We demonstrate a technique for IR action spectroscopy that enables measuring IR spectra in a background-free fashion for low-temperature neutral gas-phase molecules of arbitrary structure. The method is exemplified experimentally for $N$-methylacetamide molecules in the mid-IR spectral range of $1000–1800{\mathrm{cm}}^{-1}$, utilizing the free electron laser FELIX. The technique involves the resonant absorption of multiple mid-IR photons, which induces molecular dissociation. The dissociation products are probed with 10.49 eV vacuum ultraviolet photons and analyzed with a mass spectrometer. We also demonstrate the capability of this method to record, with unprecedented ease, mid-IR spectra for the molecular associates, such as clusters and oligomers, present in a molecular beam. In this way the mass-selected spectra of low-temperature gas-phase dimers and trimers of $N$-methylacetamide are measured in the full amide I–III range.

• Received 11 May 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.118101