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 demonstrate a technique to reduce first-order Doppler shifts in crossed atomic/molecular and laser beam setups by aligning two counterpropagating laser beams as part of a Sagnac interferometer. Interference fringes on the exit port of the interferometer reveal minute deviations from perfect antiparallelism. Residual Doppler shifts of this method scale with the ratio v / ͑4d͒ of the typical atomic/molecular velocity v and the laser beam diameter d. The method is implemented for precision frequency calibration studies at deep-UV wavelengths, both in one-and two-photon excitation schemes: the 6s 2 → 6s30p 3/2 J =1 line in Yb at 199 nm and the 4p 6 → 4p 5 p͓1/2͔ 0 transition in Kr at = 212 nm. The achieved precision of 6 ϫ 10 −10 is limited by the characteristics of the laser system.

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.

We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (∼ 1 liter), robust and lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX prototype has been developed and calibrated, showing excellent agreement with end-to-end performance simulations. Calibration tests show that the precision of the polarization measurements is at least 2 · 10 −4 . We performed multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with one of AERONET's sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring network.