Vacuum-ultraviolet frequency-modulation spectroscopy

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing ($\nu_{\mathrm{VUV}}=2\nu_{\mathrm{UV}}+\nu_2$) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies $\nu_{\mathrm{UV}}$ and $\nu_2$. Sidebands generated in the output of the second laser ($\nu_2$) using an electro-optical modulator operating at the frequency $\nu_{\mathrm{mod}}$ are directly transfered to the VUV and used to record FM spectra. Demodulation is demonstrated both at $\nu_{\mathrm{mod}}$ and $2\nu_{\mathrm{mod}}$. The main advantages of the method are that its sensitivity is not reduced by pulse-to-pulse fluctuations of the VUV laser intensity, compared to VUV absorption spectroscopy is its background-free nature, the fact that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages we present VUV FM spectra of Ar, Kr, and N$_2$ in selected regions between 105000cm$^{-1}$ and 122000cm$^{-1}$.Comment: 23 pages, 10 figure

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

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 c

Indication of a cosmological variation of the proton-electron mass ratio based on laboratory measurement and reanalysis of H-2 spectra

Based on highly accurate laboratory measurements of Lyman bands of H-2 and an updated representation of the structure of the ground X (1)Sigma(+)(g) and excited B (1)Sigma(+)(u) and C (1)Pi(u) states, a new set of sensitivity coefficients K-i is derived for all lines in the H-2 spectrum, representing the dependence of their transition wavelengths on a possible variation of the proton-electron mass ratio mu=m(p)/m(e). Included are local perturbation effects between B and C levels and adiabatic corrections. The new wavelengths and K-i factors are used to compare with a recent set of highly accurate H-2 spectral lines observed in the Q 0347-383 and Q 0405-443 quasars, yielding a fractional change in the mass ratio of Delta mu/mu=(2.4 +/- 0.6)x10(-5) for a weighted fit and Delta mu/mu=(2.0 +/- 0.6)x10(-5) for an unweighted fit. This result indicates, at a 3.5 sigma confidence level, that mu could have decreased in the past 12 Gyr