Close-lying Electronic States Research Articles

Impact of Cavity on Molecular Ionization Spectra.

Ionization phenomena have been widely studied for decades. With the advent of cavity technology, the question arises how quantum light affects molecular ionization. As the ionization spectrum is recorded from the neutral ground state, it is usually possible to choose cavities which exert negligible effect on the neutral ground state, but have significant impact on the ion and the ionization spectrum. Particularly interesting are cases where the ion exhibits conical intersections between close-lying electronic states, which gives rise to substantial nonadiabatic effects. Assuming single-molecule strong coupling, we demonstrate that vibrational modes irrelevant in the absence of a cavity play a decisive role when the molecule is in the cavity. Here, dynamical symmetry breaking is responsible for the ion-cavity coupling and high symmetry enables control of the coupling via molecular orientation relative to the cavity field polarization. Significant impact on the spectrum by the cavity is found and shown to even substantially increase for less symmetric molecules.

Dissociation Dynamics of Anionic Nitrogen Dioxide in the Low-Lying Resonant States.

Experimental studies of the dynamics near the molecular dissociation threshold are frequently frustrated, due to the small cross sections and the demand for identification of close-lying electronic states or nuclear motions with multiple degrees of freedom. Using the high-resolution anion velocity map imaging technique, here we report a dynamics study of the dissociations of anionic nitrogen dioxide in low-lying resonant states formed by electron attachment [e- + NO2 (X2A1) → NO2- → NO (X2Π) + O- (2P)]. The long-term puzzling issues about the near-threshold dissociations of NO2- are settled. We suggest that three low-lying resonant states (1B1, 3B1, and 3B2) of NO2- contribute to the production of O- at attachment energies of <5 eV. Furthermore, B1 and B2 symmetries of the resonant states lead to different anisotropies of the O- angular distribution. At the relatively high electron attachment energies, a prompt fragmentation in the molecular bent conformation competes with an indirect dissociation in the straightened conformation.

Electronic Substitution Effect on the Ground and Excited State Properties of Indole Chromophore: A Computational Study.

Indole, being the main chromophore of amino acid tryptophan and several other biologically relevant molecules like serotonin, melatonin, has prompted considerable theoretical and experimental interest. The current work focuses on the investigation of substitution effect on the ground and excited electronic states of indole using computational quantum chemistry. Having three close-lying excited electronic states, the vibronic coupling effect becomes extremely important yet challenging for the photophysics and photochemistry of indole. Here, we have evaluated the performance of time-dependent density functional theory against available experimental and ab initio results from the literature. The electronic effects on the excited states of indole and indole derivatives e. g. tryptophan, serotonin and melatonin are reported. A bathochromic shift has been observed in the absorption spectrum for the La state. The absorption wavelength increases in the order of indole<tryptophan <serotonin <melatonin. While the contribution of the in-plane small adjacent groups increases the electron density of the indole ring, the out-of-plane long substituent groups have minor effect. The absorption spectra calculated including the vibronic coupling are in good agreement with experiments. These results can be used to estimate the error in photophysical observables of indole derivatives calculated considering indole as a prototypical system.

Gas-Phase Fluorescence of Proflavine Reveals Two Close-Lying, Brightly Emitting States.

Surprising excitation-dependent, dual emission from a small organic model fluorophore is reported. Gas-phase fluorescence spectra of proflavine (a diaminoacridine) ions reveal two long-lived emitting states, with distinct bands separated by just 1700 cm-1. The relative intensities of these two bands depend on the excitation wavelength. Time-dependent density functional theory (TD-DFT) calculations support the existence of two close-lying singlet electronic states, with excitation into S2 predicted to be >1000-fold more likely than into S1. These data strongly suggest that internal conversion (IC) rates are suppressed relative to solvated proflavine, and that IC is competitive with intramolecular vibrational relaxation (IVR). This work offers an in-depth assessment of the gas-phase photophysics of a simple fluorophore that could open a new pathway to understanding dual emission in fluorophores.

Taming Disulfide Bonds with Laser Fields. Nonadiabatic Surface-Hopping Simulations in a Ruthenium Complex.

Laser control of chemical reactions is a challenging field of research. In particular, the theoretical description of coupled electronic and nuclear motion in the presence of laser fields is not a trivial task and simulations are mostly restricted to small systems or molecules treated within reduced dimensionality. Here, we demonstrate how the excited state dynamics of [Ru(S–Sbpy)(bpy)2]2+ can be controlled using explicit laser fields in the context of fewest-switches surface hopping. In particular, the transient properties along the excited state dynamics leading to population of the T1 minimum energy structure are exploited to define simple laser fields capable of slowing and even completely stopping the onset of S–S bond dissociation. The use of a linear vibronic coupling model to parametrize the potential energy surfaces showcases the strength of the surface-hopping methodology to study systems including explicit laser fields using many nuclear degrees of freedom and a large amount of close-lying electronic excited states.

Photoelectron Spectra of Gd2O2- and Nonmonotonic Photon-Energy-Dependent Variations in Populations of Close-Lying Neutral States.

Photoelectron spectra of Gd2O2- obtained with photon energies ranging from 2.033 to 3.495 eV exhibit numerous close-lying neutral states with photon-energy-dependent relative intensities. Transitions to these states, which fall within the electron binding energy window of 0.9 and 1.6 eV, are attributed to one- or two-electron transitions to the ground and low-lying excited neutral states. An additional, similar manifold of electronic states is observed in an electron binding energy window of 2.1-2.8 eV, which cannot be assigned to any simple one-electron transitions. This study expands on previous work on the Sm2O- triatomic, which has a more complex electronic structure because of the 4f6 subshell occupancy of each Sm center. Because of the simpler electronic structure from the half-filled 4f7 subshell occupancy in Gd2O2 and Gd2O2-, the numerous close-lying transitions observed in the spectra are better resolved, allowing a more detailed view of the changes in relative intensities of individual transitions with photon energy. With supporting calculations on numerous possible close-lying electronic states, we suggest a potential description of the strong photoelectron-valence electron interactions that may result in the photon-energy-dependent changes in the observed spectra.

A guinea pig for conformer selectivity and mechanistic insights into dissociative ionization by photoelectron photoion coincidence: fluorocyclohexane.

We studied fluorocyclohexane (C6H11F, FC6) by double imaging photoelectron photoion coincidence spectroscopy in the 9.90-13.90 eV photon energy range. The photoelectron spectrum can identify species isomer and, in this case, even conformer selectively. Ab initio results indicated that the axial conformer has two, close-lying cation electronic states. With the help of Franck-Condon simulations of the vibrational fine structure, we determined the origin of three transitions, (i) axial FC6 → axial FC6+ of C1 symmetry (X[combining tilde]+, A'' in CS), (ii) equatorial FC6 → equatorial FC6+ of C1 symmetry (X[combining tilde]+, A'' in CS), and (iii) axial FC6 → A' axial FC6+ of CS symmetry (Ã+) as 10.12 ± 0.01, 10.15 ± 0.01 and 10.15 ± 0.02 eV, respectively. At slightly higher energies, the FC6 cation starts fragmenting by HF loss (E0 = 10.60 eV), followed by sequential CH3 (E0 = 10.71 eV) or C2H4 (E0 = 11.06 eV) loss. Surprisingly, the methyl-loss step has an effective barrier of only 0.11 eV, and yet it is a slow process at threshold. Based on the statistical model, this is explained by isomerization and stabilization of the C6H10+ intermediate. The highest energy channel observed, vinyl fluoride (C2H3F) loss yielding C4H8+ appears in the breakdown diagram at 12 eV, which agrees with the computed threshold to cyclobutane cation formation. However, the model predicted a ca. 1 eV competitive shift for this parallel channel, i.e., an E0 = 11.23 eV. This led us to explore the potential energy surface to find a lower-lying fragmentation channel including H-transfer steps. Rate constant measurements and statistical modeling thus yield fundamental insights into the reaction mechanism beyond what is immediately seen in the mass spectra.

Electronic Spectra of Diacetylene Cations (HC4H+) Tagged with Ar and N2

Electronic spectra of mass-selected HC4H+-Arn (n = 1-3) and HC4H+-(N2)n (n = 1-2) complexes are measured over the 290-530 nm range using resonance-enhanced photodissociation spectroscopy in a tandem mass spectrometer. Vibronic transitions in the visible region are compared with previous experimental and theoretical results for the Ã2Πu ← X̃2Πg band system of HC4H+. Hole burning experiments confirm that transitions over the 290-340 nm range involve the diacetylene cation (HC4H+). On the basis of previous experiments and comparison with spectra of isoelectronic molecules the peaks are assigned to the 22Πu ← X̃2Πg band system, with the origin transition for HC4H+-Ar occurring at 29723 cm-1. The main progression has a spacing of 906 cm-1 and is assigned to the symmetric C-C stretch vibrational mode (ν3). The assignment of additional bands is complicated by spectral congestion, the possible presence of energetically close-lying electronic states, vibronic coupling effects, and by the fact that HC4H+ possibly becomes nonlinear in the 22Πu state.

Ultrafast Photodynamics of Glucose.

We have investigated the photodynamics of β-d-glucose employing our field-induced surface-hopping (FISH) method, which allows us to simulate the coupled electron-nuclear dynamics, explicitly including nonadiabatic effects and light-induced excitation. Our results reveal that from the initially populated S1 and S2 states, glucose returns nonradiatively to the ground state within about 200 fs. This takes place mainly via conical intersections (CIs), whose geometries in most cases involve the elongation of a single O-H bond, whereas in some instances, ring-opening due to dissociation of a C-O bond is observed. Experimentally, excitation to a distinct excited electronic state is improbable due to the presence of a dense manifold of states bearing similar oscillator strengths. Our FISH simulations, explicitly including a UV laser pulse of 6.43 eV photon energy, reveal that after initial excitation, the population is almost equally spread over several close-lying electronic states. This is followed by a fast nonradiative decay on the time scale of 100-200 fs, with the final return to the ground state proceeding via the S1 state through the same types of CIs as observed in the field-free simulations.

Direct Observation of a Dark State in the Photocycle of a Light-Driven Molecular Motor.

Controlling the excited-state properties of light driven molecular machines is crucial to achieving high efficiency and directed functionality. A key challenge in achieving control lies in unravelling the complex photodynamics and especially in identifying the role played by dark states. Here we use the structure sensitivity and high time resolution of UV-pump/IR-probe spectroscopy to build a detailed and comprehensive model of the structural evolution of light driven molecular rotors. The photodynamics of these chiral overcrowded alkene derivatives are determined by two close-lying excited electronic states. The potential energy landscape of these “bright” and “dark” states gives rise to a broad excited-state electronic absorption band over the entire mid-IR range that is probed for the first time and modeled by quantum mechanical calculations. The transient IR vibrational fingerprints observed in our studies allow for an unambiguous identification of the identity of the “dark” electronic excited state from which the photon’s energy is converted into motion, and thereby pave the way for tuning the quantum yield of future molecular rotors based on this structural motif.

Non-adiabatic effects in the Ã2B2 and states of CH2F2+ through coupling vibrational modes

Vibronic coupling between the two energetically close-lying excited electronic states (Ã2B2 and ) of CH2F2+ is studied in this article. A reduced dimensional model Hamiltonian is constructed in a diabatic representation and using standard vibronic coupling theory. A detailed topographical analysis of the coupled surfaces is presented here and nuclear dynamics study on these surfaces is also studied by using time-dependent wavepacket propagation approach.

Photoelectron spectra of CeO(-) and Ce(OH)2 (-).

The photoelectron spectrum of CeO(-) exhibits what appears to be a single predominant electronic transition over an energy range in which numerous close-lying electronic states of CeO neutral are well known. The photoelectron spectrum of Ce(OH)2 (-), a molecule in which the Ce atom shares the same formal oxidation state as the Ce atom in CeO(-), also exhibits what appears to be a single transition. From the spectra, the adiabatic electron affinities of CeO and Ce(OH)2 are determined to be 0.936 ± 0.007 eV and 0.69 ± 0.03 eV, respectively. From the electron affinity of CeO, the CeO(-) bond dissociation energy was determined to be 7.7 eV, 0.5 eV lower than the neutral bond dissociation energy. The ground state orbital occupancies of both CeO(-) and Ce(OH)2 (-) are calculated to have 4f 6s(2) Ce(+) superconfigurations, with open-shell states having 4f5d6s superconfiguration predicted to be over 1 eV higher in energy. Low-intensity transitions observed at higher electron binding energies in the spectrum of CeO(-) are tentatively assigned to the (1)Σ(+) (Ω = 0) state of CeO with the Ce+26s2 superconfiguration.

Ammonia-modified Co(II) sites in zeolites: IR spectroscopy and spin-resolved charge transfer analysis of NO adsorption complexes.

IR spectroscopic studies and quantum chemical modeling (aided by the analysis of charge transfer processes between co-adsorbed ammonia and the Co(II)-NO adduct) evidence that donor ammonia molecules, ligated to extraframework Co(2+) centers in zeolites, vitally affect the strength of the N-O bond. Calculations indicate that versatility of ammine nitrosyl complexes, differing in the number of NH3 ligands as well as in the geometry and electronic structure of the Co-N-O unit (showing variable activation of NO) may co-exist in zeolite frameworks. However, only combined analysis of experimental and calculation results points to the adducts with three or five NH3 coligands as decisive. The novel finding concerning the interpretation of discussed IR spectra is the assignment of the most down-shifted bands at 1600-1615 cm(-1) to the N-O stretch in the singlet [Co(NH3)3(NO)](2+) adduct, in place of tentative ascription to pentaammine adducts. Theory indicates also that the Co(ii) center (with manifold of close-lying electronic and spin states) acts as a tunable electron donor where the spin state may open or close specific channels transferring electron density from the donor ligands (treated as the part of environment) to the NO molecule.

The electronic structure of spintronic materials as seen by spin-polarized angle-resolved photoemission

The key quantity in spintronic devices is the spin polarization of the current flowing through the various device components, which in turn is closely determined by the components’ electronic structure. Modern spin- and angle-resolved photoemission spectroscopy (spin-ARPES) can map the details of the spin-polarized electronic structure in many novel material systems – both magnetic and nonmagnetic. In order to separate close-lying electronic states, however, an improvement in energy and angular resolution as well as information depth is still mandatory. We review several types of modern photoemission spectrometers capable of spin analysis and discuss the application of the technique for several physical systems including ferromagnetic thin films and topological insulators.

The ground states of iron(III) porphines: Role of entropy–enthalpy compensation, Fermi correlation, dispersion, and zero-point energies

Porphyrins are much studied due to their biochemical relevance and many applications. The density functional TPSSh has previously accurately described the energy of close-lying electronic states of transition metal systems such as porphyrins. However, a recent study questioned this conclusion based on calculations of five iron(III) porphines. Here, we compute the geometries of 80 different electronic configurations and the free energies of the most stable configurations with the functionals TPSSh, TPSS, and B3LYP. Zero-point energies and entropy favor high-spin by ~ 4 kJ/mol and 0–10 kJ/mol, respectively. When these effects are included, and all electronic configurations are evaluated, TPSSh correctly predicts the spin of all the four difficult phenylporphine cases and is within the lower bound of uncertainty of any known theoretical method for the fifth, iron(III) chloroporphine. Dispersion computed with DFT-D3 favors low-spin by 3–53 kJ/mol (TPSSh) or 4–15 kJ/mol (B3LYP) due to the attractive r − 6 term and the shorter distances in low-spin. The very large and diverse corrections from TPSS and TPSSh seem less consistent with the similarity of the systems than when calculated from B3LYP. If the functional-specific corrections are used, B3LYP and TPSSh are of equal accuracy, and TPSS is much worse, whereas if the physically reasonable B3LYP-computed dispersion effect is used for all functionals, TPSSh is accurate for all systems. B3LYP is significantly more accurate when dispersion is added, confirming previous results.

Conformer-specific vibronic spectroscopy and vibronic coupling in a flexible bichromophore: Bis-(4-hydroxyphenyl)methane

The vibronic spectroscopy of jet-cooled bis-(4-hydroxyphenyl)methane has been explored using fluorescence excitation, dispersed fluorescence (DFL), UV-UV hole-burning, UV depletion, and fluorescence-dip infrared spectroscopies. Calculations predict the presence of three nearly isoenergetic conformers that differ in the orientations of the two OH groups in the para positions on the two aromatic rings (labeled uu, dd, and ud). In practice, two conformers (labeled A and B) are observed, with S(0)-S(1) origins at 35,184 and 35,209 cm(-1), respectively. The two conformers have nearly identical vibronic spectra and hydride stretch infrared spectra. The low-frequency vibronic structure is assigned to bands involving the phenyl torsions (T and T), ring-flapping (R and R), and butterfly (β) modes. Symmetry arguments lead to a tentative assignment of the two conformers as the C(2) symmetric uu and dd conformers. The S(0)-S(2) origins are assigned to bands located 132 cm(-1) above the S(0)-S(1) origins of both conformers. DFL spectra from the S(2) origin of the two conformers display extensive evidence for vibronic coupling between the two close-lying electronic states. Near-resonant coupling from the S(2) origin occurs dominantly to S(1) R(1) and S(1) R(1)β(1) levels, which are located -15 and +31 cm(-1) from it. Unusual vibronic activity in the ring-breathing (ν(1)) and ring-deformation (ν(6a)) modes is also attributed to vibronic coupling involving these Franck-Condon active modes. A multimode vibronic coupling model is developed based on earlier theoretical descriptions of molecular dimers [Fulton and Gouterman, J. Chem. Phys. 35, 1059 (1961)] and applied here to flexible bichromophores. The model is able to account for the ring-mode activity under conditions in which the S(2) origin is strongly mixed (60%/40%) with S(1) 6a(1) and 1(1) levels. The direct extension of this model to the T/T and R/R inter-ring mode pairs is only partially successful and required some modification to lower the efficiency of the S(1)/S(2) mixing compared to the ring modes.

Quantum Dynamics of the Ultrafast ππ*/nπ* Population Transfer in Uracil and 5-Fluoro-Uracil in Water and Acetonitrile

We present the first fully quantum-mechanical dynamical study of the pi pi* --> n pi* decay in photoexcited uracil derivatives in solution. The dynamics of this process for uracil (U) and 5-fluoro-uracil (5FU) in acetonitrile and aqueous solution is investigated, by treating both electrons and nuclei at the quantum-mechanical level, and solving the time-dependent Schrodinger equation for the evolution of the photoexcited system. The potential energy surfaces along the most relevant nuclear degrees of freedom have been obtained at the time-dependent density functional theory level, accounting for the solvent effect by mixed atomistic/continuum models. The obtained results nicely agree with experimental evidence. For U, we predict an ultrafast (<50 fs) pi pi* --> n pi* decay with 10-25% yields, weakly dependent on the solvent. This finding strongly indicates that the npi* state is responsible for the decay channel observed in experiments for U in polar solvents, yielding to a final recovery of the ground state in tens of picoseconds. At variance, our dynamical calculations predict a remarkable solvent effect for 5FU, showing that the pi pi* --> n pi* decay channel is open in acetonitrile and closed in water, in agreement with the much faster decay observed in experiments in the first solvent. The analysis of our theoretical simulations, also including explicitly the laser excitation step, allows us to point out a number of interesting features for the decay dynamics of photoexcited molecules with close-lying electronic states, whose general interest goes beyond the specific system under investigation.

Metals in biology: Iron complexes modeling electron transfer

Complexes of iron-bisglyoxal, [Fe(C2H2O2)2)]n n = +3, + 2, +1,0, –1, –2, have been studied by a quantum chemical ab initio RHF-MO-SCF method. These complexes have basically the same geometric structure as certain iron complexes of catechols and ascorbic acid, and have been employed as models in various studies of metal ion catalysis of chemical oxidation reactions. Our computational results indicate the presence of low-lying excited states. The lowest value of the total energy was obtained for n = –1. This case was studied in more detail. Partial geometry variation was carried out for two close-lying electronic states. It was found that these two states obtain their lowest values for different geometries, and that the two surfaces have an intersection close to these minima. The electronic distribution of the iron in the different states remains unchanged along the energy surfaces, but differs considerably between the states. Both charge distributions and orbital energies indicate that the iron is Fe(II) in the one state and Fe(III) in the other. A transfer of the complex from the one energy surface to the other is therefore equivalent to the transfer of an electron between iron and ligand dimer.

Gas-phase electronic spectrum of the C14ring

The electronic spectrum of a cyclic C(14) in the visible range has been detected in the gas phase by a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Absorption is localized in the 19 000 to 20 000 cm(-1) region and appears as a dozen of 3-7 cm(-1) narrow peaks belonging to one or two close-lying electronic states. Bands have structures which for the narrowest ones is likely to be the rotational profile contour. The spectrum is attributed to a cyclic form of C(14) based on time-dependent density-functional calculations and reactivity with H(2). The spectral pattern differs from that previously seen in the larger C(4n+2) member rings, C(18) and C(22), indicating some sort of a structural crossover.

Rotational and vibrational structure in the 288 nm band system of the FeCl 2 radical

The laser excitation spectrum of the 288 nm band system of FeCl 2, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. Vibronic transitions are observed from the ground state to two close-lying excited electronic states that differ in inversion ( g, u) parity. Two extensive progressions in the symmetric stretching vibration have been identified, referred to as Progressions A and B. The main features of Progression A, which is based on the ν 0 0 band, are allowed transitions to the excited electronic state of ungerade symmetry. Progression B is built on the 3 0 1 band and consists of vibronically induced transitions to the gerade excited state. A substantial decrease in the symmetric stretching vibrational wavenumber is observed on excitation ( ν 1 ′ = 200.43 ( 18 ) , ν 1 ″ = 352.39 ( 13 ) cm - 1 ) . Local perturbations are found to cause relative shifts between the different isotopomers. Several vibronic bands have been recorded and analysed at rotational resolution for the three isotopomers Fe 35Cl 2, Fe 35Cl 37Cl, and Fe 37Cl 2 in natural abundance. All bands show perpendicular rotational structure of a linear molecule, and have been unambiguously assigned to a Ω = 5–4 transition, consistent with the inverted 5Δ g ground state predicted by ab initio and DFT calculations. The zero-point averaged Fe Cl bond length is determined to be r 0 ′ = 2.24506 ( 46 ) and r 0 ″ = 2.13198 ( 38 ) Å in the upper and lower electronic states. The results show that the molecule is linear in both states.