1B2 State Research Articles

High-level studies of the ionic states of norbornadiene and quadricyclane, including analysis of new experimental photoelectron spectra by configuration interaction and coupled cluster calculations.

Synchrotron-based photoelectron spectra (PES) of norbornadiene (NBD) and quadricyclane (QC) differ significantly from those in previous studies. The adiabatic ionization energy (AIE1) for NBD, assigned to the 2B1 state at 8.279 eV, shows a progression of 18 members with decreasing vibration frequency from 390 cm-1 to 340 cm-1; our calculated frequency is 381 cm-1. Similarly, the AIE1 for QC at 7.671 eV, assigned to the 2B2 state, discloses a vibrational progression of nine or more members with vibration frequency decreasing from 703 cm-1 to 660 cm-1; our calculated vibration frequency is 663 cm-1. These AIEs, determined by coupled cluster and fourth order Møller-Plesset perturbation theory, were very similar to the corresponding second order perturbation theory results. The calculated AIE symmetry sequences are 2B1 < 2A1 < 2A2 < 2B2 for NBD and 2B2 < 2A2 < 2B1 < 2A1 for QC. The overall PES vertical ionization energy profiles for both compounds were closely reproduced by Tamm-Dancoff approximation energies and intensities. The vibrational structure of the ionic states, determined using Franck-Condon methods, gave a good account of the observed spectra, but the observed envelopes for both IE1 are complex sets of vibrations, rather than single progressions. The NMR spectra for QC showed residual second order properties at 300 MHz; both QC and NBD have been theoretically analyzed in greater detail using AA/BB/CC/XX/ spectra, where all H are coupled; the magnetic shielding and spin-spin coupling constants obtained are similar to experimental values.

Vibronic coupling in the ground and excited states of the pyridine radical cation.

Vibronic interactions in the pyridine radical cation ground state, 2A1, and its lowest excited states, 2A2 and 2B1, are studied theoretically. These states originate from the ionization out of the highest occupied orbitals of pyridine, 7a1 (nσ), 1a2 (π), and 2b1 (π), respectively, and give rise to the lowest two photoelectron maxima. According to our previous high-level ab initio calculations [Trofimov et al., J. Chem. Phys. 146, 244307 (2017)], the 2A2 (π-1) excited state is very close in energy to the 2A1 (nσ-1) ground state, which suggests that these states could be vibronically coupled. Our present calculations confirm that this is indeed the case. Moreover, the next higher excited state, 2B1 (π-1), is also involved in the vibronic interaction with the 2A1 (nσ-1) and 2A2 (π-1) states. The three-state vibronic coupling problem was treated within the framework of a linear vibronic coupling model employing parameters derived from the ionization energies of pyridine computed using the linear response coupled-cluster method accounting for single, double, and triple excitations (CC3). The potential energy surfaces of the 2A1 and 2A2 states intersect in the vicinity of the adiabatic minimum of the 2A2 state, while the surfaces of the 2A2 and 2B1 states intersect near the 2B1 state minimum. The spectrum computed using the multi-configuration time-dependent Hartree (MCTDH) method accounting for 24 normal modes is in good qualitative agreement with the experimental spectrum of pyridine obtained using high-resolution He I photoelectron spectroscopy and allows for some assignment of the observed features.

LDA+Usc calculations of phase relations in FeO

Using the LDA+$\textit{U}_\text{sc}$ method, we present calculations phase relations of iron monoxides involving five polytypes in multiple spin-state configurations. The Hubbard parameter $U$ is determined self-consistently simultaneously with the occupation matrix and structures at arbitrary pressures. The Hubbard parameter strongly depends on pressure, structure, and spin state. Comparison with experimental structural data indicates the LDA+$\textit{U}_\text{sc}$ can predict structure, compression curves, phase relations, and transition pressures very well for the insulating B1 and iB8 states. However, it requires additional calculations using the Mermin functional that includes the electronic entropic contribution to the free energy to obtain an nB8 metallic state and a consistent iB8 to nB8 insulator-to-metal transition pressure.

Electronic excitation of the 3B2 state of thiophene molecule by low-energy electron collisions

The Schwinger multichannel method is employed to study elastic and electronically inelastic collisions of low-energy electrons by the thiophene molecule. It is well known that the elastic integral cross section for electron-thiophene scattering presents three shape resonances, some of which located around the opening region of the first excited state. The main purpose of this work is to evaluate the influence of polarization effects on the positions of the resonances appearing in the elastic channel and its potential impact on the description of the electronic excitation process involving the transition from ground state to the 3B2 excited state of thiophene. Our cross section results indicate that target polarization may affect the electronic excitation process in the region of the threshold of the 3B2 excited state, but its impact seems to be related to the effect of polarization on the resonances in the elastic channel. More specifically, there are cases (A2 and B1 symmetries) where the overlap of the shape resonance with the threshold changes significantly with the inclusion of polarization resulting in a considerable modification in the magnitude of the electronically inelastic cross section, while in other case (B2 symmetry) a similar overlap is not accompanied by sensible changes in the magnitude of the excitation cross section. Our excitation functions at scattering angles of 90° and 135° are compared with the calculations and measurements of Loupas et al (2018 J. Phys. Chem. A 122 1146) and show in general a qualitative agreement.

High resolution vibronic state-specific dissociation of NO2+ in the 10.0-15.5 eV energy range by synchrotron double imaging photoelectron photoion coincidence.

Vacuum ultraviolet (VUV) photoionization and dissociative photoionization of NO2 in the 10.0-15.5 eV energy range have been investigated in detail by using high-resolution double imaging photoelectron photoion coincidence (i2PEPICO) at synchrotron SOLEIL. Five low-lying electronic states of the NO2+ cation, X1Σg+, a3B2, b3A2, A1A2 and B1B2, are prepared with well-resolved vibronic structures and their state-specific dissociation mechanisms are unraveled and discussed. The present experimental results clarify that except the X1Σg+ ground electronic state and the first three vibrational levels of the a3B2 electronic state, the other cationic states of NO2+ within the present energy range are totally dissociative towards the NO+(X1Σ+) + O(3P) and/or NO+(X1Σ+) + O(1D) dissociation limits. An energy barrier exists along the direct dissociation route of the a3B2 state, and the b3A2 electronic state is a quasi-bound state with a very shallow well, both of which adiabatically correlate to the NO+(X1Σ+) + O(3P) dissociation limit. The a3B2 state mainly with bending vibration excitations undergoes a non-adiabatic transition to the 23A''(3B1) repulsive state along its bending potential energy curve and then predissociates into the NO+(X1Σ+) + O(3P) products. Our experimental results firstly demonstrate that the NO+(X1Σ+, v) fragment ions produced from individual vibronic levels of the dissociative NO2+(a3B2, b3A2) states are produced at the v = 0 ground vibrational level with a high rotational population due to the excitation of the vibrational bending mode of NO2+ and the associated imparted torque upon dissociation. The slower predissociations of the A1A2 and B1B2 electronic states via their spin-orbit couplings with the repulsive 23A''(3B1) state produce the NO+(X1Σ+) and O(3P) fragments with a long vibrational progression. In addition, the B1B2 state can also undergo a radiationless transition such as internal conversion into the hot X1Σg+ state and then dissociate into the second dissociation channel correlated to the NO+(X1Σ+) and O(1D) products.

Ground and Low-Lying Excited States of NbC3–/0 Clusters: Assignment of the Anion Photoelectron Spectra from Multiconfigurational Calculations

The BP86 DFT, CCSD(T), and CASPT2 methods were employed to study the low-lying states of NbC3-/0 clusters. The anionic and neutral ground states were determined to be the 1A1 and 12A1 of the cyclic-NbC3-/0 isomers. All bands in the photoelectron spectra of the NbC3- cluster were interpreted based on electron detachment processes from the 1A1 of the cyclic-NbC3- isomer. The X, A, B, and C bands in the spectra are, respectively, ascribed to the transitions from 1A1 to the 12A1, 2B1, 22A1, and 2B2 states. The wave function of the initial 1A1 anionic state exhibits a pronounced multireference character that plays to some extent a role in the 1A1 → 12A1 and the 1A1 → 22A1 ionizations. Only the 1A1 → 2B2 transition appears as a clear one-electron detachment process. The Franck-Condon factor simulations for the 1A1 → 12A1 and 1A1 → 2B1 transitions are consistent with the observed vibrational progressions of the X and A bands in the spectra.

The role of open-volume defects in the annihilation of antisites in a B2-ordered alloy

The atomic arrangement in certain ordered alloys, such as B2–Fe60Al40 determines intrinsic material properties for instance, the saturation magnetization. Here we have investigated the influence of open-volume defects on the atomic ordering process at elevated temperatures in Fe60Al40 thin films. A dependence of the ordering process on the type and concentration of defects is observed by positron annihilation spectroscopy combined with ab-initio calculations. Comparing the lifetimes of positrons in the alloy for different annealing and irradiation treatments reveals the role of mono-vacancies, triple defects as well as large vacancy clusters: The rate of atomic ordering to the ordered B2 state is increased in the presence of mono-vacancies whereas triple defects and vacancy complexes decrease the ordering rate. Furthermore, an agglomeration of vacancies during annealing to di-vacancies and larger vacancy clusters is observed. The distribution of open-volume defects can be modified in such a way as to control the thermal stability via ion-irradiation and thermal pre-treatments.

Theoretical Study on the Spectroscopic Observation of Intersystem Crossing between 3B1 and 1A1 States of GeH2 Using the GeH2- (2B1) Anion.

We performed nonadiabatic quantum wave packet dynamics calculations to simulate the photodetachment spectrum of the GeH2- (2B1) anion. We developed the (4 × 4) diabatic potential energy surfaces to describe the intersystem crossing transitions between the neutral 1A1 and 3B1 states induced by spin-orbit interactions based on ab initio calculations. The spin-orbit coupling matrix elements were calculated using the Breit-Pauli Hamiltonian with the spin-free states obtained from the multireference configuration interaction method. The calculated photodetachment spectrum showed many intense peaks that could be assigned to the vibrational states mostly associated with the pure singlet or triplet spin states. However, we also found weak satellite peaks that could be assigned to vibrational states consisting of the highly excited vibrational state on the singlet surface and the low-lying vibrational state on the triplet surface.

Measurements of ultrafast dissociation in resonant inelastic x-ray scattering of water.

There has been a discussion on the interpretation of the resonant inelastic x-ray scattering (RIXS) spectra of liquid water in terms of either different structural environments or that core hole dynamics can generate well-resolved dissociative spectral components. We have used RIXS with high resolution in the OH stretch vibration energy part, at extremely high overtones going toward the continuum of full OH bond breakage, to identify the amount of dissociative contributions in the valence band RIXS spectra at different excitation energies. We observe that at low excitation energies, corresponding to population of states with strongly antibonding character, the valence band RIXS spectra have a large contribution from a well-resolved dissociative feature. Instead, at higher excitations, this spectral component diminishes and becomes a weak structure on the high-energy side of one of the spectral peaks related to the 1b1 state from tetrahedral configurations. This result brings both interpretations to be essential for the understanding of RIXS spectra of liquid water.

Ion-Pair Formation in Neutral Potassium-Neutral Pyrimidine Collisions: Electron Transfer Experiments.

We report novel data on ion-pair formation in hyperthermal (30–800 eV) neutral potassium collisions with neutral pyrimidine (Pyr, C4H4N2) molecules. In this collision regime, negative ions formed by electron transfer from the alkali atom to the target molecule were time-of-flight mass analyzed and the fragmentation patterns and branching ratios have been obtained. The most abundant product anions have been assigned to CN− and C2H− and the electron transfer mechanisms are comprehensively discussed. Particular importance is also given to the efficient loss of integrity of the pyrimidine ring in the presence of an extra electron, which is in contrast to dissociative electron attachment experiments yielding the dehydrogenated parent anion. Theoretical calculations were performed for pyrimidine in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process. In order to further our knowledge about the collision dynamics, potassium cation (K+) energy loss spectrum has been obtained and within this context, we also discuss the role of the accessible electronic states. A vertical electron affinity of (−5.69 ± 0.20) eV was obtained and may be assigned to a (b1) state that leads to CN− formation.

Accurate characterization of the lowest triplet potential energy surface of SO2 with a coupled cluster method.

The near-equilibrium potential energy surface (PES) of the ã 3B1 state of SO2 is developed from explicitly correlated spin-unrestricted coupled cluster calculations with single, double, and perturbative triple excitations with an augmented triple-zeta correlation-consistent basis set. The lowest-lying ro-vibrational energy levels of several sulfur isotopologues have been determined using this PES. It is shown that the new ab initio PES provides a much better description of the low-lying vibrational states than a previous PES determined at the multi-reference configuration interaction level. In particular, the theory-experiment agreement for the three lowest-lying vibrational transitions is within 1-3 cm-1.

Gas Phase Oxidation of Carbon Monoxide by Sulfur Dioxide Radical Cation: Reaction Dynamics and Kinetic Trend With the Temperature.

Gas phase ion chemistry has fundamental and applicative purposes since it allows the study of the chemical processes in a solvent free environment and represents models for reactions occurring in the space at low and high temperatures. In this work the ion-molecule reaction of sulfur dioxide ion with carbon monoxide CO is investigated in a joint experimental and theoretical study. The reaction is a fast and exothermic chemical oxidation of CO into more stable CO2 by a metal free species, as , excited into ro-vibrational levels of the electronic ground state by synchrotron radiation. The results show that the reaction is hampered by the enhancement of internal energy of sulfur dioxide ion and the only ionic product is SO.+. The theoretical approach of variational transition state theory (VTST) based on density functional electronic structure calculations, shows an interesting and peculiar reaction dynamics of the interacting system along the reaction path. Two energy minima corresponding to [SO2–CO].+ and [OS–OCO].+ complexes are identified. These minima are separated by an intersystem crossing barrier which couples the bent 3B2 state of CO2 with C2v symmetry and the 1A1 state with linear D∞h symmetry. The spin and charge reorganization along the minimum energy path (MEP) are analyzed and eventually the charge and spin remain allocated to the SO.+ moiety and the stable CO2 molecule is easily produced. There is no bottleneck that slows down the reaction and the values of the rate coefficient k at different temperatures are calculated with capture theory. A value of 2.95 × 10−10 cm3s−1molecule−1 is obtained at 300 K in agreement with the literature experimental measurement of 3.00 × 10−10 ± 20% cm3s−1molecule−1, and a negative trend with temperature is predicted consistently with the experimental observations.

Potential Energy Surface of Group 11 Trimers (Cu, Ag, Au): Bond Angle Isomerism in Au3.

Potential energy surfaces for the group 11 trimers were generated at various levels of coupled-cluster theory to examine the effects of Jahn-Teller distortions. Our calculations show that the lowest-energy conformer for Cu3, Ag3, and Au3 is the 2B2 (∼65° isomer) without spin-orbit corrections. Spin-orbit corrections have negligible contributions to the relative energies for the angle dependence of the potential energy surfaces for Cu3 and Ag3. The inclusion of spin-orbit corrections for Au3 makes the 2B2 (∼65°) and 2A1 (∼55°) states approximately degenerate. A novel 2B2 isomer of Au3 at an obtuse angle of ∼125° was also characterized, providing evidence for bond angle isomerism on the same 2B2 potential energy surface. Spin-orbit corrections increase the barrier height between the 2B2 (65°) and 2B2 (125°) bond angle isomers of Au3. The calculated symmetric stretch vibrational frequencies are in good agreement with the available experimental values. All frequencies calculated for the Au3 2B2 (∼125°) state are real, and there is at least one bound bending vibration for this state. Jahn-Teller parameters are also derived for each trimer.

Ultrafast decay dynamics of water molecules excited to electronic D[combining tilde]' and D[combining tilde]'' states: a time-resolved photoelectron spectroscopy study.

The ultrafast decay dynamics of water molecules excited to D[combining tilde]'1B1 and D[combining tilde]''1A2 states is studied by combining two-photon excitation and time-resolved photoelectron imaging methods. The lifetime of the D[combining tilde]'1B1(000) state of H2O (D2O) is determined to be 1.54 ± 0.1 (22.6 ± 1.6) ps, consistent with a previous high-resolution spectroscopic study. The H2O D[combining tilde]''1A2(000) state decays with a lifetime of 4.1 ± 0.2 ps, while in the D2O D[combining tilde]''1A2(000) state, two independent decay pathways are observed, with time constants of 0.55 ± 0.1 and 13 ± 1 ps, respectively. The former is proposed to be associated with a hitherto undocumented D[combining tilde]'' → C[combining tilde] pathway, induced by Coriolis interaction.

Correspondence between electronic structure calculations and simulations: nonadiabatic dynamics in CS2.

The choice of ab initio electronic structure method is an important factor in determining the fidelity of nonadiabatic dynamics simulations. We present an in-depth comparison of two simulations of photodissociation in the CS2 molecule following excitation to the 11B2 state. The simulations account for nonadiabatic and spin-orbit coupling, and are performed using the SHARC surface-hopping approach combined with state-averaged SA8-CASSCF(8,6)/SVP and SA8-CASSCF(10,8)/SVP ab initio calculations, with additional reference calculations at the MRCI(14,10)/aug-cc-pvTZ level. The relative performance and veracity of the simulations can be assessed by inspection of the potential energy curves along specific coordinates. The simulations demonstrate direct competition between internal conversion and intersystem crossing, with strong correlation between molecular geometry, electronic state density, and dynamics.

Mechanism of the Chemiluminescent Reaction between Nitric Oxide and Ozone.

The gas phase reaction of nitric oxide with ozone to give chemiluminescence is used extensively for detection of nitrogen oxides. The molecular mechanism of chemiluminescence in this reaction is not known. So far, the only chemiluminescent systems studied in depth are certain cycloperoxides, which emit light following decomposition. Given our understanding of the mechanism of chemiluminescence in those molecules, one would expect by extension that in the NO + O3 reaction the chemiluminescent species (NO2 in this case) is formed in the excited state through a reaction pathway that diverges from the ground state pathway near the transition state. A systematic search for such a pathway leads us to conclude that such a mechanism is unlikely. Instead, our study suggests that chemiluminescence in the NO + O3 reaction is due to emission from the NO2 vibronic states associated with the ground (X̃ 2A1) and first excited (à 2B2) electronic states, which are populated in the nascent NO2 produced in the reaction. The vibronic coupling between the X̃ 2A1 and à 2B2 states of NO2 is due to a conical intersection (CI), which is geometrically and energetically close to the à 2B2 minimum energy geometry and only 1.3 eV higher than ground state NO2. Further, the CI is 1.2 eV lower than the energy of the NO + O3 reactants and therefore thermodynamically accessible following the reaction. An analysis of the product energy distribution indicates that the major fraction of the reaction energy is channeled into the vibrational modes of NO2, sufficient to populate the vibronic states of NO2 around the X̃/à CI. These vibronic states show dipole-allowed emission in a frequency range that is consistent with the observed broad chemiluminescence spectrum.

Ultrafast Excited-State Dynamics of Ligand-Field and Ligand-to-Metal Charge-Transfer States of CuCl42- in Solution: A Detailed Transient Absorption Study.

Ultrafast excited-state dynamics of CuCl42- in acetonitrile is studied by femtosecond broadband transient absorption spectroscopy following excitation of the complex into all ligand-field (LF or d-d) states and into the two ligand-to-metal charge transfer (LMCT) states corresponding to the most intense steady-state absorption bands. The LF excited states are found to be nonreactive. The lowest-lying 2E LF excited state has a lifetime less than 150 fs, and the lifetimes of the second (2B1) and the third (2A1) LF excited states are 1 and 5 ps, respectively. All three LF states decay directly into the ground 2B2 state. Such significant differences in excited-state decay time constants were rationalized computationally through time-dependent density functional theory (TD-DFT) computations. TD-DFT mapping of the relaxation pathway along the symmetric Cl-Cu-Cl umbrella bending vibration gives evidence for a conical intersection between the 2E excited state and the ground 2B2 state. The LMCT states decay within 200 fs with the primary deactivation mode consistent to be Cu-Cl stretch. A fraction of the CuCl42- complexes excited into the LMCT states undergoes ionic dissociation to form products that survive longer than 1 ns. The remaining fraction undergoes internal conversion, which can be viewed as back electron transfer, populating the lower vibrationally hot LF states. The LF states populated from the LMCT states exhibit the same lifetimes as the Franck-Condon LF states and likewise decay directly into the ground state.

Time-Resolved Photoelectron Studies of Thiophene and 2,5-Dimethylthiophene.

The photoinduced dynamics of thiophene and 2,5-dimethylthiophene (2,5-DMT) were investigated upon excitation at 200 and 255 nm (2,5-DMT only) using time-resolved photoelectron spectroscopy and compared with results from ab initio coupled cluster calculations. For thiophene, depopulation of the initially excited B2(π3π4*) state to the lower-lying A1(π2π4*) state occurs within 25 ± 20 fs, with a subsequent bifurcation into a ring-puckering channel and a ring-opening channel with lifetimes of 80 ± 20 and 450 ± 50 fs, respectively. For 2,5-DMT, the dynamics following excitation at 200 nm is described by a monoexponential decay with a time constant of 120 ± 20 fs, while that following excitation at 255 nm is best fit by a biexponential decay with time constants of 115 ± 20 fs and 15 ± 3 ps, respectively. The fast signal observed after excitation of 2,5-DMT is assigned to the ring-opening channel, which is favored with respect to thiophene due to a lower excited-state barrier along the ring-opening coordinate and an increased inertia toward the ring-puckering channel. Coupled cluster calculations have been undertaken to compare the relaxation dynamics of thiophene to thiazole and isothiazole. For the latter two molecules, we find a strong gradient along the ring-opening coordinate in the Franck-Condon region of the initially populated ππ* state and predict that ring-opening is the dominating relaxation channel after photoexcitation. We use the extracted information for a comparison of the thiophene dynamics with the light-induced processes observed in other five-membered heterocyclic molecules.

Phenylnitrene Radical Cation Rearrangements.

The electronic structure and the rearrangements of the phenylnitrene radical cation C6H5N.+ 2.+ have been investigated at DFT and CASPT2(7,9) levels of theory. The 2B2 state has the lowest energy of five identified electronic states, and it can undergo ring expansion to the 1-azacycloheptetetraene radical cation 4.+ with an activation energy of ca. 28 kcal/mol. Ring opening and recyclization provide a route to 5-cyanocyclopentadiene radical cation 8.+, which may undergo facile 1,5-hydrogen shifts. The 2-, 3-, and 4-pyridylcarbene radical cations 31.+, 35.+ , and 39.+ interconvert with the phenylnitrene radical cation via azacycloheptatetraenes with activation barriers <35 kcal/mol. The carbene-carbene and carbene-nitrene rearrangements, ring expansions, ring contractions, ring openings (e.g., to cyanopentadienylidene 28.+), and cyclizations taking place in all these radical cations are completely analogous to the thermal and photochemical rearrangements.

An experimental and theoretical study of the photoelectron spectra of cis-dichloroethene: Valence shell vertical ionization and vibronic coupling in the low-lying cationic states.

The valence shell photoelectron spectrum of cis-dichloroethene has been studied both experimentally and theoretically. Photoelectron spectra have been recorded with horizontally and vertically plane polarized synchrotron radiation, thereby allowing the anisotropy parameters, characterizing the angular distributions, to be determined. The third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green's function has been employed to compute the complete valence shell ionization spectrum. In addition, the vertical ionization energies have been calculated using the outer valence Green's function (OVGF) method and the equation-of-motion coupled-cluster, with single and double substitutions for calculating ionization potentials (EOM-IP-CCSD) model. The theoretical results have enabled assignments to be proposed for most of the structure observed in the experimental spectra, including the inner-valence regions dominated by satellite states. The linear vibronic coupling model has been employed to study the vibrational structure of the lowest photoelectron bands, using parameters obtained from ab initio calculations. The ground state optimized geometries and vibrational frequencies have been computed at the level of the second-order Møller-Plesset perturbation theory, and the dependence of the ionization energies on the nuclear configuration has been evaluated using the OVGF method. While the adiabatic approximation holds for the 2B1 state photoelectron band, the 2B2, 2A1, and 2A2 states interact vibronically and form a complex photoelectron band system with four distinct maxima. The 2B1 and 2B2 states also interact vibronically with each other. The potential energy surface of the 2B1 state is predicted to have a double-minimum shape with respect to the out-of-plane a2 deformations of the molecular structure. The single photoelectron band resulting from this interaction is characterized by a highly irregular structure, reflecting the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces forming a conical intersection close to the minimum of the 2B2 state.