Efficient generation of below-threshold harmonics for high-fidelity multi-photon physics in the VUV spectral range.

We demonstrate the generation of microjoule level, sub-20-fs, Ti:Sa fifth-harmonic pulses utilizing a loose-focusing geometry in a long Ar gas cell. The VUV pulses centered at 161.8 nm reach pulse energies of 1.1 µJ per pulse, while the corresponding pulse duration is measured with a second-order, fringe-resolved autocorrelation scheme to be 18±1 fs. Nonresonant, two-photon ionization of Kr and three-photon ionization of Ne verify the fifth-harmonic pulse high-intensity content and indicate the feasibility of multi-photon VUV pump-VUV probe studies of ultrafast atomic and molecular dynamics.