With powerful, pulsed laser systems the generation of narrowband and tunable coherent radiation at short wavelengths (in the extreme ultraviolet and soft-röntgen part of the electro-magnetic spectrum) via non-linear upconversion processes in dense gaseous media is investigated (generation of high harmonics). The thus produced energetic photons (energies in the range 10-15 eV) were used in recent years to investigate the Lamb shift in the helium atom, and highly-excited states of light molecules of astrophysical interest such as H2 and CO. Currently we have a project to investigate the dipole-allowed absorption spectrum of the N2 molecule and its isotopes against the background of the absorption of solar XUV in the highest layers (ionosphere) of the Earth's atmosphere. In addition the narrowband XUV-laser setup is used to accurately calibrate some transitions in H2 molecules and in various atoms, that are also observed in highly redshifted quasar absorptions. These data can be used to put constraints on possible variations of fundamental constants.
The XUV-Laser Project aims at developing a source of narrowband and tunable radiation in the wavelength domain of the extreme ultraviolet, i.e. shortward of 100 nm. Nonlinear upconversion of tunable radiation from dye lasers, and frequency doubled dye lasers, at power levels of 100 mJ/pulse in approx. 5 ns pulses is employed. Via third and fifth harmonic conversion the entire wavelength domain between 54 and 100 nm is presently covered. When using a commercial dye laser at the fundamental a bandwidth of 0.3 cm-1 in the XUV is obtained, thus beating the resolution of any synchrotron source. The output at short wavelengths is generated by third and fifth harmonic conversion process, starting with either red or blue dye lasers, pumped by either the 2nd or 3rd harmonic of the Nd-YAG laser (a GCR 5 QuantaRay system). In the figure the yield in terms of photons per pulse is shown as a function of wavelength.
Below some pictures of the experimental setup are displayed. A pulsed-dye-amplifier (PDA)-laser produces nearly Fourier-transform limited pulses (at 5 ns). After nonlinear upconversion to the fifth harmonic radiation XUV radiation of a bandwidth as narrow as 750 MHz (or 0.025 cm-1) can be generated at a wavelength as short at 58 nm.
Laser system used for the laboratory experiments. On the left: CW-ring dye laser delivering the tunable frequency at 1 MHz bandwidth; this frequency is accurately calibrated. On the right: The Pulsed-Dye amplifier for the generation of powerful laser pulses in the visible domain; subsequent harmonic conversion then yields tunable and narrowband extreme ultraviolet (XUV) radiation.
Vacuum setup; three differentially pumped chambers for XUV generation, production of a collimated H2 beam, and the interaction-detection chamber.