S. Hannemann

Frequency metrology on the EF (1)Sigma(+)(g)<- X (1)Sigma(+)(g)(0,0) transition in H2, HD, and D2

We present a frequency metrology study on the lowest rotational levels of the hydrogen EF Σg+1 ←X Σg+1 (0,0) 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 (CW) 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 Q (0) to Q (2) lines in H2 and D2, and the Q (0) and Q (1) lines in HD are determined with an absolute accuracy at the 10-9 level.

A narrow-band injection-seeded pulsed titanium : sapphire oscillator-amplifier system with on-line chirp analysis for high-resolution spectroscopy

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.

Deep-ultraviolet cavity ringdown spectroscopy

The sensitive optical detection technique of cavity ringdown spectroscopy is extended to the wavelength range 197-204 nm. A novel design narrowband Fourier-transform-limited laser is used, and the technique is applied to gas-phase extinction measurements in CO2, SF6, and O2. Further demonstration of the system capabilities is given in high-resolution recordings of the Schumann-Runge (0, 0), (1,0), and (2, 0) bands in O2.

Reducing the first-order Doppler shift in a Sagnac interferometer.

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/(4d) 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)p[1/2](0) transition in Kr at lambda=212 nm. The achieved precision of 6 x 10(-10) is limited by the characteristics of the laser system.

Production of narrowband tunable extreme-ultraviolet radiation by noncollinear resonance-enhanced four-wave mixing

Fourier-transform-limited extreme-ultraviolet (XUV) radiation (bandwidth approximately < 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 omega1 + omega2 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 H2 and N2.

Pressure broadening and fine-structure-dependent predissociation in oxygen B-3 Sigma(-)(u), v=0

Both laser-induced fluorescence and cavity ring-down spectral observations were made in the Schumann-Runge band system of oxygen, using a novel-type ultranarrow deep-UV pulsed laser source. From measurements on the very weak (0,0) band pressure broadening, pressure shift, and predissociation line-broadening parameters were determined for the B 3sigma(u)-, v = 0,F(i) fine-structure components for various rotational levels in O2. The information content from these studies was combined with that of entirely independent measurements probing the much stronger (0,10), (0,19), and (0,20) Schumann-Runge bands involving preparation of vibrationally excited O2 molecules via photolysis of ozone. The investigations result in a consistent set of predissociation widths for the B 3sigma(u)-, v = 0 state of oxygen.

On a variation of the proton-electron mass ratio

Recently indication for a possible variation of the proton-to-electron mass ratio μ=mp/me was found from a comparison between laboratory H2 spectroscopic data and the same lines in quasar spectra. This result will be put in perspective of other spectroscopic activities aiming at detection of variation of fundamental constants, on a cosmological as well as on a laboratory time scale. Furthermore the opportunities for obtaining improved laboratory wavelength positions of the relevant H2 absorption lines, as well as the prospects for obtaining a larger data set of H2 absorptions at high redshift will be presented. Also an experiment to detect Δμ on a laboratory time scale will be discussed.

Frequency metrology on the Mg 3s{sup 2} {sup 1}S{yields}3s4p {sup 1}P line for comparison with quasar data

We report a frequency metrology study on the

3s4p P-1 line for comparison with quasar data

\mathrm{Mg}\phantom{\rule{0.3em}{0ex}}3{s}^{2}\phantom{\rule{0.2em}{0ex}}^{1}S\ensuremath{\rightarrow}3s4p\phantom{\rule{0.2em}{0ex}}^{1}P

Isotopically resolved calibration of the 285-nm Mgi resonance line for comparison with quasar absorptions

transition near