IRIS experiments were performed with a 3D ion trap mass spectrometer (Bruker, AmaZon Speed ETD) modified for spectroscopy experiments (details on the modifications and synchronization between MS and laser instrument are described elsewhere ) and the FELIX infrared free electron laser (see Ref. ). FELIX was set to produce radiation in the ~600-1900 cm -1 range in the form of ~10 µs macropulses at a 10 Hz repetition rate. The bandwidth was ~0.4% of the centre frequency and the (wavelength-dependent) macropulse energy was 20-150 mJ. Mass-isolated ions were irradiated with a single FELIX macropulse. Photon absorption by the trapped mass-selected ions occurs when the laser frequency is on resonance with one of the vibrational transitions of the ion. Multiple-photon absorption leads to an increase in internal energy and eventually dissociation, which is detected by recording the MS spectrum after irradiation. Thus, the fraction of dissociated ions is a proxy for IR absorption. Recording a series of photofragmentation mass spectra as a function of the laser frequency and plotting the fragmentation yield [IRIS intensity = ln(ΣI(precursor + fragment ions)/I(precursor ion))] produces an IR spectrum. Each IR point was recorded as an average of six MS spectra and the IRIS intensity was linearly corrected for frequency-dependent macropulse energy variations . The IR frequencies were calibrated using a grating spectrometer.