra were recorded at several levels of laser pulse energy
attenuation.^{53 (link)}