A molecular hydrogen absorber at a lookback time of 12.4 billion years, corresponding to 10$%$ of the age of the universe today, is analyzed to put a constraint on a varying proton--electron mass ratio, $mu$. A high resolution spectrum of the J1443$+
$2724 quasar, which was observed with the Very Large Telescope, is used to create an accurate model of 89 Lyman and Werner band transitions whose relative frequencies are sensitive to $mu$, yielding a limit on the relative deviation from the current laboratory value of $Deltamu/mu=(-9.5pm5.4_{textrm{stat}} pm 5.3_{textrm{sys}})times 10^{-6}$.
We report Karl G. Jansky Very Large Array (VLA) absorption spectroscopy in four methanol (CH$_3$OH) lines in the $z = 0.88582$ gravitational lens towards PKS1830-211. Three of the four lines have very different sensitivity coefficients $K_mu$ to chan
ges in the proton-electron mass ratio $mu$; a comparison between the line redshifts thus allows us to test for temporal evolution in $mu$. We obtain a stringent statistical constraint on changes in $mu$ by comparing the redshifted 12.179 GHz and 60.531 GHz lines, $[Delta mu/mu] leq 1.1 times 10^{-7}$ ($2sigma$) over $0 < z leq 0.88582$, a factor of $approx 2.5$ more sensitive than the best earlier results. However, the higher signal-to-noise ratio (by a factor of $approx 2$) of the VLA spectrum in the 12.179 GHz transition also indicates that this line has a different shape from that of the other three CH$_3$OH lines (at $> 4sigma$ significance). The sensitivity of the above result, and that of all earlier CH$_3$OH studies, is thus likely to be limited by unknown systematic errors, probably arising due to the frequency-dependent structure of PKS1830-211. A robust result is obtained by combining the three lines at similar frequencies, 48.372, 48.377 and 60.531 GHz, whose line profiles are found to be in good agreement. This yields the $2sigma$ constraint $[Delta mu/mu] lesssim 4 times 10^{-7}$, the most stringent current constraint on changes in $mu$. We thus find no evidence for changes in the proton-electron mass ratio over a lookback time of $approx 7.5$ Gyrs.
Using a Ramsey-type setup, the lambda-doublet transition in the $J=1,, Omega=1$ level of the $a^3Pi$ state of CO was measured to be 394 064 870(10) Hz. In our molecular beam apparatus, a beam of metastable CO is prepared in a single quantum level by
expanding CO into vacuum and exciting the molecules using a narrow-band UV laser system. After passing two microwave zones that are separated by 50 cm, the molecules are state-selectively deflected and detected 1 meter downstream on a position sensitive detector. In order to keep the molecules in a single $m_J^B$ level, a magnetic bias field is applied. We find the field-free transition frequency by taking the average of the $m_J^B = +1 rightarrow m_J^B = +1$ and $m_J^B = -1 rightarrow m_J^B = -1$ transitions, which have an almost equal but opposite Zeeman shift. The accuracy of this proof-of-principle experiment is a factor of 100 more accurate than the previous best value obtained for this transition.
Novel VUV sources and techniques for VUV spectroscopy are reviewed. Laser-based VUV sources have been developed via non-linear upconversion of laser pulses in the nanosecond (ns), the picosecond (ps), and femtosecond (fs) domain, and are applied in h
igh-resolution gas phase spectroscopic studies. While the ns and ps pulsed laser sources, at Fourier-transform limited bandwidths, are used in wavelength scanning spectroscopy, the fs laser source is used in a two-pulse time delayed mode. In addition a Fourier-transform spectrometer for high resolution gas-phase spectroscopic studies in the VUV is described, exhibiting the multiplex advantage to measure many resonances simultaneously.
Direct frequency comb spectroscopy of trapped ions is demonstated for the first time. It is shown that the 4s^2S_(1/2)-4p^2P_(3/2) transition in calcium ions can be excited directly with a frequency comb laser that is upconverted to 393 nm. Detection
of the transition is performed using a shelving scheme to suppress background signal from non-resonant comb modes. The measured transition frequency of f=761 905 012.7(0.5) MHz presents an improvement in accuracy of more than two orders of magnitude.
