High-order harmonic generation yielding tunable extreme-ultraviolet radiation of high spectral purity

Production of extreme-ultraviolet radiation by high-order harmonic generation is demonstrated to yield unprecedented spectral purity of lambda/Delta lambda=2.5 x 10(5) at wavelengths covering the entire range 40-100 nm. Tunability and sub-cm(-1) bandwidth of the harmonics are demonstrated in recordings of the He (1s4p) and Ar (3p(5)3d') resonance lines at 52.2 and 86.6 nm. Frequency shift of the harmonics due to chirp-induced phenomena are investigated and found to be small, resulting in a frequency accuracy of about 5 x 10(-7) in the domain of extreme-ultraviolet radiation.