Energy and angular distributions in dissociative photodetachment of O−4

Anion Photoelectron Imaging Spectroscopy of C6HF5-, C6F6-, and the Absence of C6H2F4. J Phys Chem A 2023;127:8556-8565. [PMID: 37816145 DOI: 10.1021/acs.jpca.3c04016] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Substituents have a profound effect on the electronic structure of the benzene molecule. In this paper, we present new photoelectron spectra of the C5HF5- molecular anion, to test predictions [ Int. J. Quant. Chem. 2017, 188, e25504] that pentafluorobenzene has a positive electron affinity, as hexafluorobenzene was already known to have. The PE spectrum of C6HF5- exhibits a broad and vibrationally unresolved band due to significant differences between the structure of the anion and the neutral. The vertical detachment energy (VDE) of C5HF5- is determined to be 1.33 ± 0.05 eV, and the lowest binding energy at which the signal is observed is 0.53 ± 0.05 eV, which, if taken as the electron affinity, is in good agreement with the computed value. In addition, we attempted to generate intact C6H2F4- molecular ions using the 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, and 1,2,4,5-tetrafluorobenzene precursors, as tetrafluorobenzene was predicted to have a near-zero but marginally positive electron affinity. Using a photoemission anion source, we were not able to produce the intact tetrafluorobenzene anion. Density functional theory calculations support a more detailed discussion of the impact of fluorine substitution on the electronic structure of these species.

Probing Anion-Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

Photoelectron Imaging Spectra of O2-·VOC and O4-·VOC Complexes

The anion photoelectron imaging spectra of O₂^-·VOC and O₄^-·VOC (VOC = hexane, isoprene, benzene, and benzene-d₆) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O₂^-·VOC spectral features compared to the bare O₂^- indicate that photodetachment of the anion accesses a modestly repulsive region of the O₂-VOC potential energy surface, with subtle VOC dependence on the relative energies of the O₂ (X ³Σ_g^-)·VOC ground state and O₂ (a ¹Δ_g)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O₂ (a ¹Δ_g)·VOC excited state relative to the O₂ (X ³Σ_g^-)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O₄^-·benzene and O₄^-·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.

Photoelectron-photofragment coincidence studies of the tert-butoxide anion (CH3)3CO((-)), the carbanion isomer (CH3)2CH2COH((-)), and corresponding radicals

A study of the photodetachment and dissociative photodetachment (DPD) of the C(4)H(9)O(-) isomers tert-butoxide, (CH(3))(3)CO(-), and the α-hydroxy carbanion (CH(3))(2)C(CH(2))OH(-) is reported. Photoelectron-photofragment coincidence spectroscopy was used to study these anions at 387, 537, and 600 nm. Supported by CBS-QB3 ab initio calculations, the product mass and translational energy distributions were found to be consistent with dissociation of either highly excited (CH3)(3)CO radicals or (CH(3))(2)C(CH2)OH alkylhydroxy radicals. Vibrationally resolved photoelectron spectra of stable radicals at 537 and 600 nm in conjunction with Franck-Condon simulations were used to assign the dominant channel to tert-butoxide ((CH(3)3)CO(-)) anions thermalized to a vibrational temperature of 550 K. DPD is assigned to highly vibrationally excited radicals produced by photodetachment of unrelaxed tert-butoxide products formed at an effective source temperature of 1400 K. The higher energy carbanion was found to be a minor channel and was not observed to dissociate. Calculated energetics for photodetachment and DPD of (CH(3))(3)CO(-) and (CH(3))(2)C(CH(2))OH(-) are discussed and compared with the experimental results.

Oxygen cluster anions revisited: solvent-mediated dissociation of the core O4(-) anion

The electronic structure and photochemistry of the O(2n)(-)(H(2)O)(m), n = 1-6, m = 0-1 cluster anions is investigated at 532 nm using photoelectron imaging and photofragment mass-spectroscopy. The results indicate that both pure oxygen clusters and their hydrated counterparts with n ≥ 2 form an O(4)(-) core. Fragmentation of these clusters yields predominantly O(2)(-) and O(2)(-)·H(2)O anionic products, with the addition of O(4)(-) fragments for larger parent clusters. The fragment autodetachment patterns observed for O(6)(-) and larger O(2n)(-) species, as well as some of their hydrated counterparts, indicate that the corresponding O(2)(-) fragments are formed in excited vibrational states (v ≥ 4). Yet, surprisingly, the unsolvated O(4)(-) anion itself does not show fragment autodetachment at 532 nm. It is hypothesized that the vibrationally excited O(2)(-) is formed in the intra-cluster photodissociation of the O(4)(-) core anion via a charge-hopping electronic relaxation mechanism mediated by asymmetric solvation of the nascent photofragments: O(4)(-) → O(2)(-)(X(2)Π(g)) + O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) + O(2)(-)(X(2)Π(g)). This process depends on the presence of solvent molecules and leads to vibrationally excited O(2)(-)(X(2)Π(g)) products.

Femtosecond study of Cu(H2O) dynamics

The short-time nuclear dynamics of Cu(H(2)O) is investigated using femtosecond photodetachment-photoionization spectroscopy and time-dependent quantum wave packet calculations. The Cu(H(2)O) dynamics is initiated in the electronic ground state of the complex by electron photodetachment from the Cu(-)(H(2)O) complex, where hydrogen atoms are oriented toward Cu. Several time-resolved resonant multiphoton ionization schemes are used to probe the ensuing reorientation and dissociation. Immediately following photodetachment, the neutral complex is far from its minimum energy geometry and possesses an internal energy comparable to the Cu-H(2)O dissociation energy and undergoes both large-amplitude H(2)O motion and dissociation. Dissociation is observed to occur on three distinct time scales: 0.6, 8, and 100 ps. These results are compared to the results of time-dependent J=0 wave packet calculations, propagating the initial anion vibrational wave functions on the ground-state potential of the neutral complex. An excellent agreement is obtained between the experimental results and the ionization signals derived from the calculated probability amplitudes. Related experiments and calculations are carried out on the Cu(D(2)O) complex, with results very similar to those of Cu(H(2)O).

Coincidence spectroscopy

The application of coincidence techniques to the study of the reaction dynamics of isolated molecules is reviewed. Coincidence spectroscopy is a powerful approach for carrying out a number of measurements. At its most basic level, coincidence techniques can identify the source of a specific signal, as in the well-known photoelectron-photoion coincidence approach used for several years. By carrying out coincidence experiments in an increasingly differential manner, correlated energy and angular distributions of reaction products may be recorded. Completely energy- and angle-resolved measurements of photoelectrons and ionic or neutral products can reveal molecular-frame photoelectron and photofragment angular distributions and aid in the characterization of dissociative states of molecules and ions. Recent work in this area is reviewed, including examples from studies of dissociative photodetachment, dissociative photoionization, time-resolved studies of dissociative photoionization, and three-body dissociation processes.