We report a frequency metrology study on the Mg 3s 2 1 S → 3s4p 1 P transition near 202.5 nm. For this purpose, the fourth harmonic of the output from an injection-seeded Ti: sapphire pulsed laser is employed in a Mg atomic beam experiment with laser-induced fluorescence detection. Absolute frequency calibration with a frequency comb laser is performed on the cw seeding radiation, while the chirp-induced frequency shift between the pulsed output and the seed light is monitored on line. The resulting transition frequency for the main isotope 24 Mg is determined at 49 346.756 809͑35͒ m −1 . This value is three orders of magnitude more precise than the best value in the literature. The line positions of the other isotopes 25 Mg and 26 Mg are also measured at comparable accuracy, giving rise to very exact values for the isotopic shifts. The achieved precision for the transition frequency at the 7 ϫ 10 −10 level makes this second resonance line of Mg I an additional candidate for inclusion in many-multiplet methods, aimed at detecting a possible temporal variation of the fine-structure constant ␣ from comparison with quasar spectra. The isotopic shifts obtained are also important to correct for possible systematic shifts due to evolution of isotopic abundances, which may mimic ␣-variation effects.

A narrow-band tunable injection-seeded pulsed titanium:sapphire laser system has been developed for application in high-resolution spectroscopic studies at the fundamental wavelengths in the near infrared as well as in the ultraviolet, deep ultraviolet, and extreme ultraviolet after upconversion. Special focus is on the quantitative assessment of the frequency characteristics of the oscillator-amplifier system on a pulse-to-pulse basis. Frequency offsets between continuous-wave seed light and the pulsed output are measured as well as linear chirps attributed mainly to mode pulling effects in the oscillator cavity. Operational conditions of the laser are found in which these offset and chirp effects are minimal. Absolute frequency calibration at the megahertz level of accuracy is demonstrated on various atomic and molecular resonance lines.

We present a frequency metrology study on the lowest rotational levels of the hydrogen EF 1 ⌺ g + ← X 1 ⌺ g + ͑0,0͒ two-photon transition near 202 nm. For this purpose, the fourth harmonic of an injection-seeded titanium:sapphire pulsed oscillator is employed in a Doppler-free REMPI-detection scheme on a molecular beam of hydrogen. A frequency comb laser is used to perform the absolute frequency calibration on the continuous-wave ͑CW͒ laser that injection-seeds the oscillator. Chirp-induced frequency differences between the output of the pulsed oscillator and the seeding light are monitored on-line, while possible systematic shifts related to the AC-Stark and Doppler effects are addressed in detail. The transition frequencies of the Q͑0͒ to Q͑2͒ lines in H 2 and D 2 , and the Q͑0͒ and Q͑1͒ lines in HD are determined with an absolute accuracy at the 10 −9 level.

Using the third harmonic of a cw titanium:sapphire laser, the hyperfine structure (HFS) and isotope shift (IS) of three deep-UV transitions of neutral Yb have been measured for the first time. By exploiting the angular distribution of fluorescence radiation, accurate and complete results are obtained for the HFS and IS of the 398.8 nm transition of Yb. From the measured data, normal and specific mass shift as well as field shift values for all transitions considered have been derived.

Fourier-transform-limited extreme-ultraviolet (XUV) radiation (bandwidth 300 MHz) tunable around 91 nm is produced by use of two-photon resonance-enhanced four-wave mixing on the Kr resonance at 94 093 cm −1 . Noncollinear phase matching ensures the generation of an XUV sum frequency 2 1 + 2 that can be filtered from auxiliary laser beams and harmonics by an adjustable slit. Application of the generated XUV light is demonstrated in spectroscopic investigations of highly excited states in H 2 and N 2 . © 2005 Optical Society of America OCIS codes: 190.4380, 140.7240, 300.6400. Nonlinear optical schemes for the production of narrowband and tunable extreme-ultraviolet (XUV) radiation, either through third-order harmonic conversion or resonant sum-and difference-frequency mixing, date back to the 1970s and 1980s. A description of phase matching and properties of the ͑3͒ nonlinear susceptibility tensor was given by Bjorklund. 2 performed pioneering studies of the resonance enhancement effect by two-photon transitions in noble gases, it was later found that the 4p 6 −5p͓1/2͔ 0 transition in Kr gas is most effective in enhancing the XUV yield.3,4 The replacement of grating-based pulsed lasers by Fourier-transformlimited lasers, generally in the form of pulsed-dye amplifiers (PDAs) opened the possibility of generating ultranarrowband XUV radiation 5,6 even at wavelengths as short as 58 nm. 7 In applications in which the generated XUV beam needs to be separated from the incident laser beams, as well as from the auxiliary harmonic and mixed frequencies, usually a grating is employed, with the drawback of intensity loss of an order of magnitude or more. Here we demonstrate that phase matching in a noncollinear beam configuration can combine the advantage of resonance enhancement with the production of a separated XUV beam, filtered geometrically by the insertion of an adjustable slit. The scheme is somewhat similar to that of BOXCARS, which is applied for geometric filtering of a generated beam in coherent antiStokes Raman spectroscopy. 8All the measurements are performed in a threechamber differentially pumped vacuum setup that has been described in connection with thirdharmonic generation 9 (THG) and fifth-harmonic generation.7 Here we produce tunable XUV light by mixing the output of two different lasers in a pulsed jet of Kr. One provides the resonant light 1 for the Kr 4p 6 −5p͓1/2͔ 0 two-photon transition at 94 093 cm −1 ; the other supplies the tunable component 2 in a resonance-enhanced four-wave-mixing scheme. For all the results presented, the sum frequency 2 1 + 2 is used, and the repetition rate is 10 Hz. To filter out undesired wavelengths, a simple geometric scheme is applied as depicted in Fig. 1. Aligning the resonant light 1 at 212 nm and the tunable output 2 of the second laser to overlap in the Kr jet under an angle of approximately 80 mrad allows phase-matching conditions for the sum frequency 2 1 + 2 , the difference frequency 2 1 − 2 , and the third harmonic 3 1 to be fulfilled at angles as dis...