Session U06: Spectroscopy, Lifetimes, and Oscillator Strengths

Resolving high order icosahedral tensor interactions in C60 fullerenes

C60's stiff bonds and high symmetry provide the unique opportunity to study a large polyatomic molecule (60 atoms) in individual ro-vibrational states at a temperature of ∼100 K. Simultaneously, the thermal occupation of 100's of rotational states reveals extensive J-dependence of rotational perturbations, elucidating the interplay of angular momentum, symmetry, and rovibrational coupling in a high-symmetry molecule. While previous work established the well-resolved, regular rigid rotor-like R-branch spectrum in the 8.4 μm vibrational band, the P-branch appeared anomalously fractionated and was hitherto unresolved and unexplained. We have experimentally measured and assigned individual rotational sublevels in the P-branch and identified their origin in tensor interactions driven by the molecule's icosahedral symmetry. This constitutes the first observation of intramolecular icosahedral tensor interac tions since their initial prediction for C60 three decades ago. Additionally, we obtain strong evidence that the P-branch perturbations arise from intramolecular vibrational coupling, providing a direct window into the emergence of complexity in C60.   

Electric Octupole Transitions in Ag-like High-Z ions

We report measurements and identifications of electric octupole (E3) transitions between the 4f_7/2,5/2 and 5s_1/2 levels of Ag-like W²⁷⁺, Re²⁸⁺, and Ir³⁰⁺ ions. We used a flat-field grazing-incidence extreme-ultraviolet (EUV) spectrometer on the electron beam ion trap (EBIT) of the National Institute of Standards and Technology (NIST) to record spectra that included lines from Ag-like and neighboring charge states. Our results confirm the occurrence of E3 transitions in W²⁷⁺, first reported by Sakaue et al. [1], and provide higher precision results for the wavelengths that our multi-configuration Dirac-Fock (GRASP2K) calculations can reproduce. 

Our collisional-radiative calculations based on the NOMAD [2] and FAC [3] codes give an insight into the population kinetics for Ag-like ions of heavy elements, including the electron density dependence of the relative line intensities. Our measurements of the forbidden lines in highly charged ions provide information on quantum-electrodynamic and relativistic effects of atomic structure and offer new diagnostic techniques for hot low electron density plasmas.

References

[1] H.A. Sakaue et al, Phys. Rev. 100, 052515 (2019)

[2] Yu. Ralchenko, Y. Maron, JQSRT 71, 609 (2001)

[3] M.F. Gu, Can. J. Phys. 86, 675 (2008)   

Populating high-n Rydberg states in highly-charged Ar ions

Rydberg Highly Charged Ions (RyHCI) are interesting systems to study as they provide excellent test beds for precision tests of quantum electrodynamics and precision x-ray wavelength standards. Moreover, certain high angular momentum states are considered to be potentially useful for measuring fundamental constants. Dielectronic Recombination (DR) is a significant process that results in the production of Rydberg states. DR is a process in which a free electron is captured by an ion, non-radiatively, by exciting a bound electron in the ion. DR is a resonant process, occurring only at specific free electron energies. The resulting doubly excited ion then radiatively decays.

Experiments were conducted at the National Institute of Standards and Technology's (NIST) Electron Beam Ion Trap (EBIT) facility utilizing the Transition Edge Sensing (TES) microcalorimeter [1]. We present highly charged Ar x-ray spectra as a function of the electron beam energy that show the signatures of different processes populating electronic levels including DR. We measure DR cross-sections to compare with the absolute, precisely measured radiative recombination (RR) cross-sections as well as to compare with theoretical cross-sections. These measurements are expected to be valuable diagnostics for many areas of plasma physics.

[1]    P. Szypryt, et al. Rev. Sci. Instrum 90, 123107 (2019)   

Relativistic Ritz Approach to Hydrogen-like Atoms

The Rydberg formula along with the Ritz quantum defect ansatz has been a standard theoretical tool used in atomic physics since before the advent of quantum mechanics, yet this approach has remained limited by its nonrelativistic foundation. Here I present a long-distance relativistic effective theory describing hydrogenlike systems with arbitrary mass ratios, thereby extending the canonical Ritz-like approach. Fitting the relativistic theory to the hydrogen energy levels predicted by bound-state QED indicates that it is superior to the canonical, nonrelativistic approach. An analytic analysis reveals nonlinear consistency relations within the bound-state QED level predictions that relate higher-order corrections to those at lower order, providing guideposts for future perturbative calculations as well as insights into the asymptotic behavior of the Bethe logarithm. Applications of the approach include fitting to atomic spectroscopic data, allowing for the determination of the Rydberg constant from large spectral data sets and also to check for internal consistency of the data -- all independently from bound-state QED.   

Spectral Measurements of HF and VHF Radio Frequency Electric Fields Using Rydberg Rabi Matching

Rydberg atoms are useful as calibration-free sensors of electric field amplitude when target radio frequencies are resonant to a Rydberg-Rydberg transition. However, detecting sub-GHz carriers remains challenging. In this work, we generate atomic quasi-energy sidebands using a field in the range of 3 MHz to 300 MHz, the HF and VHF bands, then probe these states using a Rydberg-resonant microwave field to controllably split Rydberg states. By matching the resonant microwave field’s Rabi frequency to the HF/VHF field’s frequency, we convert Townes-Merritt sideband transmission peaks into a secondary line splitting within each Autler-Townes peak that grows non-linearly with HF/VHF field amplitude. This experiment demonstrates reception of meters-long radio waves in a subwavelength centimeter-sized vapor cell and reception of amplitude-modulated (AM) information on MHz-range carriers, useful for long-range reception. We present theory using only two levels and effective polarizabilities, which predicts low field effects well, but is insufficient for the state mixing evident with higher fields approaching 1 V/cm. We compare our sensor to other similar designs, noting that this scheme applies an inherent band-pass filter for the carrier frequency, which is automatically downconverted by the atoms.   

Precision measurement of molecular rotational spectra in excited vibrational modes

Rotational spectroscopy is an important field in molecular physics with widespread applications in radio astronomy. However majority of rotational spectroscopy measurements have been performed only in the ground vibrational mode of molecules because the thermal population of excited vibrational modes drops exponentially with vibrational energy. Rotational spectra in each excited vibrational mode is uniquely altered by the associated vibrational motions and coupling among neighboring ro-vibrational energy levels. This makes measurements of rotational spectra in excited vibrational modes of high interest for reconstructing the complex intramolecular energy transfer pathways within these molecules. Standard techniques to obtain rotational spectrum in excited vibrational modes typically suffer from complexity or reduced precision compared to that routinely achieved for rotational spectra in the ground vibrational mode. Here, we demonstrate how an infrared QCL may be used to measure rotational transitions in highly excited vibrational modes by enhancing absorption strength or inducing lasing of terahertz rotational transitions in these vibrational modes. We used a tunable QCL to excite v₃ R-branch transitions in nitrous oxide and either enhanced absorption or induced lasing on 20 v₃ rotational transitions, whose frequencies between 299 and 772 GHz were then measured using either heterodyne or modulation spectroscopy. The spectra were fitted to obtain the rotational constants B₃ and D₃, which reproduce the measured spectra to within the experimental uncertainty of ± 5 kHz.   

Xenon-helium gas mixtures as thermalization mediator for vacuum-ultraviolet photon Bose-Einstein condensation

Bose-Einstein condensation, previously mostly associated with cold atomic ensembles, has in our and other groups in recent years been realized with visible spectral range photons in material-filled optical microcavities. A quantum gas of light, confined to the microcavity inducing a non-trivial low-energy ground state, is subject to repeated absorption and emission cycles in a liquid dye solution, driving the photon gas to thermal equilibrium at ambient temperature, as understood from the collision-induced thermalization of the rovibronic substructure of the involved molecular states. We propose an experimental approach for the realization of a Bose-Einstein condensate of photons in the vacuum-ultraviolet (100 – 200 nm), a spectral regime in which the construction of lasers is inherently difficult due to short excited state lifetimes. Intriguingly, a photon condensate requires no population inversion, while nevertheless constituting a source of coherent and monochromatic light. Our current proposal envisages transitions between the 5p⁶ and 5p⁵6s states of lightly bound xenon-helium quasi-molecules at around 100 bar total pressure to be employed in the thermalization scheme. We here report on spectroscopic absorption and emission data, exploring suitable pressure and admixture conditions. Further, the competing influence of homonuclear xenon-xenon excimers is assessed.   

Algebraic relations arising from mass polarization in one-photon and two-photon transitions of heliumlike ions

Decay rates for two-photon transitions can be expressed as a power series in powers of μ/M for the case of finite nuclear mass M and reduced mass μ.  As shown previously [1], gauge invariance implies the existence of algebraic relationships which relate the expansion coefficients for the length and velocity forms.   These expansions provide a sensitive test of agreement between calculations in the length and velocity gauges at any desired order in μ/M.  In the present work, the algebraic relations are derived for arbitrary n-photon electric dipole (nE1) decay rates and tested in two-electron atoms up to the third-order terms powers of μ/M ≈ 10^-4. Both one-photon (singlet and triplet) and two-photon spontaneous decay rates for the heliumlike ions are shown numerically to satisfy the derived algebraic relations within the accuracy of the calculations. 

[1] A. T. Bondy, D. C. Morton, G. W. F. Drake, Phys. Rev. A 102, 052087 (2020).   

Theoretical ionization energy for the 1s24p1P of helium to test a 10σ discrepancy with experiment

There is a 10σ discrepancy between theory and experiment for the ionization energy of the 1s2s³S₁ state of helium [1]. In order to provide an additional check, Clausen et al. [2] have performed measurements for the Rydberg P-states of helium from n = 24 to n = 100 and extrapolated to n = ∞ to find the ionization energy. In the present work, we extend previous high-precision variational calculations [3] up to n = 24 using triple basis sets in Hylleraas coordinates. With the inclusion of relativistic and QED corrections, the results provide a direct theoretical test against the Clausen measurement at n = 24. The results are in excellent agreement, thereby confirming the 10σ discrepancy between theory and experiment for the ionization energy of the 1s2s³S₁ state of helium.

[1] V. Patkos, V. A. Yerokhin and K. Pachucki, Phys. Rev. A 103, 042809 (2021).

[2] G. Clausen et al. Phys. Rev. Lett. 127, 093001 (2021).

[3] G. W. F. Drake, M. M. Cassar and R. A. Nistor, Phys. Rev. A, 65, 054501 (2002).   

Precision theoretical determination of electric dipole matrix elements in atomic cesium

We perform high-precision calculations of the electric dipole matrix elements 1/2||D||n'P_1/2,3/2> with n=6,7 and n'=6,7,...,12 in atomic cesium using the relativistic coupled-cluster method. Our approach, which include nonlinear core singles, doubles, perturbative core triples, and valence triples, is the most sophisticated method to date. The uncertainties of our results is ~0.2% for transitions between low-lying state and at most ~2% for transitions to states with large principal quantum number.