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1. Resolving the F ₂ bond energy discrepancy using coincidence ion pair production (cipp) spectroscopy

   https://doi.org/10.1039/D1CP00140J  

   Matthíasson, Kristján; Kvaran, Ágúst; Garcia, Gustavo A.; Weidner, Peter; Sztáray, Bálint (April 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Coincidence ion pair production (cipp) spectra of F 2 were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975–126 210 cm −1 . The F + + F − signal shows a rotational band head structure, corresponding to F 2 Rydberg states crossing over to the ion pair production surface. Spectral simulation and quantum defect analysis allowed the characterization of five new molecular Rydberg states (F 2 **): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed ( T 0 = 125 999 cm −1 ) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.6229 4 ± 0.0004 3 eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F–F dissociation energy of 1.6012 9 ± 0.0004 4 eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F 2 and the dissociative photoionization threshold to F + + F was determined as 19.0242 ± 0.0006 eV. Using the atomic fluorine ionization energy, this can be converted to an F 2 dissociation energy of 1.6013 2 ± 0.0006 2 eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.6013 0 ± 0.0003 6 eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.25 1 ± 0.01 7 kJ mol −1 . This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three. 

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2. Long-range model of vibrational autoionization in core-nonpenetrating Rydberg states of NO

   https://doi.org/10.1063/5.0070879  

   Barnum, Timothy_J; Clausen, Gloria; Jiang, Jun; Coy, Stephen_L; Field, Robert_W (December 2021, The Journal of Chemical Physics)

   In high orbital angular momentum (ℓ ≥ 3) Rydberg states, the centrifugal barrier hinders the close approach of the Rydberg electron to the ion-core. As a result, these core-nonpenetrating Rydberg states can be well described by a simplified model in which the Rydberg electron is only weakly perturbed by the long-range electric properties (i.e., multipole moments and polarizabilities) of the ion-core. We have used a long-range model to describe the vibrational autoionization dynamics of high-ℓ Rydberg states of nitric oxide (NO). In particular, our model explains the extensive angular momentum exchange between the ion-core and the Rydberg electron that had been previously observed in vibrational autoionization of f (ℓ = 3) Rydberg states. These results shed light on a long-standing mechanistic question around these previous observations and support a direct, vibrational mechanism of autoionization over an indirect, predissociation-mediated mechanism. In addition, our model correctly predicts newly measured total decay rates of g (ℓ = 4) Rydberg states because for ℓ ≥ 4, the non-radiative decay is dominated by autoionization rather than predissociation. We examine the predicted NO+ ion rotational state distributions generated by vibrational autoionization of g states and discuss applications of our model to achieve quantum state selection in the production of molecular ions. 

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3. Non-adiabatic dissociation dynamics of Ar⋯I2 (E, v) intermolecular vibrational levels probed using velocity-map imaging

   https://doi.org/10.1063/5.0166512  

   Makarem, Camille; Loomis, Richard A. (September 2023, The Journal of Chemical Physics)

   Ion time-of-flight velocity-map imaging was used to measure the kinetic-energy distributions of the I2 ion-pair fragments formed after photoexcitation of Ar⋯I2 complexes to intermolecular vibrational levels bound within the Ar + I2 (E, vE = 0–2) potential energy surfaces. The kinetic-energy distributions of the I2 products indicate that complexes in the Ar⋯I2 (E, vE) levels preferentially dissociate into I2 in the D and β ion-pair states with no change in I2 vibrational excitation. The energetics of the levels prepared suggest that there is a non-adiabatic coupling of the initially prepared levels with the continuum of states lying above the Ar + I2 (D, vD = vE) and Ar + I2 (β, vβ = vE) dissociation limits. The angular anisotropies of the I2 product signals collected for many of the Ar⋯I2 (E, vE) levels have maxima parallel to the laser polarization axis. This contradicts expectations for the prompt dissociation of complexes with T-shaped geometries, which would result in images with maxima perpendicular to the polarization axis. These anisotropies suggest that there is a perturbation of the transition moment in these clusters or there are additional intermolecular interactions, likely those sampled while traversing above the attractive wells of the lower-energy potentials during dissociation. I2 (D′, vD′) products are also identified when preparing several of the low-lying levels localized in the T-shaped well of the Ar + I2 (E, vE = 0–2) potentials, and they are formed in multiple νD′ vibrational levels spanning energy ranges up to 500 cm−1. 

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4. Valence shell electronically excited states of norbornadiene and quadricyclane

   https://doi.org/10.1063/5.0187707  

   Cooper, Joseph_C; Holland, David_M_P; Ingle, Rebecca_A; Bonanomi, Matteo; Faccialà, Davide; De_Oliveira, Nelson; Abid, Abdul_R; Bachmann, Julien; Bhattacharyya, Surjendu; Borne, Kurtis; et al (February 2024, The Journal of Chemical Physics)

   The absolute photoabsorption cross sections of norbornadiene (NBD) and quadricyclane (QC), two isomers with chemical formula C7H8 that are attracting much interest for solar energy storage applications, have been measured from threshold up to 10.8 eV using the Fourier transform spectrometer at the SOLEIL synchrotron radiation facility. The absorption spectrum of NBD exhibits some sharp structure associated with transitions into Rydberg states, superimposed on several broad bands attributable to valence excitations. Sharp structure, although less pronounced, also appears in the absorption spectrum of QC. Assignments have been proposed for some of the absorption bands using calculated vertical transition energies and oscillator strengths for the electronically excited states of NBD and QC. Natural transition orbitals indicate that some of the electronically excited states in NBD have a mixed Rydberg/valence character, whereas the first ten excited singlet states in QC are all predominantly Rydberg in the vertical region. In NBD, a comparison between the vibrational structure observed in the experimental 11B1–11A1 (3sa1 ← 5b1) band and that predicted by Franck–Condon and Herzberg–Teller modeling has necessitated a revision of the band origin and of the vibrational assignments proposed previously. Similar comparisons have encouraged a revision of the adiabatic first ionization energy of NBD. Simulations of the vibrational structure due to excitation from the 5b2 orbital in QC into 3p and 3d Rydberg states have allowed tentative assignments to be proposed for the complex structure observed in the absorption bands between ∼5.4 and 7.0 eV. 

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5. Photofragment imaging differentiates between one- and two-photon dissociation pathways in MgI⁺

   https://doi.org/10.1063/5.0134668  

   Lockwood, Schuyler P; Metz, Ricardo B (February 2023, The Journal of Chemical Physics)

   The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm−1, the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg+ (2S) + I (2P3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D0(Mg+-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm−1). At photon energies of 33 000–41 000 cm−1, exclusively I+ fragments are formed. Over most of this region, the formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm−1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm−1. This is consistent with absorption to the (3) Ω = 0+ state from ν = 0, 1 of the (1) Ω = 0+ ground state; from the (3) Ω = 0+ state, absorption of a second photon results in dissociation to Mg* (3P°J) + I+ (3PJ). From 38 500 to 41 000 cm−1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI+ at low energy and ground state MgI+ at higher energy to form Mg (1S) + I+ (3PJ) products. 

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