Vacuum-ultraviolet frequency-modulation spectroscopy

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing (ν_VUV=2 ν_UV +ν₂ ) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies ν_UV and ν₂. Sidebands generated in the output of the second laser (ν₂) using an electro-optical modulator operating at the frequency ν_mod are directly transferred to the VUV and used to record FM spectra. Demodulation is demonstrated both at ν_mod and 2 ν_mod . The main advantages of the method compared to VUV absorption spectroscopy are its background-free nature, the fact is that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages, we present VUV FM spectra of Ar, Kr, and N₂ in selected regions between 105000 cm^-1 and 122000 cm^-1.