Precision Laser Spectroscopy in the Extreme Ultraviolet

Techniques for the production of short wavelengths in the extreme ultraviolet wavelength range aredescribed, spanning low-order harmonic generation in the perturbative regime for narrow bandwidth XUV productionto the plateau-regime with the three-step collisional model for the generation of high harmonics with broadband ultrafastpulses. In addition, the intermediate regime is discussed for pulses of 300 ps duration, as well as the regime of harmonicconversion based on continuous wave lasers aiming at the production of coherent Lyman-α radiation. Applications of XUVsources in the frequency domain typically rely on the production of XUV-light from narrowband Fourier-transform-limitedpulses for recording high-resolution spectra. Alternatively, harmonically upconverted pulses of ultrashort duration (withinter-pulse coherence) can be used for spectroscopic studies in the time and frequency domain (such as direct frequencycomb spectroscopy techniques). Examples of both approaches for atomic and molecular spectroscopy are discussed at somelength. For precision metrology on atomic systems, a focus is put on the Lamb shift determination in the He ground state. Inthe realm of molecular spectroscopy, predissociation phenomena in the nitrogen molecule are highlighted, with relevancefor dissociation processes occurring in the upper layers of the Earth atmosphere. Similar predissociation phenomena incarbon monoxide are also treated in some detail, in this case with relevance for the chemistry in interstellar clouds. PrecisionXUV spectroscopy of molecular hydrogen is discussed as well, as it relates to the possibility of detecting a variation ofthe proton–electron mass ratio on a cosmological time scale. In the last section, applications of direct frequency combspectroscopic techniques at short wavelengths are explained and prospects are given for using these techniques at extremeultraviolet wavelengths.