Computational study of the post-transition state dynamics for the OH + CH3OH reaction probed by photodetachment of the CH3O-(H2O) anion

Dissociative photodetachment dynamics simulations were conducted to study the CH3O-(H2O) → CH3O + H2O + e- reaction using classical molecular dynamics (MD) and ring-polymer molecular dynamics (RPMD) techniques on two newly formulated neutral potential energy surfaces (PES1 and PES2) by different research groups. While the dissociation dynamics exhibited similarities between classical MD and RPMD, there were noticeable differences in the fluctuation of probability densities for the internal modes due to nuclear quantum effects. Upon comparison of our findings with experimental data concerning the electron binding energy distribution and photofragment relative energy, it suggests that the potential energy landscapes of PES2 are reasonably precise. The time evolution of occupied vibrational states of the H2O photofragment is presented in this study.

Probing the activated complex of the F + NH3 reaction via a dipole-bound state

Experimental characterization of the transition state poses a significant challenge due to its fleeting nature. Negative ion photodetachment offers a unique tool for probing transition states and their vicinity. However, this approach is usually limited to Franck-Condon regions. For example, high-lying Feshbach resonances with an excited HF stretching mode (vHF = 2-4) were recently identified in the transition-state region of the F + NH3 → HF + NH2 reaction through photo-detaching FNH3- anions, but the direct photodetachment failed to observe the lower-lying vHF = 0,1 resonances and bound states due apparently to negligible Franck-Condon factors. Indeed, these weak transitions can be resonantly enhanced via a dipole-bound state (DBS) formed between an electron and the polar FNH3 species. In this study, we unveil a series of Feshbach resonances and bound states along the F + NH3 reaction path via a DBS by combining high-resolution photoelectron spectroscopy with high-level quantum dynamical computations. This study presents an approach for probing the activated complex in a reaction by negative ion photodetachment through a DBS.

Probing the dynamics and bottleneck of the key atmospheric SO2 oxidation reaction by the hydroxyl radical

SO2 (Sulfur dioxide) is the major precursor to the production of sulfuric acid (H2SO4), contributing to acid rain and atmospheric aerosols. Sulfuric acid formed from SO2 generates light-reflecting sulfate aerosol particles in the atmosphere. This property has prompted recent geoengineering proposals to inject sulfuric acid or its precursors into the Earth's atmosphere to increase the planetary albedo to counteract global warming. SO2 oxidation in the atmosphere by the hydroxyl radical HO to form HOSO2 is a key rate-limiting step in the mechanism for forming acid rain. However, the dynamics of the HO + SO2 → HOSO2 reaction and its slow rate in the atmosphere are poorly understood to date. Herein, we use photoelectron spectroscopy of cryogenically cooled HOSO2- anion to access the neutral HOSO2 radical near the transition state of the HO + SO2 reaction. Spectroscopic and dynamic calculations are conducted on the first ab initio-based full-dimensional potential energy surface to interpret the photoelectron spectra of HOSO2- and to probe the dynamics of the HO + SO2 reaction. In addition to the finding of a unique pre-reaction complex (HO⋯SO2) directly connected to the transition state, dynamic calculations reveal that the accessible phase space for the HO + SO2 → HOSO2 reaction is extremely narrow, forming a key reaction bottleneck and slowing the reaction rate in the atmosphere, despite the low reaction barrier. This study underlines the importance of understanding the full multidimensional potential energy surface to elucidate the dynamics of complex bimolecular reactions involving polyatomic reactants.

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a high-resolution variant of anion photoelectron spectroscopy that has been applied with considerable success over the years to the study of radicals, size-selected clusters, and transition states for unimolecular and bimolecular reactions. Cryo-SEVI retains the versatility of conventional anion photoelectron spectroscopy while offering sub-meV resolution, thereby enabling the resolution of vibrational structure in the photoelectron spectra of complex anions. This Feature Article describes recent experiments in our laboratory using cryo-SEVI, including a new research direction in which anions are vibrationally pre-excited with an infrared laser pulse prior to photodetachment.

Observation of resonances in the transition state region of the F + NH3 reaction using anion photoelectron spectroscopy

The transition state of a chemical reaction is a dividing surface on the reaction potential energy surface (PES) between reactants and products and is thus of fundamental interest in understanding chemical reactivity. The transient nature of the transition state presents challenges to its experimental characterization. Transition-state spectroscopy experiments based on negative-ion photodetachment can provide a direct probe of this region of the PES, revealing the detailed vibrational structure associated with the transition state. Here we study the F + NH₃ → HF + NH₂ reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled FNH₃^- anions. Reduced-dimensionality quantum dynamical simulations performed on a global PES show excellent agreement with the experimental results, enabling the assignment of spectral structure. Our combined experimental-theoretical study reveals a manifold of vibrational Feshbach resonances in the product well of the F + NH₃ PES. At higher energies, the spectra identify features attributed to resonances localized across the transition state and into the reactant complex that may impact the bimolecular reaction dynamics.

Probing Isomerization Dynamics via a Dipole-Bound State

The observation of molecular isomerization dynamics is a long-standing goal in physical chemistry. The loosely bound electron in a dipole-bound state (DBS) can be a messenger for probing the isomerization of the neutral core. Here we study the isomerization dynamics of the salt dimer (NaCl)₂ from linear to rhombic via a DBS using cryogenic photoelectron spectroscopy in combination with ab initio calculations. Although the energy level of the DBS is below the electron affinity of the linear (NaCl)₂, (NaCl)₂^- in its DBS can autodetach due to the linear-to-rhombic isomerization. (NaCl)₂^- in the ground DBS has a relatively long lifetime of a few nanoseconds due to the quantum tunneling through a potential barrier during the transformation from linear to rhombic. In contrast, the vibrationally excited DBS has a much shorter lifetime on the order of picoseconds. The energy distribution of autodetachment electrons has an unexpected characteristic of the thermionic emission.

Strong ortho/para effects in the vibrational spectrum of Cl-(H2)

The predissociation spectrum of the Cl-35(H₂) complex is measured between 450 and 800 cm^-1 in a multipole radiofrequency ion trap at different temperatures using the FELIX infrared free electron laser. Above a certain temperature, the removal of the Cl^-(p-H₂) para nuclear spin isomer by ligand exchange to the Cl^-(o-H₂) ortho isomer is suppressed effectively, thereby making it possible to detect the spectrum of this more weakly bound complex. At trap temperatures of 30.5 and 41.5 K, we detect two vibrational bands of Cl^-(p-H₂) at 510(1) and 606(1) cm^-1. Using accurate quantum calculations, these bands are assigned to transitions to the inter-monomer vibrational modes (v₁,v₂ ^l₂ ) = (0, 2⁰) and (1, 2⁰), respectively.

On-the-Fly Ring-Polymer Molecular Dynamics Calculations of the Dissociative Photodetachment Process of the Oxalate Anion

The dissociative photodetachment dynamics of the oxalate anion, C₂O₄H^- + hν → CO₂ + HOCO + e^-, were theoretically studied using the on-the-fly path-integral and ring-polymer molecular dynamics methods, which can account for nuclear quantum effects at the density-functional theory level in order to compare with the recent experimental study using photoelectron-photofragment coincidence spectroscopy. To reduce computational time, the force acting on each bead of ring-polymer was approximately calculated from the first and second derivatives of the potential energy at the centroid position of the nuclei beads. We find that the calculated photoelectron spectrum qualitatively reproduces the experimental spectrum and that nuclear quantum effects are playing a role in determining spectral widths. The calculated coincidence spectrum is found to reasonably reproduce the experimental spectrum, indicating that a relatively large energy is partitioned into the relative kinetic energy between the CO₂ and HOCO fragments. This is because photodetachment of the parent anion leads to Franck-Condon transition to the repulsive region of the neutral potential energy surface. We also find that the dissociation dynamics are slightly different between the two isomers of the C₂O₄H^- anion with closed- and open-form structures.

Study of He*/Ne*+Ar, Kr, N2, H2, D2 Chemi-Ionization Reactions by Electron Velocity-Map Imaging

The chemi-ionization of Ar, Kr, N₂, H₂, and D₂ by Ne(³P₂) and of Ar, Kr, and N₂ by He(³S₁) was studied by electron velocity map imaging (e-VMI) in a crossed molecular beam experiment. A curved magnetic hexapole was used to state-select the metastable species. Collision energies of 60 meV were obtained by individually controlling the beam velocities of both reactants. The chemi-ionization of atoms and molecules can proceed along different channels, among them Penning ionization and associative ionization. The evolution of the reaction is influenced by the internal redistribution of energy, which happens at the first reaction step that involves the emission of an electron. We designed and built an e-VMI spectrometer in order to investigate the electron kinetic energy distribution, which is related to the internal state distribution of the ionic reaction products. The analysis of the electron kinetic energy distributions allows an estimation of the ratio between the two-reaction channel Penning and associative ionization. In the molecular cases the vibrational or electronic excitation enhanced the conversion of internal energy into the translational energy of the forming ions, thus influencing the reaction outcome.

Dissociative photodetachment of H3O2-: a full-dimensional quantum dynamics study

The transition state is a central concept of chemistry. Photoelectron-photofragment coincidence (PPC) spectroscopy has been proven as an attractive method to study the transition state dynamics. Within a state-of-the-art full-dimensional quantum mechanical model, the dissociative photodetachment dynamics of H₃O₂^- is investigated on accurate anion and neutral potential energy surfaces. The calculated PPC spectrum of H₃O₂^- agrees well with the experimental measurement. The dissociative product OH is exclusively populated on the ground vibrational state, implying the character of the spectator bond. In contrast, the product H₂O is predominantly populated in the ground and fundamental states of the symmetric and antisymmetric stretching modes, which is caused by the strong coupling between the antisymmetric motion of the transferred H atom in the transient intermediate [H₃O₂]* and both stretching modes of the product H₂O.

Complex Formation in Three-Body Reactions of Cl- with H2

Three-body reaction rates of Cl^– with H₂ to form the weakly bound complex Cl^–(H₂) are measured between 10 and 26 K in a linear radio-frequency wire trap. Formation of larger clusters of the form Cl^–(H₂)₂ are also observed. The three-body (or termolecular) rate coefficients follow the form aT^–1, with a = 1.12(2) × 10^–29 cm⁶ K s^–1. Reverse reactions to repopulate the Cl^– parent ion were measured, even though the binding energy of the complex makes bimolecular dissociative collisions energetically inaccessible at low temperatures. The back-reaction was found to be proportional to the cube of the hydrogen density, suggesting that the dissociation mechanism depends on multiple collisions. Comparisons of the rate coefficients measured in a 16-pole wire trap and a 22-pole trap demonstrate significantly lower ion temperatures in the wire trap.

Modeling the Photoelectron Angular Distributions of Molecular Anions: Roles of the Basis Set, Orbital Choice, and Geometry

A study investigating the effect of the basis set, orbital choice, and geometry on the modeling of photoelectron angular distributions (PADs) of molecular anions is presented. Experimental and modeled PADs for a number of molecular anions, including both closed- and open-shell systems, are considered. Guidelines are suggested for chemists who wish to design calculations to capture the correct chemical physics of the anisotropy of photodetachment, while balancing the computational cost associated with larger molecular anions.

Formation of Hydrogen Peroxide from O-(H2O)n Clusters

Hydrogen peroxide (H₂O₂) has recently received much attention as a safe and clean energy carrier for hydrogen molecules. In this study, based on direct ab initio molecular dynamics (AIMD) calculations, we demonstrated that H₂O₂ is directly formed via the photoelectron detachment of O^-(H₂O)_n (n = 1-6) (water clusters of an oxygen radical anion). Three electronic states of oxygen atoms were examined in the calculations: O(X)(H₂O)_n (X = ³P, ¹D, and ¹S states). After the photoelectron detachment of O^-(H₂O)_n (n = 1) to the ¹S state, a complex comprising O(¹S) and H₂O, O(¹S)-OH₂, was formed. A hydrogen atom of H₂O immediately transferred to O(¹S) during an intracluster reaction to form H₂O₂ as the final product. Simulations were run to obtain a total of 33 trajectories for n = 1 that all led to the formation of H₂O₂. The average reaction time of H₂O₂ formation was calculated to be 57.7 fs in the case of n = 1, indicating that the reaction was completed within 100 fs of electron detachment. All the reaction systems O(¹S)(H₂O)_n (n = 1-6) indicated the formation of H₂O₂ by the same mechanism. The reaction times for n = 2-6 were calculated to range between 80 and 180 fs, indicating that the reaction for n = 1 is faster than that of the larger clusters, that is, the larger the cluster size, the slower the reaction is. The reaction dynamics of the triplet O(³P) and singlet O(¹D) potential energy surfaces were calculated for comparison. All calculations yielded the dissociation product O(X)(H₂O)_n → O(X) + (H₂O)_n (X = ³P and ¹D), indicating that the O(¹S) state contributes to the formation of H₂O₂. The reaction mechanism was discussed based on the theoretical results.

Quantum calculations of the photoelectron spectra of the OH-·NH3 anion: implications for OH + NH3→ H2O + NH2 reaction dynamics

We present the results of quantum dynamics calculations for analyzing the experimentally measured photoelectron spectra of the OH^-·NH₃ anion complex. Detachment of an excess electron of OH^-·NH₃ initially produces a molecular arrangement, which is close to the transition-state structure of the neutral OH + NH₃→ H₂O + NH₂ hydrogen abstraction reaction due to the Franck-Condon principle, and thus finally leads to the OH + NH₃ or H₂O + NH₂ asymptotic channel. We used both the path integral method and the reduced-dimensionality quantum wave packet method to simulate the photoelectron spectra of the OH^-·NH₃ anion. The calculated spectra were found to be in qualitative agreement with the experimental spectra. It was found that the photodetached complex mainly dissociates into the OH + NH₃ channel; however, we found that the hydrogen exchange process also contributes to the photodetachment spectra.

Diradical Interactions in Ring-Open Isoxazole

Electron capture by the σ* LUMO of isoxazole triggers the dissociation of the O-N bond and the opening of the ring. Photodetachment of the resulting anion accesses a neutral structure, in which the O· and ·N bond fragments interact through the intact remainder of the molecular ring and via a 3 Å gap created by the bond dissociation. These through-bond and through-space interactions result in a dense manifold of diradical states, including (in the order of increasing energy) a triplet, an open-shell singlet, a closed-shell singlet, and another triplet state. We report photoelectron spectra that reflect partially resolved signatures of these states. Remarkably, the structure of the isoxazole diradical manifold is qualitatively different from that of the analogous system in oxazole. The distinct properties of the two manifolds are explained by using a coupled-fragments molecular-orbital model. Consistent with the past conclusions [Culberson et al. Phys. Chem. Chem. Phys. 2014, 16, 3964-3972], the lingering through-space interactions between the O· and ·C bond fragments in ring-open oxazole are responsible for the relative stabilization of the closed-shell singlet state, which correlates with the ground-state cyclic structure. In contrast, the placement of the N atom in the terminal position within the ring-open structure of isoxazole is the key factor leading to the near degeneracy of the π and σ* orbitals, favoring a triplet-state configuration.

Photoelectron Spectroscopy and Structures of X- ⋅⋅⋅CH2 O (X=F, Cl, Br, I) Complexes

A combined experimental and theoretical approach has been used to investigate X^- ⋅⋅⋅CH₂ O (X=F, Cl, Br, I) complexes in the gas phase. Photoelectron spectroscopy, in tandem with time-of-flight mass spectrometry, has been used to determine electron binding energies for the Cl^- ⋅⋅⋅CH₂ O, Br^- ⋅⋅⋅CH₂ O, and I^- ⋅⋅⋅CH₂ O species. Additionally, high-level CCSD(T) calculations found a C_2v minimum for these three anion complexes, with predicted electron detachment energies in excellent agreement with the experimental photoelectron spectra. F^- ⋅⋅⋅CH₂ O was also studied theoretically, with a C_s hydrogen-bonded complex found to be the global minimum. Calculations extended to neutral X⋅⋅⋅CH₂ O complexes, with the results of potential interest to atmospheric CH₂ O chemistry.

Threshold photodetachment spectroscopy of the astrochemical anion CN. J Chem Phys 2020;153:184309. [PMID: 33187436 DOI: 10.1063/5.0029841]

Threshold photodetachment spectroscopy has been performed on the molecular anion CN^- at both 16(1) K and 295(2) K in a 22-pole ion trap and at 295(2) K from a pulsed ion beam. The spectra show a typical energy dependence of the detachment cross section yielding a determination of the electron affinity of CN to greater precision than has previously been known at 31 163(16) cm^-1 [3.864(2) eV]. Allowed s-wave detachment is observed for CN^-, but the dependence of the photodetachment cross section near the threshold is perturbed by the long-range interaction between the permanent dipole moment of CN and the outgoing electron. Furthermore, we observe a temperature dependence of the cross section near the threshold, which we attribute to a reduction of the effective permanent dipole due to higher rotational excitation at higher temperatures.

Franck–Condon simulations of transition-state spectra for the OH + H2O and OD + D2O reactions

Quantum wave packet calculations in reduced dimensions were performed to analyze the experimentally measured transition-state spectra of the OH + H₂O and OD + D₂O hydrogen exchange reactions.

Quantum dynamics analysis of transition-state spectrum for the SH + H2S → H2S + SH reaction

Quantum dynamics calculations were performed to understand transition-state spectroscopy of the SH + H₂S hydrogen atom transfer reaction.

Double Photodetachment of F-·H2O: Experimental and Theoretical Studies of [F·H2O]. J Phys Chem Lett 2018;9:6808-6813. [PMID: 30433784 DOI: 10.1021/acs.jpclett.8b02562]

Double photodetachment of the cluster F^-·H₂O in a strong laser field is explored in a combined experimental-theoretical study. Products are observed experimentally by coincidence photofragment imaging following double ionization by intense laser pulses. Theoretically, equation of motion coupled cluster calculations (EOM-CC), suitable for modeling strong correlation effects in the electronic wave function, shed light on the Franck-Condon region, and ab initio molecular dynamics simulations also performed using EOM-CC methods reveal the fragmentation dynamics in time on the lowest-lying singlet and triplet states of [F·H₂O]⁺. The simulations show the formation of H₂O⁺ + F, which is the predominant experimentally observed product channel. Suggestions are proposed for the formation mechanisms of the minor products, for example, the very interesting H₂F⁺, which involves significant geometrical rearrangement. Analysis of the results suggests interesting future directions for the exploration of photodetachment of anionic clusters in an intense laser field.

Quantum dynamics of ClH2O- photodetachment: Isotope effect and impact of anion vibrational excitation

Photodetachment of the ClH₂O^- anion is investigated using full-dimensional quantum mechanics on accurate potential energy surfaces of both the anion and neutral species. Detailed analysis of the photoelectron spectrum and the corresponding wavefunctions reveals that the photodetachment leads to, in the product channel of the exothermic HCl + OH → Cl + H₂O reaction, the formation of numerous Feshbach resonances due apparently to slow energy transfer from H₂O vibrational modes to the dissociation coordinate. These long-lived resonances can be grouped into two broad peaks in the low-resolution photoelectron spectrum, which is in good agreement with available experiments, and they are assigned to the ground and first excited OH stretching vibrational manifolds of H₂O complexed with Cl. In addition, effects of isotope substitution on the photoelectron spectrum were small. Finally, photodetachment of the vibrationally excited ClH₂O^- in the ionic hydrogen bond mode is found to lead to Feshbach resonances with higher stretching vibrational excitations in H₂O.

Slow Photoelectron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled anions (cryo-SEVI) is a powerful technique for elucidating the vibrational and electronic structure of neutral radicals, clusters, and reaction transition states. SEVI is a high-resolution variant of anion photoelectron spectroscopy based on photoelectron imaging that yields spectra with energy resolution as high as 1-2 cm^-1. The preparation of cryogenically cold anions largely eliminates hot bands and dramatically narrows the rotational envelopes of spectral features, enabling the acquisition of well-resolved photoelectron spectra for complex and spectroscopically challenging species. We review the basis and history of the SEVI method, including recent experimental developments that have improved its resolution and versatility. We then survey recent SEVI studies to demonstrate the utility of this technique in the spectroscopy of aromatic radicals, metal and metal oxide clusters, nonadiabatic interactions between excited states of small molecules, and transition states of benchmark bimolecular reactions.

Dynamical resonances in chemical reactions

The transition state is a key concept in the field of chemistry and is important in the study of chemical kinetics and reaction dynamics.

Partial Potential Energy Surfaces and Their Application to Reaction Resonances

Feshbach resonances are not restricted to small reactive systems such as F + H2 and I + HI but can be found in many reactive systems. In this paper, the concept of the partial potential energy surface (PPES) is introduced. It is shown that the dynamic “Lake Eyring” explains very well the existence of reactive resonances in elementary chemical reactions. In particular, the PPESs of the Cl + CH3CH2Br and Cl + CH3CH2CH2Br systems, including the minimum energy path and the vibrational potential energy curves, were constructed using quantum chemistry methods. Based on the constructed PPESs, the scattering resonance states of these reactions could be examined and the resonance state lifetimes were estimated.

Effects of vibrational excitation on the F + H2O → HF + OH reaction: dissociative photodetachment of overtone-excited [F-H-OH]. Chem Sci 2017;8:7821-7833. [PMID: 29163919 PMCID: PMC5674243 DOI: 10.1039/c7sc03364h]

Photodetaching vibrationally excited FH₂O^– channels energy into the reaction coordinate of the F + H₂O reaction, as shown in this joint experimental-theoretical study.

The reaction F + H₂O → HF + OH is a four-atom system that provides an important benchmark for reaction dynamics. Hydrogen atom transfer at the transition state for this reaction is expected to exhibit a strong dependence on reactant vibrational excitation. In the present study, the vibrational effects are examined by photodetachment of vibrationally excited F^–(H₂O) precursor anions using photoelectron-photofragment coincidence (PPC) spectroscopy and compared with full six-dimensional quantum dynamical calculations on ab initio potential energy surfaces. Prior to photodetachment at hν_UV = 4.80 eV, the overtone of the ionic hydrogen bond mode in the precursor F^–(H₂O), 2ν_IHB at 2885 cm^–1, was excited using a tunable IR laser. Experiment and theory show that vibrational energy in the anion can be effectively carried away by the photoelectron upon a Franck–Condon photodetachment, and also show evidence for an increase of branching into the F + H₂O reactant channel. The experimental results suggest a greater role for product rotational excitation than theory. Improved potential energy surfaces and longer wavepacket propagation times would be helpful to further examine the nature of the discrepancy.

A review of dynamical resonances in A + BC chemical reactions

The concept of the transition state has played an important role in the field of chemical kinetics and reaction dynamics. Reactive resonances in the transition-state region can dramatically enhance the reaction probability; thus investigation of the reactive resonances has attracted great attention from chemical physicists for many decades. In this review, we mainly focus on the recent progress made in probing the elusive resonance phenomenon in the simple A  +  BC reaction and understanding its nature, especially in the benchmark F/Cl  +  H₂ and their isotopic variants. The signatures of reactive resonances in the integral cross section, differential cross section (DCS), forward- and backward-scattered DCS, and anion photodetachment spectroscopy are comprehensively presented in individual prototype reactions. The dynamical origins of reactive resonances are also discussed in this review, based on information on the wave function in the transition-state region obtained by time-dependent quantum wave-packet calculations.

Nonadiabatic quantum dynamics calculations of transition state spectroscopy of I + HI and I + DI reactions: the existence of long life vibrational bonding resonances

Nonadiabatic quantum dynamics calculations were performed to understand the transition state spectroscopy of I + HI and I + DI reactions.

Dynamics of transient speciesviaanion photodetachment

Recent experimental and theoretical advances in transient reaction dynamics probed by photodetachment of polyatomic anions are reviewed.

HOCCO versus OCCO: Comparative spectroscopy of the radical and diradical reactive intermediates

We present a photoelectron imaging study of three glyoxal derivatives: the ethylenedione anion (OCCO(-)), ethynediolide (HOCCO(-)), and glyoxalide (OHCCO(-)). These anions provide access to the corresponding neutral reactive intermediates: the OCCO diradical and the HOCCO and OHCCO radicals. Contrasting the straightforward deprotonation pathway in the reaction of O(-) with glyoxal (OHCCHO), which is expected to yield glyoxalide (OHCCO(-)), OHCCO(-) is shown to be a minor product, with HOCCO(-) being the dominant observed isomer of the m/z = 57 anion. In the HOCCO/OHCCO anion photoelectron spectrum, we identify several electronic states of this radical system and determine the adiabatic electron affinity of HOCCO as 1.763(6) eV. This result is compared to the corresponding 1.936(8) eV value for ethylenedione (OCCO), reported in our recent study of this transient diradical [A. R. Dixon, T. Xue, and A. Sanov, Angew. Chem., Int. Ed. 54, 8764-8767 (2015)]. Based on the comparison of the HOCCO(-)/OHCCO(-) and OCCO(-) photoelectron spectra, we discuss the contrasting effects of the hydrogen connected to the carbon framework or the terminal oxygen in OCCO.

Photoelectron wave function in photoionization: plane wave or Coulomb wave?

The calculation of absolute total cross sections requires accurate wave functions of the photoelectron and of the initial and final states of the system. The essential information contained in the latter two can be condensed into a Dyson orbital. We employ correlated Dyson orbitals and test approximate treatments of the photoelectron wave function, that is, plane and Coulomb waves, by comparing computed and experimental photoionization and photodetachment spectra. We find that in anions, a plane wave treatment of the photoelectron provides a good description of photodetachment spectra. For photoionization of neutral atoms or molecules with one heavy atom, the photoelectron wave function must be treated as a Coulomb wave to account for the interaction of the photoelectron with the +1 charge of the ionized core. For larger molecules, the best agreement with experiment is often achieved by using a Coulomb wave with a partial (effective) charge smaller than unity. This likely derives from the fact that the effective charge at the centroid of the Dyson orbital, which serves as the origin of the spherical wave expansion, is smaller than the total charge of a polyatomic cation. The results suggest that accurate molecular photoionization cross sections can be computed with a modified central potential model that accounts for the nonspherical charge distribution of the core by adjusting the charge in the center of the expansion.