The rotational spectra of five isotopic species of the Ar-SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectra of the normal species is dominated by a strong progression of a-type rotational transitions arising from increasing quanta in the Si-O stretch. A rotational analysis of these lines and a hyperfine analysis of Ar-Si¹⁷O suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally-averaged structure close to T-shaped in which the oxygen atom lies closer to argon than the silicon atom, much like Ar-CO. Newly performed calculations of the rovibrational level pattern are in good agreement with the experimentally-derived rotational constants of normal and isotopic Ar-SiO up to v=12 ( ∼ 14,500 cm⁻¹) in the Si-O stretch suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential.

r0pt Figure The rotational spectra of five isotopic species of the Ar–SiO complex have been observed at high-spectral resolution, employing various techniques to obtain spectra between 8 and 35 GHz. Progressions of rotational transitions were recorded for a range of quanta in the Si-O stretch which correspond to resonance states of the complex since the vibrational frequency of the diatomic exceeds the binding energy of the complex. A complementary theoretical study was performed in which variational rovibrational calculations were performed using a series of potential energy surfaces (PESs) representing the SiO + Ar interaction and describing a range of vibrational quanta in the SiO(v=0,1…7) fragment. As seen in the Figure, the global minimum (V = -152.2 cm⁻¹) is nearly T-shaped, but a barrier of only 7.2 cm⁻¹ leads to a second minimum in a long channel along the angular coordinate. The relative energy of the T-shaped minimum and the channel toward linearity, varies with the number of quanta in SiO (progressively favoring the more linear structure), and for SiO(v=7), the T-shaped structure is no longer the global minimum. To compute the rovibrational levels and wavefunctions, the RV3 three-atom variational code of Wang and Carrington was used.

Using an infrared and millimeter double resonance action spectroscopic scheme in a cryogenic ion trap, high resolution rotational transitions are recorded for the (near) symmetric top CH₃⁺-He. Eighteen rotational transitions up to J"=6 are recorded in the range 60 - 410 GHz. Small unexpected splittings are resolved for K=1. Advantages of this novel approach of high-resolution ion spectroscopy and potential future applications to spectra of cold cation-helium complexes are discussed.

Slow photoelectron velocity-map imaging spectroscopy of cryogenically-cooled anions (cryo-SEVI) is a powerful technique for elucidating the vibrational and electronic structure of exotic neutral species. SEVI is a high-resolution variant of anion photoelectron imaging that yields spectra with energy resolution as high as 1 cm⁻¹. The preparation of cold anions eliminates hot bands and narrows rotational envelopes, enabling the acquisition of well-resolved photoelectron spectra for complex and spectroscopically challenging species.^1,2 Recently, cryo-SEVI has been applied as a spectroscopic probe of transition state dynamics on neutral reactive surfaces, through photodetachment of a bound anion similar in geometry to the desired transition state. In the benchmark F + H₂ reaction, we probe the transition state region through detachment of FH₂⁻ and directly observe new reactive resonances. Comparison to new theory allows for the assignment of resonances associated with quasi-bound states of the transition state and products.³ We also report spectra of the F + CH₃OH hydrogen abstraction reaction through photodetachment of the CH₃OHF⁻ van der Waals cluster. We gain insight into the energetics and vibrational structure of transient complexes along the reaction coordinate of this complex polyatomic system.⁴ Finally, we report a new cryo-SEVI study of vinylidene (H₂CC), a high energy isomer of acetylene, which is accessed directly through detachment of H₂CC⁻. We find spectroscopic evidence that the isomerization of vinylidene to acetylene is highly state-specific, with excitation of the ν₆ in-plane rocking mode resulting in appreciable tunneling-facilitated mixing with highly vibrationally excited states of acetylene.⁵

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¹Hock et al. JCP 137, 244201 (2012); ²Weichman et al. PNAS 113, 1698 (2016); ³Kim et al. Science 349, 510 (2015); ⁴Weichman et al. Nat. Chem. 9, 950 (2017); ⁵DeVine et al. Science 358, 336 (2017)

The dissociation energy (D₀) of ortho H₂ is a benchmark value in quantum chemistry, with recent QED calculations now approaching accuracies achievable in simple atoms. A precision measurement of the GK-X molecular transition, in combination with other precision measurements (see also part two), provides an improved value for D₀.[1,2] The GK-X transition is excited through Doppler-free two-photon spectroscopy using 179-nm radiation, based on frequency up-conversion using a special KBBF crystal. The optical frequency of the fundamental (716 nm), which is the output of a narrowband pulsed Ti:Sa laser system, is locked to a frequency comb. This enables accuracies at the sub-MHz level, leading to an order-of-magnitude improvement for D₀ to the 10⁻⁹ level of accuracy. The comparison of this accurate experimental result with the best calculations may provide a test of the Standard Model of Physics.[3]

[1] D. Sprecher, Ch. Jungen, W. Ubachs and F. Merkt, Faraday Discussions 150, 51-70 (2011)
[2] W. Ubachs, J.C.J. Koelemeij, K.S.E. Eikema and E.J. Salumbides, J. Mol. Spectr. 320, 1-12 (2016)
[3] J. Liu, E. J. Salumbides, U. Hollenstein, J. C. J. Koelemeij, K. S. E. Eikema, W. Ubachs and F. Merkt, J. Chem. Phys. 130 (17), 174306 (2009)

The dissociation energy D₀ of ortho H₂ is a benchmark quantity in quantum chemistry, with recent QED calculations now approaching accuracies achievable in simple atoms. In the light of recent discrepancies between experiment and theory [1], a combined effort (see also part one) has been undertaken to provide an improved experimental value for D₀. We report the transition frequency from the GK ¹Σ_g⁺ (v=1, N=1) state to the 56p (N=1, S=0,F=0−2) Rydberg state belonging to the series converging on the X^+ 2Σ_g⁺ (v⁺=0,N⁺=1) ground state of ortho H₂⁺. A resonant three-photon excitation scheme was employed, using pulsed VUV and VIS laser sources to reach the intermediate GK state and a continuous-wave near-infrared (NIR) laser source for the transition to the Rydberg state. To reach the desired accuracy, the procedure involved [2]: (i) minimizing the Doppler width through the use of a doubly skimmed, supersonic molecular beam produced by a cryogenic pulsed valve, (ii) minimizing stray electric and magnetic fields, (iii) cancelling the first-order Doppler shift using two counterpropagating laser beams, (iv) calibrating the NIR-laser frequency using a frequency comb referenced to an atomic clock. The ionization energy of the intermediate GK state was obtained by adding the binding energy of the Rydberg state determined previously by millimeter-wave spectroscopy and multichannel quantum-defect theory [3]. In combination with the GK ¹Σ_g⁺ (v=1,N=1) ← X ¹Σ_g⁺ (v=0,N=1) transition frequency presented in part one, an order-of magnitude improvement for D₀ at the 10⁻⁹ level of accuracy has been achieved, while remaining consistent with the previously most precise determination [4].

With the recent efficient implementation of coupled-cluster techniques with the inclusion of quadruple excitations [1] as well as the development of relativistic exact two-component theory [2], calculations of core-ionized/excited states aiming at high accuracy have become feasible [3]. Here we extend a preliminary study presented during the last ISMS to an extensive benchmark study involving twenty-six K-edge ionization energies of first-row elements (C, O, N, F) in fifteen molecules. Core-valence separation [4] has been used to facilitate the convergence of the equation-of-motion coupled-cluster equation for the core-ionized states. Effects from high-level correlation, geometrical relaxation, and vibrational corrections are critically analyzed, and the computed results are carefully compared with the corresponding experimental data.

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D. Matthews, and J. F. Stanton, J. Chem. Phys. 142, 064108 (2015).

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K. G. Dyall, J. Chem. Phys. 106, 9618 (1997).

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For example, see R. H. Myhre, T. J. A. Wolf, L. Cheng, S. Nandi, S. Coriani, M. Gühr and H. Koch, J. Chem. Phys. 148, 064106 (2018).

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S. Coriani, and H. Koch, J. Chem. Phys. 143, 181103 (2015).