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1. Characterizing the origin band spectrum of isoquinoline with resonance enhanced multiphoton ionization and electronic structure calculations

   https://doi.org/10.1063/5.0168421  

   Krogmeier, Timothy J.; Pappas, Emerson S.; Reardon, Kylie A.; Rivera, Marcos R.; Head-Marsden, Kade; Parsons, Bradley F.; Schlimgen, Anthony W. (October 2023, The Journal of Chemical Physics)

   We report the experimental resonance enhanced multiphoton ionization spectrum of isoquinoline between 315 and 310 nm, along with correlated electronic structure calculations on the ground and excited states of this species. This spectral region spans the origin transitions to a π–π* excited state, which previous work has suggested to be vibronically coupled with a lower lying singlet n–π* state. Our computational results corroborate previous density functional theory calculations that predict the vertical excitation energy for the n–π* state to be higher than the π–π* state; however, we find an increase in the C–N–C angle brings the n–π* state below the energy of the π–π* state. The calculations find two out-of-plane vibrational modes of the n–π* state, which may be brought into near resonance with the π–π* state as the C–N–C bond angle increases. Therefore, the C–N–C bond angle may be important in activating vibronic coupling between the states. We fit the experimental rotational contour with a genetic algorithm to determine the excited state rotational constants and orientation of the transition dipole moment. The fits show a mostly in-plane polarized transition, and the projection of the transition dipole moment in the a-b plane is about 84° away from the a axis. These results are consistent with the prediction of our electronic structure calculations for the transition dipole moment of the π–π* excited state. 

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2. Mechanisms and dynamics of the NH ⁺ ₂ + H ⁺ and NH ⁺ + H ⁺ + H fragmentation channels upon single-photon double ionization of NH ₃

   https://doi.org/10.1088/1361-6455/abc3aa  

   Larsen, Kirk A; Rescigno, Thomas N; Streeter, Zachary L; Iskandar, Wael; Heck, Saijoscha; Gatton, Averell; Champenois, Elio G; Severt, Travis; Strom, Richard; Jochim, Bethany; et al (November 2020, Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract We present state-selective measurements on the N H 2 + + H + and NH + + H + + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH 3 , where the two photoelectrons and two cations are measured in coincidence using 3D momentum imaging. Three dication electronic states are identified to contribute to the N H 2 + + H + dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold N H 2 + fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited N H 2 + fragment with roughly 1 eV of internal energy. The NH + + H + + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the N H 2 + + H + channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states. 

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3. Photoionization of neutral iron from the ground and excited states

   Zatsarinny O, Bartschat (February 2019, Physical review, A)

   The B-spline R-matrix method is used to investigate the photoionization of neutral iron from the ground and excited states in the energy region from the ionization thresholds to 2 Ry. The multiconfiguration Hartree-Fock method in connection with adjustable configuration expansions and term-dependent orbitals is employed for an accurate representation of the initial states of Fe I and the target wave functions of Fe II. The close-coupling expansion contains 261 LS states of Fe II and includes all levels of the 3d^6 4s, 3d^5 4s^2, 3d^7, 3d^6 4p, and 3d^5 4s4p configurations. Full inclusion of all terms from the principal configurations considerably changes both the lowenergy resonance structure and the energy dependence of the background cross sections. Partial cross sections are analyzed in detail to clarify the most important scattering channels. Comparison with other calculations is used to place uncertainty bounds on our final photoionization cross sections and to assess the likely uncertainties in the existing data sets. 

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4. Experimental and theoretical study of highly excited states of the cesium dimer

   https://doi.org/10.1063/5.0280444  

   Rowe, B A; Stahovich, J T; Magnier, S; Sovkov, V B; Nikolov, A B; Whang, S; Hersh, A D; Wardach, P L; Keen, J D; Lyyra, A M; et al (July 2025, The Journal of Chemical Physics)

   In this work, we report the results of a combined ab initio and experimental investigation of highly excited Σg+1 and Πg1 states of the cesium dimer in a previously unobserved energy region of the molecule. The structure of these high-lying electronic states was predicted via calculations in the framework of the pseudopotential method and observed via the optical–optical double resonance technique. Understanding the rovibronic structure of the cesium dimer at this high energy regime has significance for the formation of ultracold Cs2 ground state molecules and their detection using resonantly enhanced multiphoton ionization (REMPI) techniques. Using the ab initio results and the selection rules for dipole allowed transitions, the experimentally observed rovibrational levels were identified as belonging to the 11Σg+1 and 61Πg electronic states. The Dunham–RKR method was utilized to generate experimental potential energy curves for the two electronic states. 

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5. State selectivity in core ionization of methane

   https://doi.org/10.1103/PhysRevResearch.7.L022021  

   Rajput, J; Stindl, J; Cassimi, A; Gervais, B; Kircher, M; Kastirke, G; McGinnis, O D; Enoki, R; Williams, J B; Davis, V; et al (April 2025, Physical Review Research)

   A clear experimental signature of the population of the lowest triplet state ${}^{3}T_1$of the methane dication is identified in a photoionization experiment. This state is populated only in valence ionization and is absent when the dication is formed by core ionization followed by Auger-Meitner decay. For valence ionization, the total internal energy of the ${\mathrm{CH}}_3^{+}$fragment, formed during the deprotonation of ${\mathrm{CH}}_4{}^{2+}$, is evaluated. Notably, the distribution of this internal energy peaks at the same value regardless of the initially populated electronic state of ${\mathrm{CH}}_4{}^{2+}$. We find that excited electronic states of ${\mathrm{CH}}_3^{+}$are predominantly populated with significant rovibrational excitation. Published by the American Physical Society2025 

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