IR spectroscopy on gas-phase molecules with a free electron laser

Mass selective gas-phase infrared spectroscopy using pulsed, high power InfraRed (IR) radiation produced by the Free Electron Laser (FEL) `Free Electron Laser for Infrared eXperiments' (FELIX) is demonstrated. Two complementary excitation schemes are used. In the first experiment, para amino benzoic acid (PABA) is infrared excited using FELIX and then ionized using a vacuum ultraviolet (VUV) F2 laser. In the second experiment, C60 is multiphoton excited by FELIX to energies high enough that the molecule can undergo autoionization. In both experiments, spectra are obtained by monitoring the ion of interest as a function of IR wavelength in the 100–2000 cm−1 range. It is demonstrated that these spectra contain the same information as direct IR absorption spectra. The sensitivity of these methods, however, is many orders of magnitude higher than that of conventional IR absorption spectroscopy. Details of the two different ionization mechanisms are discussed.