Ground State Products Research Articles

Subpicosecond HI elimination in the 266 nm photodissociation of branched iodoalkanes.

The 266 nm photodissociation dynamics of 1-iodopropane and 2-iodopropane were studied using photofragment translational energy spectroscopy using vacuum ultraviolet (VUV) photoionization and electron impact ionization detection of products. The photochemistry of 1-iodopropane was found to be similar to that of iodomethane and iodoethane, with dominant production of I*(2P1/2), and no evidence (<0.21%) for HI + alkene formation. Significantly different behavior was observed for 2-iodopropane, with dominant production of ground state I(2P3/2), and a HI yield >10.5%. The anisotropy (β) parameters for all channels approached the limiting value of 2.0, indicating that 1,2-HI elimination occurs on subpicosecond timescales, like direct C-I bond fission, following excitation to 3Q0. The HI translational energy and angular distributions were similar to those for I(2P3/2), suggesting that motion of the heavy I atom in HI is largely derived from the repulsive nature of the 1Q1 surface correlating to R + I with the light H atom picked up by ground state I late in the exit channel producing highly vibrationally excited HI.

Thermochemical studies of reactions of Re+ with SO2 using guided ion beam experiments and theory.

The kinetic energy dependent reactions of Re+ with SO2 were studied with guided ion beam tandem mass spectrometry. ReO+, ReO2+, and OReS+ species were observed as products, all in endothermic reactions. Modeling of the kinetic energy dependent cross sections yields 0 K bond dissociation energies (BDEs, in eV) of 4.78 ± 0.06 (Re+-O), 5.75 ± 0.02 (Re+-O2), and 4.35 ± 0.14 (Re+-SO). The latter two values can be combined with other information to derive the additional values 6.05 ± 0.05 (ORe+-O) and 4.89 ± 0.19 (ORe+-S). BDEs of ReO+ and ReO2+ agree with literature values whereas the values for OReS+ are the first measurements. The former result is obtained even though formation of ground state ReO+ is spin-forbidden. Quantum mechanical calculations at the B3LYP level of theory with a def2-TZVPPD basis set yield results that agree reasonably well with experimental values. Additional calculations at the BP86 and CCSD(T) levels of theory using def2-QZVPPD and aug-cc-pVxZ (x = T, Q, and 5) basis sets were performed to compare thermochemistry with experiment to determine that ReO2+ rather than the isobaric ReS+ is formed. Product ground states are 3Δ3(ReO+), 3B1(OReO+), 5Π-1(ReS+), and 3A''(OReS+) after including empirical spin-orbit corrections, which means that formation of ground state products is spin-forbidden for all three product channels. The potential energy surfaces for the ReSO2+ system were also explored at the B3LYP/def2-TZVPPD level and exhibited no barriers in excess of the endothermicities for all products. BDEs for rhenium oxide and sulfide diatomics and triatomics are compared and discussed. The present result for formation of ReO+ is compared to that for formation of ReO+ in the reactions of Re+ + O2 and CO, where the former system exhibited interesting dual cross section features. Results are consistent with the hypothesis that the distinction of in-plane and out-of-plane CS symmetry in the triatomic systems might be the explanation for the two endothermic features observed in the Re+ + O2 reaction.

Atomic Physics at the 5 MV Tandem at Demokritos: the UoC APAPES project

University of Crete (UoC) has initiated the research initiative APAPES funded by THALES‡ that has already set up a new experimental station with a beam line dedicated solely on basic atomic physics research. This new experiment utilizing Zero-degree Auger Projectile Spectroscopy (ΖΑPS) is located at the 5 MV TANDEM accelerator of the National Center for Scientific Research (NCSR) “Demokritos” in Athens, and has been put together to perform high resolution studies of electrons emitted during ion-atom collisions. The apparatus consists of a Hemispherical Deflector Analyzer (HDA) combined with a 2-dimensional Position Sensitive Detector (PSD) and a doubly-differentially pumped gas cell containing the gas-target. The goal is to perform a systematic isoelectronic investigation of K-Auger spectra emitted from pre-excited and ground state He-like ions in collisions with gas targets using novel techniques. So far, various Auger electron spectra produced through collisions of mixed state (1s2, 1s2s3S) C4+ ion beam with various gas targets have been recorded. In addition, detailed simulations using SIMION have also explored the optimal lens voltages and the solid angle correction factors for long-lived metastable states. A terminal gas stripper system is scheduled to be installed in the accelerator, extending its range of available charge states and enabling the production of pure ground state as well as mixed state beams with different metastable fractions, a measurement vital to APAPES. Here, we report on the progress made up to date on the APAPES project, the description of the apparatus, updated results and plans for the near future.

Photodissociation dynamics of the methylsulfinyl radical at 248 nm

The photodissociation dynamics of jet-cooled methylsulfinyl radicals, CH3SO, at 248 nm have been investigated using molecular beam photofragment translational spectroscopy. The primary channel is CH3S + O, which occurs via the initially prepared excited CH3SO state by rapid cleavage of the S-O bond to produce ground state products. The minor SO + CH3 channel has two components in comparable proportions: a fast feature corresponding to rapid C-S cleavage on the excited state to produce CH3 and electronically excited SO, and a slow feature due to internal conversion of CH3SO followed by statistical dissociation on the ground electronic state. Statistical ground state dissociation also produces small amounts of CH2SO, likely sulfine, and H-atoms.

Iodide Photoredox and Bond Formation Chemistry.

Iodide redox chemistry is intimately coupled with the formation and breaking of chemical bonds that are relevant to emerging solar energy technologies. In this Account, recent advances in dye-sensitized iodide oxidation chemistry in organic solutions are described. Here RuII sensitizers with high cationic charge, tuned reduction potentials, and specific iodide receptor site(s) are shown to self-assemble in organic solvents and yield structures that rapidly oxidize iodide and generate I-I bonds when illuminated with visible light. These studies provided new insights into the fascinating behavior of our most polarizable and easily oxidized monatomic anion. Sensitized iodide photo-oxidation in CH3CN solutions consists of two mechanisticsteps. In the first step, an excited-state sensitizer oxidizes iodide (I-) to aniodine atom (I•) through diffusional encounters. The second step involves the reaction of I• with I- to form the I-I bond of diiodide, I2•-. The overall reaction converts a green photon into about 1.64 eV of free energy in the form of I2•- and the reduced sensitizer. The free energy is only transiently available, as back-electron transfer to yield ground-state products is quantitative. Interestingly, when the free energy change is near zero, iodide photo-oxidation occurs rapidly with rate constants near the diffusion limit, i.e., >1010 M-1 s-1. Such rapid reactivity is in line with anecdotal knowledge that iodide is an outstanding electron donor and is indicative of adiabatic electron transfer through an inner-sphere mechanism. In low-dielectric-constant solvents, dicationic RuII sensitizers were found to form tight ion pairs with iodide. Diimine ligands with additional cationic charge, or "binding pockets" that recognize halides, have been utilized to position one or more halides at specific locations about the sensitizer before light absorption. Diverse photochemical reactions observed with these supramolecular assemblies range from the photorelease of halides to the formation of I-I bonds where both iodides present in the ground-state assembly react. Natural population analysis through density functional theory calculations accurately predicts the site(s) of iodide ion-pairing and provides information on the associated free energy change. The ability to direct light-driven bond formation in these ionic assemblies is extended to chloride and bromide ions. The structure-property relationships identified, and those that continue to emerge, may one day allow for the rational design of molecules and materials that drive desired halide transformations when illuminated with light.

K38 isomer production via fast fragmentation

In radioactive ion beam experiments, beams containing isomers can be of interest in probing nuclear structure and informing astrophysical reaction rates. While the production of mixed in-flight ground state and isomer beams using nucleon transfer can be generally understood through distorted wave Born approximation methodology, low-spin isomer production via fast fragmentation is relatively unstudied. To attain a practical understanding of low-spin isomer production using fast fragmentation beams, a test case of $^{38}\mathrm{K}/^{38m}\mathrm{K}$ was studied at the National Superconducting Cyclotron Laboratory's ReAccelerated Beam facility. Starting from lise++ predictions, the fragmentation momentum distribution was sampled to determine isomer production. In addition, the effects of the gas stopper gradient and charge breeding times were examined. In the case of $^{38}\mathrm{K}$, isomer production peaks at $\ensuremath{\sim}57%$. This maximum is observed just off the lise++ predicted optimum magnetic rigidity, with only small losses in beam intensity within a few percent of this optimum rigidity setting. Control of the isomer fraction was also achieved through the modification of charge breeding times. Fast fragmentation appears to be a feasible method for production of low-spin isomeric beams, but additional study is necessary to better describe the mechanism involved.

Results on quarkonia production in Heavy Ion collisions from the ATLAS Experiment

The observed quarkonia states in heavy ion collisions provide a powerful tool to probe the dynamics of the Quark Gluon Plasma. Measurements performed on the ground and excited quarkonia states, as well as their separation into prompt and non-prompt components, reveal the effects of colour screening and colour recombination. In addition, quarkonium production rates and their excited to ground state production ratios in lighter p+Pb collisions are an interesting probe of cold nuclear matter effects. In these proceedings, the latest results of the ATLAS experiment at the LHC on these observables will be presented.

Computational Inverse Method for Constructing Spaces of Quantum Models from Wave Functions

Traditional computational methods for studying quantum many-body systems are "forward methods," which take quantum models, i.e., Hamiltonians, as input and produce ground states as output. However, such forward methods often limit one's perspective to a small fraction of the space of possible Hamiltonians. We introduce an alternative computational "inverse method," the Eigenstate-to-Hamiltonian Construction (EHC), that allows us to better understand the vast space of quantum models describing strongly correlated systems. EHC takes as input a wave function $|\psi_T\rangle$ and produces as output Hamiltonians for which $|\psi_T\rangle$ is an eigenstate. This is accomplished by computing the quantum covariance matrix, a quantum mechanical generalization of a classical covariance matrix. EHC is widely applicable to a number of models and in this work we consider seven different examples. Using the EHC method, we construct a parent Hamiltonian with a new type of antiferromagnetic ground state, a parent Hamiltonian with two different targeted degenerate ground states, and large classes of parent Hamiltonians with the same ground states as well-known quantum models, such as the Majumdar-Ghosh model, the XX chain, the Heisenberg chain, the Kitaev chain, and a 2D BdG model.

Infrared spectroscopy reveals multi-step multi-timescale photoactivation in the photoconvertible protein archetype dronpa.

Photochromic fluorescent proteins play key roles in super-resolution microscopy and optogenetics. The light-driven structural changes that modulate the fluorescence involve both trans-to-cis isomerization and proton transfer. The mechanism, timescale and relative contribution of chromophore and protein dynamics are currently not well understood. Here, the mechanism of off-to-on-state switching in dronpa is studied using femtosecond-to-millisecond time-resolved infrared spectroscopy and isotope labelling. Chromophore and protein dynamics are shown to occur on multiple timescales, from picoseconds to hundreds of microseconds. Following excitation of the trans chromophore, a ground-state primary product is formed within picoseconds. Surprisingly, the characteristic vibrational spectrum of the neutral cis isomer appears only after several tens of nanoseconds. Further fluctuations in protein structure around the neutral cis chromophore are required to form a new intermediate, which promotes the final proton-transfer reaction. These data illustrate the interplay between chromophore dynamics and the protein environment underlying fluorescent protein photochromism.

Reactions of electronically excited molecular nitrogen with H2 and H2O molecules: theoretical study

Comprehensive quantum chemical analysis with the usage of the second-order perturbation multireference XMCQDPT2 approach was carried out to study the processes in the + H2 and + H2O systems. The energetically favorable reaction pathways have been revealed based on the exploration of potential energy surfaces. It has been shown that the reactions + H2 and + H2O occur with small activation barriers and, primarily, lead to the formation of N2H + H and N2H + OH products, respectively. Further, the interaction of these species could give rise to the ground state and H2 (or H2O) products, however, the estimations, based on RRKM theory and dynamic reaction coordinate calculations, exhibited that the + H2 and + H2O reactions lead to the dissociative quenching predominately. Appropriate rate constants for revealed reaction channels have been estimated by using a canonical variational theory and capture approximation. Corresponding three-parameter Arrhenius expressions for the temperature range T = 300 − 3000 K were reported.

Kinetics of First-Row Transition Metal Cations (V+, Fe+, Co+) with OCS at Thermal Energies.

The temperature-dependent kinetics for reactions of V+, Fe+, and Co+ with OCS are measured using a selected ion flow tube apparatus heated to 300-600 K. All three reactions proceed solely by C-S activation at thermal energies, resulting in metal sulfide cation formation. Previously calculated reaction pathways were employed to inform statistical modeling of these reactions for comparison to the data. As surmised previously, all three reactions at thermal energies require spin crossing, with the Fe+ reaction crossing once circumventing a prohibitive transition state, before crossing again to form ground state products. The Fe+ and Co+ reaction efficiencies increase with energy. For the Co+ reaction, and to a lesser extent the Fe+ reaction, the apparent activation energies are less than the reaction endothermicities, possibly indicating increasing diabatic behavior of the spin crossings with energy. The V+ reaction was well modeled assuming an entirely adiabatic spin crossing, such that the resultant avoided crossing behaves similarly to a tight transition state. The subsequent reaction of VS+ with OCS producing VS2+ is also investigated; the rate-limiting transition state energy derived from statistical modeling is poorly reproduced by quantum calculations using a variety of methods, highlighting the large (1-2 eV) uncertainty in calculated energetics of transition-metal containing species.

Activation of CO2 by Gadolinium Cation (Gd+): Energetics and Mechanism from Experiment and Theory

The exothermic and barrierless activation of CO2 by the lanthanide gadolinium cation (Gd+) to form GdO+ and CO is investigated in detail using guided ion beam tandem mass spectrometry (GIBMS) and theory. Kinetic energy dependent product ion cross sections from collision-induced dissociation (CID) experiments of GdCO2 + are measured to determine the energetics of OGd+(CO) and Gd+(OCO) intermediates. Modeling these cross sections yields bond dissociation energies (BDEs) for OGd+–CO and Gd+–OCO of 0.57 ± 0.05 and 0.38 ± 0.05 eV, respectively. The OGd+–CO BDE is similar to that previously measured for Gd+–CO, which can be attributed to the comparable electrostatic interaction with CO in both complexes. The Gd+(OCO) adduct is identified from calculations to correspond to an electronically excited state. The thermochemistry here and the recently measured GdO+ BDE allows for the potential energy surface (PES) of the Gd+ reaction with CO2 to be deduced from experiment in some detail. Theoretical calculations are performed for comparison with the experimental thermochemistry and for insight into the electronic states of the GdCO2 + intermediates, transition states, and the reaction mechanism. Although the reaction between ground state Gd+ (10D) and CO2 (1Σg +) reactants to form ground state GdO+ (8Σ−) and CO (1Σ+) products is formally spin-forbidden, calculations indicate that there are octet and dectet surfaces having a small energy gap in the entrance channel, such that they can readily mix. Thereby, the reaction can efficiently proceed along the lowest energy octet surface to yield ground state products, consistent with the experimental observations of an efficient, barrierless process. At high collision energies, the measured GdO+ cross section from the Gd+ reaction with CO2 exhibits a distinct feature, attributed to formation of electronically excited GdO+ products along a single dectet PES in a diabatic and spin-allowed process. Modeling this high-energy feature gives an excitation energy of 3.25 ± 0.16 eV relative to the GdO+ (8Σ−) ground state, in good agreement with calculated excitation energies for GdO+ (10Π, 10Σ−) electronic states. The reactivity of Gd+ with CO2 is compared with the group 3 transition metal cations and other lanthanide cations and periodic trends are discussed.

Color screening and regeneration of bottomonia in high-energy heavy-ion collisions

The production of ground-state and excited bottomonia in ultrarelativistic heavy-ion collisions is investigated within a kinetic-rate equation approach including regeneration. We augment our previous calculations by an improved treatment of medium effects, with temperature-dependent binding energies and pertinent reaction rates, $B$-meson resonance states in the equilibrium limit near the hadronization temperature, and a lattice-QCD based equation of state for the bulk medium. In addition to the centrality dependence of the bottomonium yields we compute their transverse-momentum ($p_T$) spectra and elliptic flow with momentum-dependent reaction rates and a regeneration component based on $b$-quark spectra from a nonperturbative transport model of heavy-quark diffusion. The latter has noticeable consequences for the shape of the bottomonium $p_T$ spectra. We quantify how uncertainties in the various modeling components affect the predictions for observables. Based on this we argue that the $\Upsilon(1S)$ suppression is a promising observable for mapping out the in-medium properties of the QCD force, while $\Upsilon(2S)$ production can help to quantify the role of regeneration from partially thermalized $b$ quarks.

N (2P) production in electron-N2 collisions

A unique detector which is selectively sensitive to low energy metastable atoms, has been used to study the production of ground state N (2P) atoms following collisions of low energy (0–200 eV) electrons with molecular nitrogen. Time-of-flight techniques have revealed the existence of at least two distinct mechanisms yielding this dissociation product. Released kinetic energies in the dissociation have allowed positioning of the parent molecular states in the Franck–Condon region. This, together with excitation probability curves, has allowed probable parent states, such as B′ b′ and C′ 3Πu, to be identified making use of recent theoretical calculations. Both direct and pre-dissociation processes are shown to be involved.

Full-dimensional multi-state simulation of the photodissociation of thioanisole.

The photodissociation of thioanisole is very interesting because the experiments of Lim and Kim provide evidence for mode-specific effects on the product distribution. They showed that, with a specific S-CH3 stretching mode being excited as the reagent is excited to the S1 electronic state, there is a sharp increase in the proportion of the ground-state product to the excited-state product. In the present work, we report 78 011 full-dimensional semiclassical multi-state trajectories of the photodissociation process using the coherent switching with decay of mixing dynamics method. The potential surfaces and couplings are based on electronic structure calculations that include dynamic correlation through second order perturbation theory. We report results for four sets of initial conditions, one corresponding roughly to 0-0 excitation and three corresponding to exciting one vibrational mode, to look for mode-specific effects. The simulations show no significant mode-specific effect on the product energy distributions, but they do show an effect on the distribution of minimum-energy gaps in the trajectories and on the lifetime for dissociation. In particular, excitation of the S-CH3 stretching mode leads to trajectories passing closer to the S1-S2 conical intersection and to shorter lifetimes. This provides a possible explanation of why experimental results are different for excitation of this vibration.

Light Excitation of a Bismuth Iodide Complex Initiates I-I Bond Formation Reactions of Relevance to Solar Energy Conversion.

The titration of iodide into acetonitrile solutions of BiI3 resulted in the formation of [BiI6]3-. Ligand-to-metal charge transfer (LMCT) excitation of [BiI6]3- yielded a transient species assigned as the diiodide anion I2•- directly ligated to Bi, [Bi(I2•-)Ix]n. With 20 ns time resolution, transient absorption measurements revealed the appearance of two species assigned on the analysis of the iodine molecular orbitals as an η2 ligated I2•-, [(η2-I2)BiI4]3- (λmax = 640 nm), and an η1 species [(η1-I2)BiI4]3- (λmax = 750 nm). The rapid appearance of this intermediate was attributed to intramolecular I-I bond formation. The [(η2-I2)BiI4]3- subsequently reacted with 1 equiv of iodide to yield [(η1-I2)BiI5]4-. Interestingly, [(η1-I2)BiI5]4- decayed to ground state products with a first-order rate constant of k = 2 × 103 s-1. Under the same experimental conditions, I2•- in CH3CN rapidly disproportionates with a tremendous loss of free energy, ΔGo = -2.6 eV. The finding that metal ligation inhibits this energy wasting reaction is of direct relevance to solar energy conversion. The photochemistry itself provides a rare example of one electron oxidized halide species coordinated to a metal ion of possible relevance to reductive elimination/oxidation addition reaction chemistry of transition metal catalysts.

A theoretical study on the photodissociation mechanism of acetyl fluoride (CH3C(O)F) involving S0, S1, and T1 states

The dissociation potential energy surfaces of acetyl fluoride (CH3C(O)F) in the lowest three electronic states (S0, S1, and T1) have been calculated at the CASPT2-f12/VTZ-f12 level of theory. Combining with the surface intersection points, four mechanistic channels have been elucidated, namely, intersystem crossing (ISC) to the T1 state, internal conversion (IC) to the ground state, H-atom transfer, and direct dissociation along the S1 pathway. Unlike other acetyl halides, it is found that the α-CC bond cleavage in T1 surface after ISC to yield ground-state products CH3(2A′)+COF(2A′) is the most probable mechanism upon the excitation at 248nm, which agrees well with the experimental observation. Both the S1 and T1 potential energy surfaces are reported systematically for the first time. A comparison of the reactivity among CH3C(O)F, CH3C(O)Cl, CH3C(O)Br, CH3C(O)I and CH3C(O)CH3 has been made.

Guided ion beam and theoretical studies of the reactions of Re+, Os+, and Ir+ with CO.

The kinetic-energy dependences of the reactions M+ + CO where M+ = Re+, Os+, and Ir+ are studied using guided ion-beam tandem mass spectrometry. Formation of both MO+ and MC+ was observed in endothermic processes for all three metals. Modeling of the data provides thresholds that yield 0 K bond dissociation energies (BDEs, in eV) of 4.67 ± 0.09 (Re+-O), 4.82 ± 0.14 (Os+-O), 4.25 ± 0.11 (Ir+-O), 5.13 ± 0.12 (Re+-C), 6.14 ± 0.14 (Os+-C), and 6.58 ± 0.12 (Ir+-C). These BDEs agree well with literature values within experimental uncertainties demonstrating that ground state products are formed for all cases even though some of the reactions are formally spin forbidden. Quantum mechanical calculations at several levels of theory and using several basis sets were performed for MC+ and MO+ (with comparable results taken from the literature in some cases). B3LYP and CCSD(T) calculated ground state BDEs agree reasonably well with experimental values. The ground states in B3LYP and CCSD(T)/CBS calculations are Σ-3 (ReC+), Δ2 (OsC+), and Σ+1 or Δ3 (IrC+) after including spin-orbit considerations. Relaxed potential energy surfaces (PESs) for the M+ + CO reactions show crossings between surfaces of different spin states such that products can be formed with no barriers in excess of the substantial endothermicities. Unlike results for these metal cations reacting with O2, the kinetic energy dependent cross sections for the formation of MO+ in the M+ + CO reactions exhibit only one feature. Reasons for this differential behavior are discussed in detail.

Multi-Component Ginzburg-Landau Theory: Microscopic Derivation and Examples

This paper consists of three parts. In part I, we microscopically derive Ginzburg--Landau (GL) theory from BCS theory for translation-invariant systems in which multiple types of superconductivity may coexist. Our motivation are unconventional superconductors. We allow the ground state of the effective gap operator $K_{T_c}+V$ to be $n$-fold degenerate and the resulting GL theory then couples $n$ order parameters. In part II, we study examples of multi-component GL theories which arise from an isotropic BCS theory. We study the cases of (a) pure $d$-wave order parameters and (b) mixed $(s+d)$-wave order parameters, in two and three dimensions. In part III, we present explicit choices of spherically symmetric interactions $V$ which produce the examples in part II. In fact, we find interactions $V$ which produce ground state sectors of $K_{T_c}+V$ of arbitrary angular momentum, for open sets of of parameter values. This is in stark contrast with Schr\"odinger operators $-\nabla^2+V$, for which the ground state is always non-degenerate. Along the way, we prove the following fact about Bessel functions: At its first maximum, a half-integer Bessel function is strictly larger than all other half-integer Bessel functions.

Quantum Interference Effects Theoretically Found in the Photodissociation Processes of the Second Absorption Bands of ICl and IBr Molecules.

Some quantum interference effects are exposed directly in experiments, but others are not and remain just hidden and thus require thorough theoretical analysis to be exposed. In this respect, the second absorption bands of IX (X = Cl, Br) molecules show an interesting behavior in the photofragment anisotropy of the lowest I((2)P3/2)+X((2)P3/2) product channel; it changes from strongly parallel distribution on the shorter wavelength side to strongly perpendicular distribution on the longer wavelength side. Because the responsible perpendicular third Ω = 1 (1(III)) excited state correlating adiabatically to this product channel has only a weak absorption, the parallel component flux yielding the same products must be comparatively weak, even though the responsible parallel excitations to the 0(+)(III) and/or 0(+)(IV) excited states have strong absorptions. In the present theoretical study, the branching ratios and the anisotropy parameters have been obtained using the spin-orbit configuration interaction method combined with the quantum mechanical wavepacket and semiclassical approaches. Significant quantum interference effect between the two dissociative de Broglie waves on the 0(+)(III) and 0(+)(IV) potential curves has been found through the avoided crossing, and the weak parallel flux to the ground-state product channel has been explained by their destructive interference character. This interference effect has weak excitation energy dependence due to rather unique behavior of their almost parallel potential curves from the Franck-Condon to avoided crossing regions.