Ion-pair States Research Articles

Multiphoton Ion Pair Spectroscopy (MPIPS) with Ultrashort Laser Pulses for the H2 Molecule

Time-dependent Schrödinger equation, TDSE, simulations have been performed in order to prepare and study via MPIPS the evolution of vibrational wave packets on the ion pair electronic state potentials B''B1Sigma(u)(+) and Hh1Sigma(g)(+) of the H2 molecule. Using ab initio potential surfaces and transition moments, we present two- and three-photon excitation schemes with ultrashort pulses (tau <or= 10 fs) to prepare coherent superpositions of the two ion pair H(+)H(-) and H(-)H(+) states from the doorway B1Sigma(u)(+) state, which result from the strong radiative coupling between these two electronic states. The simulations are used to estimate the time evolution and recursion times of vibrational wave packets at large internuclear distances, usually not accessible by single-photon spectroscopy. Conditions for the localization of the ion pair states are proposed.

Energy Gap Law of Electron Transfer in Nonpolar Solvents

We investigate the energy gap law of electron transfer in nonpolar solvents for charge separation and charge recombination reactions. In polar solvents, the reaction coordinate is given in terms of the electrostatic potentials from solvent permanent dipoles at solutes. In nonpolar solvents, the energy fluctuation due to solvent polarization is absent, but the energy of the ion pair state changes significantly with the distance between the ions as a result of the unscreened strong Coulomb potential. The electron transfer occurs when the final state energy coincides with the initial state energy. For charge separation reactions, the initial state is a neutral pair state, and its energy changes little with the distance between the reactants, whereas the final state is an ion pair state and its energy changes significantly with the mutual distance; for charge recombination reactions, vice versa. We show that the energy gap law of electron-transfer rates in nonpolar solvents significantly depends on the type of electron transfer.

Evidence for amplified spontaneous emission from the [formula omitted] and γ1 u( 3P 2) ion-pair states of I 2 excited by optical–optical double resonance

Indirect evidence for amplified spontaneous emission (ASE) between two pairs of ion-pair states of I 2, namely E0 g + ( 3 P 2 ) / D0 u + ( 3 P 2 ) and γ1 u( 3P 2)/β1 g( 3P 2) has been obtained. Direct E0 g + ( 3 P 2 ) → B0 u + and β1 g( 3P 2) → A1 u ASE has also been observed based on the narrowed time profile of the fluorescence, the latter as a secondary process following γ1 u( 3P 2) → β1 g( 3P 2) ASE.

Optical population of free iodine molecule ion-pair states and their RgI 2(IP) complexes via RgI 2 vdW complexes of valence states correlating with the third dissociation limit

Spectral and kinetic analysis of luminescence in the λ = 220–500 nm spectral range observed after three-step, 532.124 + 1024.48 + 532.24 nm, laser excitation of the I 2 + Rg mixtures, Rg = He–Xe, in wide 2. 7–100 Torr Rg pressures shows that one cannot describe the spectral features as well as their dependences on Rg pressures in framework of competition of spontaneous and collision-induced processes in a free iodine molecule. The same conclusion has been drawn after analysis of the papers concerning one-photon laser photolysis of these mixtures. To explain all these data, one has to take into consideration photoexcitation of the RgI 2(IP) complexes via the complexes and colliding pairs of rare gases and valence state iodine molecules.

A Tightly Coupled Bis(zinc(II) phthalocyanine)−Perylenediimide Ensemble To Yield Long-Lived Radical Ion Pair States

A conjugated donor-acceptor array composed of two phthalocyanines connected to the bay region of a perylenediimide is assembled by using palladium chemistry. Excitation of the phthalocyanine produces a nanosecond lived charge-separated state.

Hyperfine coupling of the iodine E0+g, vE = 19 and γ1u, vγ = 18 ion-pair states

A considerable difference of iodine emission spectra after optical–optical double resonance excitation of the E0+g, vE = 19, JE ≈ 55 and JE ≈ 85 rovibronic levels is observed. The latter include strong transitions from the γ1u ion-pair state to valence ones. This feature is interpreted as a result of hyperfine coupling of near-resonant E0+g, vE = 19, JE = 81 and γ1u, vγ = 18, Jγ = 80 states. The hyperfine matrix element and dipole moment functions of transitions from the γ state are estimated. The hyperfine g/u mixing of the iodine ion-pair states is observed for the first time.

Negative-ion formation and mutual neutralization in atom–atom and ion–ion collisions below keV energies

Dynamics and cross-section values for ion-pair formation and its reverse process, mutual neutralization in atom–atom and ion–ion collisions are reviewed for hydrogen–hydrogen atom collisions and hydrogen–alkali atom collisions. For ion-pair formation process, the initial state shows a weak polarization potential, while the final ion-pair state arises from a strong Coulomb potential. Hence, the final state shows a series of avoided crossings with many intermediate states which correspond to electronic excited states of one of pairs. Therefore, the ladder climbing mechanism is the driving dynamics for ion-pair formation. This fact, that is, many sates are involved in dynamics, makes theoretical treatments more complex and cumbersome for an accurate description. We overview the current experimental and theoretical status of these processes.

Cold collisions between OH and Rb: The field-free case

We have calculated elastic and state-resolved inelastic cross sections for cold and ultracold collisions in the $\mathrm{Rb}(^{1}S)+\mathrm{O}\mathrm{H}(^{2}\ensuremath{\Pi}_{3∕2})$ system, including fine-structure and hyperfine effects. We have developed a set of five potential energy surfaces for $\mathrm{Rb}\text{\ensuremath{-}}\mathrm{O}\mathrm{H}(^{2}\ensuremath{\Pi})$ from high-level ab initio electronic structure calculations, which exhibit conical intersections between covalent and ion-pair states. The surfaces are transformed to a quasidiabatic representation. The collision problem is expanded in a set of channels suitable for handling the system in the presence of electric and/or magnetic fields, although we consider the zero-field limit in this work. Because of the large number of scattering channels involved, we propose and make use of suitable approximations. To account for the hyperfine structure of both collision partners in the short-range region we develop a frame-transformation procedure which includes most of the hyperfine Hamiltonian. Scattering cross sections on the order of ${10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ are predicted for temperatures typical of Stark decelerators. We conclude that spin orientation of the partners is completely disrupted during the collision. Implications for both sympathetic cooling of OH molecules in an environment of ultracold Rb atoms and experimental observability of the collisions are discussed.

Collisional energy transfer in the intermediate states used for optical–optical double resonance excitation of ion-pair states in I2

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.

Characterization of a shallow-bound 0g+ valence state of I2 using emission from the D 0u+(3P2) and F′ 0u+(1D2) ion-pair states populated by amplified spontaneous emission

A range of vibrational levels of the D 0(u)(+)((3)P(2)) and F' 0(u)(+)((1)D(2)) ion-pair states of I(2) is shown to be easily generated by amplified spontaneous emission (ASE) from their more accessible partners, E 0(g)(+)((3)P(2)) and f' 0(g)(+)((1)D(2)), in sufficient concentration for dispersed fluorescence studies of the D 0(u)(+)((3)P(2)) --> 0(g)(+)(bb) and F' 0(u)(+)((1)D(2)) --> 0(g)(+)(bb) transitions to be carried out. T(0) (J = 49) of this shallow-bound 0(g)(+)(bb) valence state is unambiguously determined and an improved R(e) value of 3.952 +/- 0.005 A is obtained from optimizing the fit of the intensities of the vibrational progressions in the 0(g)(+)(bb) state, and T(e) is found to be 27311.3 +/- 2 cm(-1), leading to D(e) = 442.0 +/- 2 cm(-1).

The dynamics of nonadiabatic transitions in collisions between the I2(E) and I2(X) molecules

The paper presents the results of a theoretical study of the dynamics of nonadiabatic transitions between the ion-pair states E0g+ and D0u+ of the I2 molecule induced by collisions with the I2 molecule in the ground electronic state X0g+. The potential energy surfaces and diabatic coupling matrix elements of electronic states were obtained using a model based on the diatomics-in-molecule approximation. Special perturbation theory for intermolecular interaction was used to show that the large transition dipole moment between the E0g+ and D0u+ states caused the appearance of additional long-range corrections, an electrostatic dipole-quadrupole correction to the diabatic coupling matrix elements and induction dipole-dipole correction to the potential energy surface. The influence of these corrections on nonadiabatic dynamics was studied at the level of the semiclassical approximation. The electrostatic correction was found to sharply increase the contribution of resonance (accompanied by minimum kinetic energy changes) vibronic transitions at large distances between the colliding molecules. The induction correction had the opposite effect because of the high transition probability at short distances. The results obtained were in qualitative agreement with experimental data. The conclusion was drawn that obtaining quantitative agreement required a more balanced inclusion of interactions at short and long distances.

Direct observation of the D′ 2 g( 3P2)– A′ 3Π(2 u) system for Cl 2 by laser induced fluorescence spectroscopy: Determination of the absolute position of the A′ state

In a discharged supersonic jet of Cl 2, transitions of the D′ 2 g ( 3 P 2)– A′ 3Π(2 u ) system for 35Cl 2 were observed directly by laser induced fluorescence spectroscopy. By a discharge in Cl 2, the Cl 2 molecules were populated into the A′ state, which is a metastable and optically forbidden state, from the X 1 Σ g + ( 0 g + ) state. An ultraviolet laser radiation excites the molecules to the D′ ion-pair state. A set of Dunham parameters for the A′ state is determined from a global least-squares fitting for 59 vibronic bands with v″ = 0–7. In the fitting, the previously reported data, T( v) and B( v) for the v = 14 and 15 bands of the A′ state [T. Ishiwata, A. Ishiguro, K. Obi, J. Mol. Spectrosc. 147 (1991) 300–320], were included. Y 00 = 57295.723(5) cm −1 of the D′ state [J.-H. Si, T. Ishiwata, K. Obi, J. Mol. Spectrosc. 147 (1991) 334–345] was also included in the global fitting in order to determine the absolute position of the A′ state. The determined parameters of the A′ state are Y 00 = 17171.506(14), Y 10 = 255.915(85), Y 20 = −4.465(70), Y 30 = −8.7(23) × 10 −2, Y 40 = 6.3(35) × 10 −3, Y 50 = −4.9(26) × 10 −4, Y 60 = 1.43(69) × 10 −5, Y 01 = 0.16282(15), Y 11 = −2.363(68) × 10 −3, Y 21 = −5.01(93) × 10 −5, and Y 31 = −3.01(36) × 10 −6 (in cm −1 and one standard deviations of the fit in parentheses). The absolute position of the A′ state is determined with good accuracy.

Non-adiabatic E → D, D′, β, γ, δ transitions in the first ion-pair tier of molecular iodine induced by collisions with I 2, He, Ar, Kr, Xe

Optical–optical double resonance spectroscopy and quantum scattering calculations are used to obtain total rate constants, electronic branching ratios, and vibrational product state distributions for non-adiabatic transitions among the first-tier ion-pair states of the I 2 molecule excited to the E 0 g + , v E = 8 , 19 , J E ≈ 55 rovibronic levels in collisions with I 2, He, Ar, Kr and Xe. Theoretical predictions agree well with the experiment and reveal that the near-resonant E 0 g + → D0 u + transition dominates for collisions with He, while somewhat more statistical distributions are typical of collisions with heavier rare gas atoms. This trend is correlated to the dipole polarizability of the collision partner that determines the strength of intermolecular interactions.

Computational Study of CO2 Reduction by Amines

Calculations at the MP2/aug-cc-pVDZ//MPWB1K/aug-cc-pVDZ level are reported for the reduction of CO2 by amines--primarily triethylamine. A polarizable continuum model is used to represent acetonitrile solvent for the reaction. Starting from a photochemically generated radical ion pair state, the mechanism of reduction is deduced to be one in which the CO2(.-) begins to use one of its oxygens to abstract a hydrogen from an alpha carbon of the amine radical cation. During this event, and before the transition state for H transfer is reached, the system encounters a surface crossing, which provides pathways for unproductive back electron transfer and for productive reduction, with the latter involving attachment of the hydrogen to the carbon of CO2. The result of the reduction is a closed-shell iminium formate ion pair, which completes the reaction by proton transfer between the ions, to give an eneamine and formic acid. On the basis of the calculations, approaches for improving the efficiency of the reduction and increasing the wavelength of the light used to drive the reaction are discussed. One of these modifications involves the use of a bicyclic amine as reductant.

Long-range interaction and the dynamics of nonadiabatic transitions in collisions of the I2(E) molecule with inert gas atoms

A special form of perturbation theory based on intermolecular interaction was used to analyze the contributions to the interaction energy between a homonuclear diatomic molecule with dipole-coupled electronic states n and n′ and a neutral particle A. At large distances, the energy of the system in both states n and n′ is determined by the induction contribution similar to the induction interaction of particle A and a polar molecule. The presence of a constant electric moment of particle A gives an electrostatic contribution to the matrix element of the nonadiabatic coupling of the states n and n′. The equations obtained were specialized to describe the interaction of the iodine molecule excited to ion-pair states with an inert gas atom and used as corrections to the potential energy surfaces of this system constructed within the diatomics-in-molecule approximation. The dynamics of transitions between the ion-pair states of the I2 molecule induced by collisions with the Ar and He atoms was calculated. The results demonstrate the importance of correctly including long-range interaction.

Rovibrational resonance effects in collision-induced electronic energy transfer: I2(E,v=–2)+CF4

Collisions of I2 in the E(0(g)+) electronic state with CF4 molecules induce electronic energy transfer to the nearby D, beta, and D' ion-pair states. Simulations of dispersed fluorescence spectra reveal collision-induced electronic energy transfer rate constants and final vibrational state distributions within each final electronic state. In comparison with earlier reports on I2(upsilon(E)=0-2) collisions with He or Ar atoms, we find markedly different dynamics when I2, excited to the same rovibronic states, collides with CF4. Final vibrational state distributions agree with the associated Franck-Condon factors with the initially prepared state to a greater degree than those found with He or Ar collision partners and suggest that internal degrees of freedom in the CF4 molecule represent a substantial means for accepting the accompanying loss of I2 vibronic energy. Comparison of the E-->D transfer of I2 excited to the J=23 and J=55 levels of the upsilon(E)=0 state reveals the onset of specific, nonstatistical dynamics as the available energy is increased above the threshold for excitation of the low frequency nu2 bending mode of CF4.

Supramolecular Bis(rutheniumphthalocyanine)−Perylenediimide Ensembles: Simple Complexation as a Powerful Tool toward Long-Lived Radical Ion Pair States

A novel supramolecular electron donor-acceptor hybrid (1) has been designed through axial coordination of a perylenebisimide moiety [BPyPDI], bearing two 4-pyridyl substituents at the imido positions, to the ruthenium(II) metal centers of two phthalocyanines [Ru(CO)Pc]. This modular protocol enables access to electron donor-acceptor hybrids with potentially great design flexibility. The new array (1) has been characterized by standard spectroscopic methods, and its photophysical behavior has been established by using ultrafast and fast time-resolved techniques. Photoexcitation of either chromophore leads to a product that is essentially identical for both pathways, that is, evolving from the [Ru(CO)Pc] or [BPyPDI] singlet excited state. Features of the photoproduct are new transient maxima at 530 and 725 nm, plus transient minima at 580 nm and 650 nm. Based on the radiolytically generated [BPyPDI*-] (i.e., one-electron reduction of [BPyPDI]) and [Ru(CO)Pc*+] (i.e., one-electron oxidation of [Ru(CO)Pc]) features, which in the 300 and 900 nm range remarkably resemble those noted for photoexcited 1, we attribute the photolytically generated species to the composite spectrum of the [Ru(CO)Pc*+ -BPyPDI*- -RuCOPc] radical ion pair state. Its lifetime, which is on the order of 115 +/- 5 ns, reveals a significant stabilization and confirms that the strongly exothermic charge recombination dynamics are placed deeply in the inverted region of the Marcus parabola.

Electronic structures of intermolecular charge-transfer states in fast electron transfers with tetrathiafulvalene donor. Thermal and photoactivation of [2 + 4] cycloaddition to o-chloranil acceptor

Tetrathiafulvalene (TTF) spontaneously forms a series of unusual charge-transfer complexes with various quinonoid acceptors such as o-chloranil (CA) that show pronounced near-IR absorption (lambda(CT) = 1100 nm). The successful isolation of the corresponding [1 : 1] donor-acceptor complex from solution and X-ray crystallographic analysis at low temperatures reveal the polarized charge-transfer state: [TTF(q+),CA(q-)] with high degree of charge-transfer (q = 0.6), which is spectrally and crystallographically distinguished from the separate redox (ion-pair) state: [TTF(+*) + CA(-*) ]. The unique interconversion of charge-transfer and electron-transfer states is theoretically well-accommodated by Mulliken theory using semi-empirical valence-bond and molecular-orbital methodologies. Mechanistic implications are discussed of both the thermally activated and the photochemically promoted processes via fast (intracomplex) electron transfer followed by collapse of the adiabatic and the non-adiabatic (vibrationally-excited) ion-pairs, respectively, to the [2 + 4] cycloadduct of tetrathiafulvalene and o-chloranil.

Ion-pair formation mechanisms in chloromethane, bromomethane and dichlorodifluoromethane

The first absolute photoion-pair cross sections have been derived from measurements of the absolute photoabsorption cross section and the photoion-pair quantum efficiency. The results for CH3Cl, CH3Br and CF2Cl2 show that direct transitions into ion-pair states play a significant role. In each molecule, the structure in the photoion-pair spectrum is different to that in the photoabsorption spectrum and often does not resemble the strong bands, due to Rydberg transitions, prominent in the latter spectra. At photon energies associated with excitations into Rydberg states, and consequently with peaks in absorption, the photoion-pair quantum efficiency usually exhibits a local minimum. This indicates that the Rydberg state decays preferentially by predissociation into a state yielding neutral fragments rather than by predissociation into a state evolving into ion pairs.

Resonant optical excitation of the I 2 ion-pair states through the RgI 2 complexes in the valence states correlating to the 2P 1/2 + 2P 1/2 limit

The luminescence spectra of I 2(β1 g, ν β ⩽ 59 and D 0 u + , ν D ⩽ 61) ion-pair states are observed in I 2 + Rg mixtures after irradiation by Nd:YAG (1064.48 and 532.24 nm) laser beams. Luminescence excitation is attributed to two-step 1064.48 + 532.24 nm pumping of RgI 2 complexes in 0 g + and 1 u valence states correlating to third valence dissociation limit 2P 1/2 + 2P 1/2. Subsequent decay of the complexes gives free I 2 ( 1 u , 0 g + ) molecules, which are excited to the β1 g and D 0 u + states by third 532.24 nm photon. This mechanism conforms all experimental findings and provides the first indication for optical population of the RgI 2 complexes in the valence states different from the B 0 u + and X 0 g + .