PRECISION XUV LASER SPECTROSCOPY OF ATOMIC AND MOLECULAR 2-ELECTRON SYSTEMS

A tunable and narrowband extreme ultraviolet (XUV) laser source in the wavelength range of 100 - 50 nm has been applied to high resolution s pectroscopy on the helium atom as well as on the hydrogen molecule. We excited the 1s(2) S-1(0) -> 1s2p P-1(1) transition in He at 58.4 nm a nd measured the absolute transition frequency with a precision of bett er than 1:10(9), allowing for a determination of the Lamb shift in the 1s(2) S-1(0) ground state (41224(45) MHz) at the same level of accura cy as the theoretical value (41233 (35) MHz). In a two-step excitation process, involving a 91 nm XUV-photon and a visible photon, we popula ted vibrational and rotational states of the hydrogen molecule and its isotopomers in the outer part of a double-well potential. Long-lived states were observed in this potential at the large internuclear dista nce of 11 a.u. Complicated dynamical processes occur in the decay of t hese highly-excited states in H-2, D-2 and HD.