Hendrick L. Bethlem

High-precision spectroscopy of simple atomic systems can be used to advance the theory of atomic energy levels but can also serve as a sensitive probe of nuclear charge radii. For this last purpose, we report an improved measurement of the 2^{3}S_{1}→2^{1}S_{0} transition frequency in ^{4}He with 48 Hz uncertainty (0.25 ppt), using a Bose-Einstein condensed sample confined in a magic-wavelength optical dipole trap. A systematic Doppler shift from condensate motion is suppressed by time-resolved ion detection, and the transition frequency is calibrated via a White Rabbit link to a remote active hydrogen maser clock. Combined with previous ^{3}He measurements and improved theory, we obtain the most precise determination to date of the charge-radius difference between the helion and alpha particle (r_{h}^{2}-r_{α}^{2}) of 1.0676(10) fm^{2}. This is consistent with other recent determinations and confirms that the current discrepancy between QED theory and experimentally observed ionization energies of excited states in helium is not apparent in the isotope shift.

Absorption spectroscopy toward high-redshift quasars provides strong constraints on the putative variation of fundamental constants of physics on cosmological timescales. The submillimeter ground-state transitions of methylidyne (CH) and water (H ₂ O), both molecules widespread and coeval in the interstellar medium, provide a sensitive test for variations in α , the fine structure constant, and μ , the proton-to-electron mass ratio, taking advantage of the unmatched spectral resolution and frequency reliability of radio techniques. We used ALMA simultaneous observations of the two species to constrain any velocity offset between their absorption profiles toward the radio-bright lensed quasars PKS 1830−211 ( z _abs = 0.89) and B 0218+357 ( z _abs = 0.68). Our observational setup minimizes instrumental errors and known sources of systematics, such as time variability of the absorption profile and frequency-dependent morphology of the background quasar. The excellent correlation between CH and H ₂ O opacities, the large number of individual narrow velocity components, and the number of independent spectra obtained due to the intrinsic time variability of the absorption profiles ensure that even the chemical segregation bias is minimized. We obtained bulk velocity shifts of δv = −0.048 ± 0.028 km s ⁻¹ and −0.13 ± 0.14 km s ⁻¹ (1 σ confidence level) between CH and H ₂ O in the direction of PKS 1830−211(NE) and B 0218+357(SW), respectively. These values convert into the 3 σ upper limits |Δ α / α |< 0.55 ppm and 1.5 ppm, respectively, taking into account the independent upper limits on |Δ μ / μ | previously obtained for these systems. These constraints on |Δ α / α |, at look-back times of about half the present age of the Universe, are two to four times deeper than previous constraints on any other single high- z system.

In this paper we report on the production and characterization of a cold beam of BaOH molecules using a cryogenic buffer-gas beam source. BaOH is a highly suitable molecule for studies of the violation of fundamental symmetries, such as the search for the electron's electric dipole moment. BaOH molecules are synthesised inside the cold source through laser ablation of a barium metal target while water vapor is seeded into the neon buffer gas. The BaOH flux is significantly enhanced (∼11 times) when laser-exciting the barium atoms inside the buffer-gas cell on the ¹ S ₀ - ³ P ₁ transition. A similar enhancement has been reported for other alkalineearth(-like) monohydroxides. For typical source conditions, the molecular beam has an average velocity of ≈ 180 m/s and an intensity of ∼ 10 ⁹ molecules s ^-1 in N = 1, which is comparable to that of cryogenic BaF beams.

Accurate spectroscopic measurements of calculable systems provide a powerful method for testing the Standard Model and extracting fundamental constants. Recently, spectroscopic measurements of finite nuclear size effects in normal and muonic hydrogen resulted in unexpectedly large adjustments of the proton charge radius and the Rydberg constant. We measured the 2 ³ S →2 ¹ S transition frequency in a Fermi gas of ³ He with an order of magnitude higher accuracy than determined previously. Together with a previous measurement in a ⁴ He Bose-Einstein condensate, a squared charge radius difference r _h ² – r _α ² = 1.0757(12) _exp (9) _theo fm ² was determined between the helion and alpha particle. This measurement provides a stringent benchmark for nuclear structure calculations.

We demonstrate a method to determine the longitudinal phase-space distribution of a cryogenic buffer gas cooled beam of barium-fluoride molecules based on a two-step laser excitation scheme. Temporal resolution is achieved by a transversely aligned laser beam that drives molecules from the ground state X 2 Σ + to the A 2 Π 1 / 2 state around 860 nm, while the velocity resolution is obtained by a laser beam that is aligned counter-propagating with respect to the molecular beam and that drives the Doppler shifted A 2 Π 1 / 2 to D 2 Σ + transition around 797 nm. Molecules in the D-state are detected background-free by recording the fluorescence from the D − X transition at 413 nm. A temporal resolution of 11 μs and a velocity resolution of 6 m s⁻¹ is obtained. In order to calibrate the absolute velocity, we have determined the Doppler free transition frequencies for the X − A and X − D transitions with an absolute accuracy below 0.3 MHz. The high resolution of the phase-space distributions allows us to observe a variation of the average velocity and velocity spread over the duration of the molecular beam pulse. Our method hence gives valuable insight into the dynamics in the source.

An electrostatic hexapole lens is used to manipulate the transverse properties of a beam of barium fluoride molecules from a cryogenic buffer gas source. The spatial distribution of the beam is measured by recording state-selective laser-induced fluorescence on an emccd camera, providing insight into the intensity and transverse position spread of the molecular beam. Although the high mass and unfavorable Stark shift of barium fluoride pose a considerable challenge, the number of molecules in the low-field seeking component of the N = 1 state that pass a 4 mm diameter aperture 712 mm behind the source is increased by a factor of 12. Furthermore, it is demonstrated that the molecular beam can be displaced by up to ±5 mm by moving the hexapole lens. Our measurements agree well with numerical trajectory simulations. We discuss how electrostatic lenses may be used to increase the sensitivity of beam experiments such as the search for the electric dipole moment of the electron.