Transition state spectroscopy experiments based on negative ion photodetachment directly probe the vibrational structure and metastable resonances that are characteristic of the neutral reactive potential energy surface (PES). Here, we study the five-atom reaction F + NH3 → HF + NH2 using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled FNH3¯ anions. Reduced-dimensional quantum dynamical simulations performed on a global potential energy surface show excellent agreement with the experimental results, allowing for the assignment of spectral structure and demonstrating that key dynamics of this bimolecular reaction are well described by this theoretical framework. Our combined experimental-theoretical study reveals features associated with a manifold of vibrational Feshbach resonances in the product well of the F + NH3 PES. At higher energies, the spectra reveal structure attributed to resonances localized across the transition state and into the reactant complex well, which may impact the bimolecular reaction dynamics.
High-resolution anion photoelectron spectra of cryogenically cooled NiO2¯ anions, obtained using slow photoelectron velocity-map imaging (cryo-SEVI), are presented in tandem with coupled cluster electronic structure calculations including relativistic effects. The experimental spectra encompass the , , and photodetachment transitions of linear ONiO0/-, revealing previously unobserved vibrational structure in all three electronic bands. The high-resolution afforded by cryo-SEVI allows for the extraction of vibrational frequencies for each state, congruent with those previously measured in the ground state and in good agreement with scalar-relativistic coupled-cluster calculations. Previously unobserved vibrational structure is observed in the and states and is tentatively assigned. Further, a refined electron affinity of 3.0464(7) eV for NiO2 is obtained as well as precise term energies for the and states of NiO2 of 0.3982(7) and 0.7422(10) eV, respectively. Numerous Franck-Condon forbidden transitions involving the doubly degenerate v2 bending mode are observed and ascribed to Herzberg-Teller coupling to an excited electronic state.
Transition state spectroscopy experiments, based on negative-ion photodechament, allow for the direct probing of the vibrational structure and metastable resonances that are characteristic of the neutral reactive surface. Here, we study the four-atom F + NH 3 → HF + NH 2 reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled NH 3 F − anions. The resulting spectra reveal features associated with a manifold of vibrational Feshbach resonances in the post-transition state product well of this reactive surface. Beyond this, the spectra contain structure reporting on reactive resonances in the pre-transition state reaction complex well. Quantum dynamical calculations performed on a full-dimensional potential surface show excellent agreement with the experimental results, allowing for the assignment of spectral structure and demonstrating that key dynamics of this bimolecular reaction are well described by this theoretical framework.
High-resolution photoelectron spectra of cryogenically cooled acetyl anions (CH3CO¯) obtained using slow photoelectron velocity-map imaging are reported. The high resolution of the photoelectron spectrum yields a refined electron affinity of 0.4352 ± 0.0012 eV for the acetyl radical as well as the observation of new vibronic structure that is assigned based on ab initio calculations. Three vibrational frequencies of the neutral radical are measured to be 1047 ± 3 cm-1 (ν6), 834 ± 2 cm-1 (ν7), and 471 ± 1 cm-1 (ν8). This work represents the first experimental measurement of the ν6 frequency of the neutral. The measured electron affinity is used to calculate a refined value of 1641.35 ± 0.42 kJ mol 1 for the gas-phase acidity of acetaldehyde. Analysis of the photoelectron angular distributions provides insight into the character of the highest occupied molecular orbital of the anion, revealing a molecular orbital with strong d-character. Additionally, details of a new centroiding algorithm based on finite differences, which has the potential to decrease data acquisition times by an order of magnitude at no cost to accuracy, are provided.
We observe that the orientational isomerization of CO on a NaCl(100) surface proceeds by thermally-activated tunneling between 19 and 24K. The rate constants of three isotopomers follow an Arrhenius temperature dependence, exhibiting activation energies below the reaction’s predicted barrier height and anomalously small prefactors. In addition, the rates depend strongly on isotope, but non-intuitively on mass. A quantum rate theory of condensed-phase tunneling qualitatively explains these observations. Vibrationally excited states, accidentally close in energy but localized on opposite sides of the isomerization barrier, provide tunneling gateways between the isomers in a process that can be many orders-of-magnitude faster than rates predicted by commonly used semi-classical models. This suggests heavy-atom condensed-phase tunneling may be more important than currently assumed.
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