IR spectroscopy with FELIX .1. Phosphorescence detection of IR transitions

The potential of the free-electron laser FELIX as a tool for IR spectroscopy has been studied. Tuning the laser by varying the undulator gap has enabled the recording of spectra between 500 and 2000 cm(-1) in a single scan. It is demonstrated that IR absorptions can be detected as FELIX-induced transient changes in the intensity of the phosphorescence from the lowest triplet state of a probe molecule. Using pyrimidine as a probe in polyethylene, n-hexane and dodecane, IR transitions in the electronic ground state and the lowest triplet state of pyrimidine have been observed. However, most of the intensity in the spectra derives from the host matrix. Detailed IR spectra of n-hexane and dodecane have been obtained at liquid-helium temperature. These show a large number of previously unobserved combination bands.