A helium nanodroplet setup for mid and far-infrared spectroscopy using pulsed-free-electron lasers: vibrational spectra of propargyl alcohol

Our microscopic understanding of intermolecular interactions has increased considerably from the studies on isolated molecular clusters. The intermolecular modes for these clusters are located in far-infrared and terahertz regions and provide a sensitive test of the potential energy surface. Here we report a helium droplet setup (BoHeNDI@FELIX), which is coupled with free electron lasers, FEL-I and FEL-II, at the FELIX laboratory in Nijmegen. These lasers provide tunable radiation in the range of 66–3600 cm−1, covering the intermolecular modes, with average output power of 0.1–0.8 W and spectral bandwidth of 0.2–5% of the central frequency. Mass-selective infrared spectra of propargyl alcohol were recorded in the frequency range of 560–1600 cm−1, which demonstrate the performance of the new setup. The observed vibrational bands could be assigned to the gauche conformer of propargyl alcohol. The high power and micropulse structure of the FELIX radiation allow multiple excitations of the embedded molecules allowing spectral detection with a signal to noise of 10, even in the low-frequency region.