Physics

The remarkable progress in the field of laser spectroscopy induced by the invention of the frequency-comb laser has enabled many new high-precision tests of fundamental theory and searches for new physics. Extending frequency-comb based spectroscopy techniques to the vacuum (VUV) and extreme ultraviolet (XUV) spectral range would enable measurements in e.g. heavier hydrogen-like systems and open up new possibilities for tests of quantum electrodynamics and measurements of fundamental constants. The main approaches rely on high-harmonic generation (HHG), which is known to induce spurious phase shifts from plasma formation. After our initial report (Physical Review Letters 123, 143001 (2019)), we give a detailed account of how the Ramsey-comb technique is used to probe the plasma dynamics with high precision, and enables accurate spectroscopy in the VUV. A series of Ramsey fringes is recorded to track the phase evolution of a superposition state in xenon atoms, excited by two up-converted frequency-comb pulses. Phase shifts of up to 1 rad induced by HHG were observed at ns timescales and with mrad-level accuracy at 110 nm. Such phase shifts could be reduced to a negligible level, enabling us to measure the 5 p 6 ?? p 5 8s 2 [3/2 ] 1 transition frequency in 132 Xe at 110 nm (seventh harmonic) with sub-MHz accuracy. The obtained value is 10 4 times more precise than the previous determination and the fractional accuracy of 2.3? 10 ??0 is 3.6 times better than the previous best spectroscopic measurement using HHG. The isotope shifts between 132 Xe and two other isotopes were determined with an accuracy of 420 kHz. The method can be readily extended to achieve kHz-level accuracy, e.g. to measure the 1S??S transition in H e + . Therefore, the Ramsey-comb method shows great promise for high-precision spectroscopy of targets requiring VUV and XUV wavelengths.

Recently, the production of ultrahigh-density (~10^{19}cm^{-3}) spin-polarized deuterium (SPD) atoms was demonstrated, from the photodissociation of deuterium iodide, but the upper density limit was not determined. Here, we present studies of spin-polarized hydrogen (SPH) densities up to 10^{20} cm^{-3}, by photodissociating 5 bar of hydrogen chloride with a focused 213 nm, 150 ps laser pulse. We extract the depolarization cross-section of hydrogen and chlorine atom collisions, which is the main depolarization mechanism at this high-density regime, to be {\sigma}_{HCl} = 7(2) x 10^{-17}cm^2. We discuss the conditions under which the ultrahigh SPH and SPD densities can be reached, and the potential applications to ultrafast magnetometry, laser-ion acceleration, and tests of polarized nuclear fusion.

State-of-the-art optical clocks achieve fractional precisions of 10 −18 and below using ensembles of atoms in optical lattices or individual ions in radio-frequency traps. Promising candidates for novel clocks are highly charged ions (HCIs) and nuclear transitions, which are largely insensitive to external perturbations and reach wavelengths beyond the optical range, now becoming accessible to frequency combs. However, insufficiently accurate atomic structure calculations still hinder the identification of suitable transitions in HCIs. Here, we report on the discovery of a long-lived metastable electronic state in a HCI by measuring the mass difference of the ground and the excited state in Re, the first non-destructive, direct determination of an electronic excitation energy. This result agrees with our advanced calculations, and we confirmed them with an Os ion with the same electronic configuration. We used the high-precision Penning-trap mass spectrometer PENTATRAP, unique in its synchronous use of five individual traps for simultaneous mass measurements. The cyclotron frequency ratio R of the ion in the ground state to the metastable state could be determined to a precision of δR=1⋅ 10 −11 , unprecedented in the heavy atom regime. With a lifetime of about 130 days, the potential soft x-ray frequency reference at ν=4.86⋅ 10 16 Hz has a linewidth of only Δν≈5⋅ 10 −8 Hz , and one of the highest electronic quality factor ( Q= ν Δν ≈ 10 24 ) ever seen in an experiment. Our low uncertainty enables searching for more HCI soft x-ray clock transitions, needed for promising precision studies of fundamental physics in a thus far unexplored frontier.

Two lowest-energy odd-parity atomic levels of actinium, 7s^27p 2P^o_1/2, 7s^27p 2P^o_3/2, were observed via two-step resonant laser-ionization spectroscopy and their respective energies were measured to be 7477.36(4) cm^-1 and 12 276.59(2) cm^-1. The lifetimes of these states were determined as 668(11) ns and 255(7) ns, respectively. In addition, these properties were calculated using a hybrid approach that combines configuration interaction and coupled-cluster methods in good agreement. The data are of relevance for understanding the complex atomic spectra of actinides and for developing efficient laser-cooling and ionization schemes for actinium, with possible applications for high-purity medicalisotope production and future fundamental physics experiments with this atom.

We present electric dipole polarizabilities ( α d ) of the alkali-metal negative ions, from H ??to Fr ??, by employing four-component relativistic many-body methods. Differences in the results are shown by considering Dirac-Coulomb (DC) Hamiltonian, DC Hamiltonian with the Breit interaction, and DC Hamiltonian with the lower-order quantum electrodynamics interactions. At first, these interactions are included self-consistently in the Dirac-Hartree-Fock (DHF) method, and then electron correlation effects are incorporated over the DHF wave functions in the second-order many-body perturbation theory, random phase approximation and coupled-cluster (CC) theory. Roles of electron correlation effects and relativistic corrections are analyzed using the above many-body methods with size of the ions. We finally quote precise values of α d of the above negative ions by estimating uncertainties to the CC results, and compare them with other calculations wherever available.

In this work, we demonstrate the use of a Rydberg atom-based sensor for determining the angle-of-arrival of an incident radio-frequency (RF) wave or signal. The technique uses electromagnetically induced transparency in Rydberg atomic vapor in conjunction with a heterodyne Rydberg atom-based mixer. The Rydberg atom mixer measures the phase of the incident RF wave at two different locations inside an atomic vapor cell. The phase difference at these two locations is related to the direction of arrival of the incident RF wave. To demonstrate this approach, we measure phase differences of an incident 19.18 GHz wave at two locations inside a vapor cell filled with cesium atoms for various incident angles. Comparisons of these measurements to both full-wave simulation and to a plane-wave theoretical model show that these atom-based sub-wavelength phase measurements can be used to determine the angle-of-arrival of an RF field.

The laser intensity dependence of the recoil energies from the Coulomb explosion of small argon clusters has been investigated by resolving the contributions of the individual charge states to the ion recoil energy spectra. Between 10 14 and 10 15 W/cm 2 the high-energy tail of the ion energy spectra changes its shape and develops into the well-known knee feature, which results from the cluster size distribution, laser focal averaging, and ionization saturation. Resolving the contributions of the different charge states to the recoil energies, the experimental data reveal that the basic assumption of an exploding homogeneously charged sphere cannot be maintained in general. In fact, the energy spectra of the high- q show distinct gaps in the yields at low kinetic energies, which hints at more complex radial ion charge distributions developing during the laser pulse impact.

The four-component relativistic Fock space coupled cluster method is used to describe the magnetic hyperfine interaction in low-lying electronic states of the KCs molecule. Both diagonal and off-diagonal matrix elements as functions of the internuclear separation R are calculated within the finite-field scheme. The resulting matrix elements exhibit very weak dependence on R for the separations exceeding 8 Å, whereas in the vicinity of the ground-state equilibrium the deviation of molecular HFS matrix elements from the atomic values reaches 15\%. The dependence of the computed HFS couplings on the level of core correlation treatment is discussed.

We present spectroscopic measurements and detailed theoretical analysis of inner-shell LMn and LNn (n ≥ 4) dielectronic resonances in highly-charged M-shell ions of tungsten. The x-ray emission from W 49+ through W 64+ was recorded at the electron beam ion trap (EBIT) facility at the National Institute of Standards and Technology (NIST) with a high-purity Ge detector for electron beam energies between 6.8 keV and 10.8 keV. The measured spectra clearly show the presence of strong resonance features as well as direct excitation spectral lines. The analysis of the recorded spectra with large-scale collisional-radiative (CR) modeling of the EBIT plasma allowed us to unambiguously identify numerous dielectronic resonances associated with excitations of the inner-shell 2s 1/2 , 2p 1/2 , and 2p 3/2 electrons.

We report direct laser cooling of a symmetric top molecule, reducing the transverse temperature of a beam of calcium monomethoxide (CaOCH 3 ) to 1.8±0.7 mK while addressing two distinct nuclear spin isomers. These results open a path to efficient production of ultracold chiral molecules and conclusively demonstrate that by using proper rovibronic optical transitions, both photon cycling and laser cooling of complex molecules can be as efficient as for much simpler linear species.