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1. Photoelectron photofragment coincidence spectroscopy of aromatic carboxylates: benzoate and p -coumarate

   https://doi.org/10.1039/d1cp02972j  

   Gibbard, J. A.; Castracane, E.; Krylov, A. I.; Continetti, R. E. (September 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Photoelectron–photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p -coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO 2 + C 6 H 5 + e − , with a very small amount of stable C 6 H 5 CO 2 showing that the radical ground state is stable and the excited states are dissociative. For p -coumarate ( p -CA − ) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO 2 + HOC 6 H 4 CHCH + e − , characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOC 6 H 5 + HCC − + CO 2 . The ion beam contained both the phenolate and carboxylate isomers of p -CA − , but DPD only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state. 

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2. Dipole effects in the photoelectron angular distributions of the sulfur monoxide anion

   https://doi.org/10.1039/D2CP03337B  

   Ru, Beverly; Hart, C. Annie; Mabbs, Richard; Gozem, Samer; Krylov, Anna I.; Sanov, Andrei (October 2022, Physical Chemistry Chemical Physics)

   Photoelectron angular distributions (PADs) in SO − photodetachment using linearly polarized 355 nm (3.49 eV), 532 nm (2.33 eV), and 611 nm (2.03 eV) light were investigated via photoelectron imaging spectroscopy. The measurements at 532 and 611 nm access the X 3 Σ − and a 1 Δ electronic states of SO, whereas the measurements at 355 nm also access the b 1 Σ + state. In aggregate, the photoelectron anisotropy parameter values follow the general trend with respect to electron kinetic energy (eKE) expected for π*-orbital photodetachment. The trend is similar to O 2 − , but the minimum of the SO − curve is shifted to smaller eKE. This shift is mainly attributed to the exit-channel interactions of the departing electron with the dipole moment of the neutral SO core, rather than the differing shapes of the SO − and O 2 − molecular orbitals. Of the several ab initio models considered, two approaches yield good agreement with the experiment: one representing the departing electron as a superposition of eigenfunctions of a point dipole-field Hamiltonian, and another describing the outgoing electron in terms of Coulomb waves originating from two separated charge centers, with a partial positive charge on the sulfur and an equal negative charge on the oxygen. These fundamentally related approaches support the conclusion that electron–dipole interactions in the exit channel of SO − photodetachment play an important role in shaping the PADs. While a similar conclusion was previously reached for photodetachment from σ orbitals of CN − (Hart, Lyle, Spellberg, Krylov, Mabbs, J. Phys. Chem. Lett. , 2021, 12 , 10086–10092), the present work includes the first extension of the dipole-field model to detachment from π* orbitals. 

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3. Photoinduced charge transfer in the Zn-methanol cation studied with selected-ion photofragment imaging

   https://doi.org/10.1063/5.0108467  

   Rittgers, Brandon M.; Marks, Joshua H.; Kellar, Douglas J.; Duncan, Michael A. (September 2022, The Journal of Chemical Physics)

   The Zn+(methanol) ion molecule complex produced by laser vaporization is studied with photofragment imaging at 280 and 266 nm. Photodissociation produces the methanol cation CH3OH+ via excitation of a charge-transfer excited state. Surprisingly, excitation of bound excited states produces the same fragment via a curve crossing prior to separation of products. Significant kinetic energy release is detected at both wavelengths with isotropic angular distributions. Similar experiments are conducted on the perdeuterated methanol complex. The Zn+ cation is a minor product channel that also exhibits significant kinetic energy release. An energetic cycle using the ionization energies of zinc and methanol together with the kinetic energy release produces an upper limit on the Zn+-methanol bond energy of 33.7 ± 4.2 kcal/mol (1.46 ± 0.18 eV). 

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4. Ultraviolet photodissociation of 2-methylallyl radical

   https://doi.org/10.1063/1674-0068/cjcp2309088  

   Lucas, Michael; Qin, Yuan; Chen, Min; Sun, Ge; Zhang, Jingsong (April 2024, Chinese Journal of Chemical Physics)

   Ultraviolet photodissociation dynamics of 2-methylallyl radical from the 3p Rydberg state were investigated in the wavelength region of 226–244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The 2-methylallyl radicals were generated by 193 nm photolysis of 3-chloro-2-methyl-1-propene precursors. The photofragment yield spectrum of H-atom products increases in intensity with decreasing wavelengths in 226–244 nm. The TOF spectra of H-atom products show a bimodal structure. The predominant product channel (with ∼98% branching ratio) has a kinetic energy release peaking at ∼7 kcal/mol, with an average ratio of ET in the total available energy, (fT), of ∼0.18 in 226–244 nm and an isotropic product angular distribution. At the low ET, isotropic component is from statistical unimolecular decomposition of highly vibrationally excited hot 2-methylallyl to the methylenecyclopropane+H products, following internal conversion from the excited electronic state. The minor product channel (with ∼2% branching ratio) has a large kinetic energy peaking at ∼50 kcal/mol, with (fT)≈0.63 and an anisotropic angular distribution (β≈−0.2). At the high ET, anisotropic component is non-statistical and is postulated to be from direct loss of H atom via the 3p Rydberg state or repulsive part of the ground state to the 1,3-butadiene+H products. 

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5. Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Trimer and Tetramer

   Jaffe, Noah B; Stanton, John F; Heaven, Michael C (September 2023, The journal of physical chemistry letters)

   Computational studies of small beryllium clusters (BeN) predict dramatic, nonmonotonic changes in the bonding mechanisms and per-atom cohesion energies with increasing N. To date, experimental tests of these quantum chemistry models are lacking for all but the Be2 molecule. In the present study, we report spectroscopic data for Be3 and Be4 obtained via anion photodetachment spectroscopy. The trimer is predicted to have D3h symmetric equilibrium structures for both the neutral molecule and the anion. Photodetachment spectra reveal transitions that originate from the X2A2″ ground state and the (1)2A1′ electronically excited state. The state symmetries were assigned on the basis of anisotropic photoelectron angular distributions. The neutral and anionic forms of Be4 are predicted to be tetrahedral. Franck−Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be4−X2A1 → Be4X1A1 transition. The electron affinities of Be3 and Be4 were determined to be 11363 ± 60 and 13052 ± 50 cm−1, respectively 

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