Time-resolved luminescence spectroscopy of self-trapped excitons in ladder type Br-bridged Pt complexes

The out-of-phase and in-phase ladder type Br-bridged Pt complexes are investigated by time-resolved luminescence spectroscopy in pico- and femtosecond time regions. The observed luminescence spectra have peaks at 0.87 and 0.94 eV in out-of-phase and in-phase materials, respectively, and are assigned to self-trapped excitons. The wave-packet oscillations in self-trapped excitons (STE) are observed in both materials. The time-evolution curves are analyzed in terms of the secondary radiation theory of strongly coupled electron-phonon system. The period and dephasing time of oscillations as well as the lifetime and spectral shape of the STE luminescence are determined. The fast dephasing or cooling of the wave-packet motion observed in the in-phase type complex is ascribed to inter-chain interactions within the ladder.     

Rotational state-dependent mixings between resonance states of vibrationally highly excited DCO (X2A')

Rotational state-dependent mixings between highly excited resonance states of DCO (X (2)A(')) were investigated by stimulated emission pumping spectroscopy via a series of intermediate rotational levels in the B (2)A(') electronic state of the radical. Two examples for such interactions, between pairs of accidentally nearly degenerate vibrational states at energies of E(v) approximately 6450 and E(v) approximately 10 060 cm(-1), respectively, were analyzed in detail. Deperturbations of the measured spectra provided the zeroth-order vibration-rotation term energies, widths, and rotational constants of the states and the absolute values of the vibrational coupling matrix elements. The coupled states turned out to have very different A rotational constants so that their mixings switch on or off as they are tuned relative to each other as function of the K(a) rotational quantum number. The respective zeroth-order states could be assigned to different interlaced vibrational polyads. Thus, when two states belonging to different polyads are accidentally nearly isoenergetic, even very weak interpolyad interactions may start to play important roles. The derived interpolyad coupling elements are small compared to the typical intrapolyad coupling terms so that their influences on the vibrational term energies are small. However, large effects on the widths (i.e., decay rates) of the states can be observed even from weak coupling terms when a narrow, long-lived state is perturbed by a broad, highly dissociative state. This influence contributes to the previously observed strong state-to-state fluctuations of the unimolecular decay rates of the DCO radical as function of vibrational excitation. Similar mechanisms are likely to promote the transition to "statistical" rates in many larger molecules.     

On the core-hole photoelectron satellites in N2 and CO

A theoretical study is given of core-level photoelectron satellites with special emphasis on the π?π* satellites in N₂ and CO. The discussion is based on an analysis of the properly constructed zeroth-order manifold of satellite states, and provides simple explanations for both bound state and photoelectron continuum aspects of core-level ionization. Similar to the frozen core static exchange approximation for the single core-hole states, we derive one-particle scattering potentials for the satellite states.     

Intruder states in multireference perturbation theory: the ground state of manganese dimer

A detailed analysis of a severe intruder state problem in the multistate multireference perturbation theory (MS-MRPT) calculations on the ground state of manganese dimer is presented. An enormous number of detected intruder states (> 5000) do not permit finding even an approximate shape of the X(1)Sigma(g) (+) potential energy curve. The intruder states are explicitly demonstrated to originate from quasidegeneracies in the zeroth-order Hamiltonian spectrum. The electronic configurations responsible for appearance of the quasidegeneracies are identified as single and double excitations from the active orbitals to the external orbitals. It is shown that the quasidegeneracy problem can be completely eliminated using shift techniques despite of its severity. The resultant curves are smooth and continuous. Unfortunately, strong dependence of the spectroscopic parameters of the X(1)Sigma(g) (+) state on the shift parameter is observed. This finding rises serious controversies regarding validity of employing shift techniques for solving the intruder state problem in MS-MRPT. Various alternative approaches of removing intruder states (e.g., modification of the basis set or changing the active space) are tested. None of these conventional techniques is able to fully avoid the quasidegeneracies. We believe that the MS-MRPT calculations on the three lowest A(g) states of manganese dimer constitute a perfect benchmark case for studying the behavior of MRPT in extreme situations.     

Coupling of identical chemical oscillators

Identical BZ oscillators, in a CSTR, modeled by the Field-Körös-Noyes (FKN) mechanism, are coupled in a diffusion-like manner. In addition to the obvious symmetric solutions, i.e. solutions in which both CSTRs are oscillating in unison, or are in the same stable steady state, unsymmetric, broken symmetry, solutions may coexist under the same set of constraints. Thus, depending on constraints and initial conditions, the combined system can be in the following states: a) stable symmetric steady state. b) symmetric oscillations, when both cells oscillate in phase. c) coexistence of symmetric and unsymmetric steady states. d) coexistence of symmetric oscillations and unsymmetric stable steady state (broken symmetry). e) coexistence of symmetric and unsymmetric oscillations. The latter differ from the former in phase, in amplitude and in period. On the other hand, no unsymmetric oscillations were found to coexist with the symmetric steady state. All the initial conditions tried ended in either of the two, possible, stable states. The change of periods and amplitude of both types of oscillations are examined as a function of the system constraints namely, concentrations and coupling rate.     

Intramolecular vibrational energy redistribution as state space diffusion: classical-quantum correspondence

We study the intramolecular vibrational energy redistribution (IVR) dynamics of an effective spectroscopic Hamiltonian describing the four coupled high frequency modes of CDBrClF. The IVR dynamics ensuing from nearly isoenergetic zeroth-order states, an edge (overtone) and an interior (combination) state, is studied from a state space diffusion perspective. A wavelet based time-frequency analysis reveals an inhomogeneous phase space due to the trapping of classical trajectories. Consequently the interior state has a smaller effective IVR dimension as compared to the edge state.     

Spherical‐harmonics expansion method for density‐matrix simulations of quantum electron dynamics in continuum states

Spherical‐harmonics expansion method is proposed to solve the quantum time‐evolution equations for density matrices numerically in momentum space. This method facilitates efficient real‐time simulations of quantum electron dynamics in continuum states through extension of our previous formalism developed for discrete states. Numerical accuracy and efficiency are demonstrated through two examples: (i) multiphoton ionization of a one‐electron atom in intense laser field, and (ii) real‐time dynamics of plasma oscillations in electron liquids. In case (i), coupled dynamics of density matrices for bound and continuum states reveals an enhancement of multiphoton ionization over the Keldysh approximation through resonant intermediate states. © 2015 Wiley Periodicals, Inc.     

Rydberg series of autoionizing states in strong laser fields

We study the behavior of Rydberg series of resonances excited by intense laser fields. Our approach is based on a semiclassical formalism for the evolution in time of the atomic states coupled by the laser. The atomic system is described by a two-channel model in a Multichannel Quantum Defect Theory approach. We calculate the total ionization and the photoelectron spectrum after a certain interaction time. We present results both for on-resonance and off-resonance excitation of the series. We employ a more or less realistic pulse shape that corresponds to a narrow Fourier bandwidth. We show that the effects of the non-resonant members of the series on the photoelectron spectra can be important and we study it both as a function of laser intensity and as a function of the interaction time. We also show that our model correctly describes the Rabi oscillations between the ground state and the excitedAI state when the field is sufficiently strong.     

Vibrational coupling in carboxylic acid dimers

The vibrational level splitting in the ground electronic state of carboxylic acid dimers mediated by the doubly hydrogen-bonded networks are investigated using pure and mixed dimers of benzoic acid with formic acid as molecular prototypes. Within the 0-2000-cm(-1) range, the frequencies for the fundamental and combination vibrations of the two dimers are experimentally measured by using dispersed fluorescence spectroscopy in a supersonic jet expansion. Density-functional-theory calculations predict that most of the dimer vibrations are essentially in-phase and out-of-phase combinations of the monomer modes, and many of such combinations show significantly large splitting in vibrational frequencies. The infrared spectrum of the jet-cooled benzoic acid dimer, reported recently by Bakker et al. [J. Chem. Phys. 119, 11180 (2003)], has been used along with the dispersed fluorescence spectra to analyze the coupled g-u vibrational levels. Assignments of the dispersed fluorescence spectra of the mixed dimer are suggested by comparing the vibronic features with those in the homodimer spectrum and the predictions of density-functional-theory calculation. The fluorescence spectra measured by excitations of the low-lying single vibronic levels of the mixed dimer reveal that the hydrogen-bond vibrations are extensively mixed with the ring modes in the S1 surface.     

The i.r. spectroscopy of some highly conjugated systems—I. Rationale of the investigation

The solvent shift method of BELLAMY is re-examined, particularly as a tool for the identification of vibrations that include and are coupled to carbonyl, all of which show solvent sensitivity. For α,β-unsaturated ketones, it is shown that s-cis and s-trans conformers are readily distinguished by plots of νCC vs. νCO. The nature of this coupling is discussed and it is concluded that, in so far as coupling occurs, νCO and νCC take on the character of out-of-phase and in-phase modes respectively. In either conformer, the in-phase (νCC) mode gains in relative intensity as the frequencies converge. While s-cis conformers couple more, conjugation is unaffected by this; the true index of conjugation is neither νCO nor νCC but mean frequency ν_m. The whole of the solvent effect appears to be relayed by carbonyl: even a highly polar carbon double bond is insensitive to solvent unless so coupled.     

LIGHT QUENCHING OF FLUORESCENCE: A NEW METHOD TO CONTROL THE EXCITED STATE LIFETIME AND ORIENTATION OF FLUOROPHORES

Abstract Experimental studies have recently demonstrated that fluorescence emission can be quenched by laser light pulses from modem high-repetition rate lasers, a phenomenon we call “light quenching.” In this overview article, we describe the possible effects of light quenching on the steady-state and time-resolved intensity and anisotropy of fluorophores. One can imagine two classes of experiments. Light quenching can occur within the single excitation pulse, or light quenching can be accomplished with a second time-delayed quenching pulse. The extent of light quenching depends on the amplitude of the emission spectrum at the quenching wavelength. Different effects are expected for light quenching by a single laser beam (within a single laser pulse) or for a time-delayed quenching pulse. Depending upon the polarization of the light quenching beam, light quenching can decrease or increase the anisotropy. Remarkably, the light quenching can break the usual z-axis symmetry of the excited state population, and the measured anisotropy (or polarization) depends upon whether the observation axis is parallel or perpendicular to the propagation direction of the light quenching beam. The polarization can increase to unity under selected conditions. Quenching with time-delayed light pulses can result in step changes in the intensity or anisotropy, which is predicted to result in oscillations in the frequency-domain intensity and anisotropy decays. These predicted effects of light quenching, including oscillations in the frequency-domain data, were demonstrated to occur using selected fluorophores. The increasing availability and use of pulsed laser sources requires consideration of the possible effects of light quenching and offers the opportunity for a new class of two-pulse or multiple-pulse time-resolved experiments where the sample is prepared by the excitation pulse and subsequent quenching pulses to modify the excited state population, followed by time- or frequency-domain measurement of the optically prepared excited fluorophores.     

Synchronization of local oscillators in oxidation reactions of C1–C4 hydrocarbons over metal catalysts

The synchronization of reaction rate oscillations in the oxidation of C₁–C₄ hydrocarbons over polycrystalline nickel, cobalt, and palladium foils has been investigated. The synchronization of foil temperature oscillations during the reaction takes place via the diffusion of the reactants in the gas phase. For the nickel catalysts, the synchronization of the oscillators occurs in the same phase, while for the palladium catalysts, both in-phase and antiphase oscillations are observed. This distinction between the dynamic behaviors of the systems of two coupled oscillators is due to the fact that the mechanism of reaction rate oscillations varies from one metal to another.     

Effectively doubling the magnetic field in spin-1/2-spin-1, HSQC, HDQC, coupled HSQC, and coupled HDQC in solution NMR

Pulse sequences for spin-1/2-spin-1 pair heteronuclear single quantum correlation (HSQC), heteronuclear double quantum correlation (HDQC), and coupled-HSQC, and coupled-HDQC NMR spectroscopies are outlined, and experimental realization for a (13)C-(2)H pair is demonstrated in solution state. In both the coupled versions, conditions for generation of in-phase and antiphase multiplets in either dimension are arrived at. The patterns and the intensity ratios are explained. The double quantum (2Q) experiments confirm doubling of both the shift frequency and the splitting due to coupling (to spin 1/2) of the 2Q coherence emanating from spin 1. The frequency doubling is equivalent to the corresponding single quantum (1Q) coherence at double the magnetic field strength. The coupling doubling, however, is independent of the magnetic field strength and a signature feature of the 2Q coherence. The ramification of the relative relaxation rates of 1Q and 2Q coherences is discussed.     

Dynamical regimes of a pH-oscillator operated in two mass-coupled flow-through reactors

We present results of experiments focused on emergent and cooperative dynamics in a system of two coupled flow-through stirred reaction cells with diffusion-like mass exchange and a strongly nonlinear chemical reaction between hydrogen peroxide and thiosulphate catalysed by cupric ions in diluted solution of sulphuric acid. Due to complex mechanism, in which a crucial role is played by hydrogen and/or hydroxide ions, dynamics in a single cell entail multiple stationary states, excitability and oscillations conveniently indicated by measuring pH. When coupled, the system shows a plethora of dynamical regimes depending on the coupling strength and flow rate. Under certain conditions both cells display dynamics close to that in the absence of coupling, but majority of the regimes are emergent and cannot be deduced from dynamics of decoupled reactors. The most prominent is a stationary state maintaining highly acidic values of pH in one of the reactors and weakly acidic in the other. When each cell is set to display excitability and the coupled system is externally perturbed, the cells may cooperate and transmit excitations elicited by pulsed perturbations in one cell to the other. Periodic pulses induce firing patterns marked by a various degree of propagated excitations and by being periodic or irregular.     

Relaxation and anharmonic couplings of the O-H stretching vibration of asymmetric strongly hydrogen-bonded complexes

We present infrared transient grating measurements of complexes of formic acid with pyridine and pyrazine at four excitation frequencies within the broad proton-stretching band. These experiments investigate the mechanism of the line broadening of the O-H stretching vibration. The transients show coherent oscillations that decay within a few hundred femtoseconds and population relaxation on two time scales. We fit the data using a simple model of three coupled oscillators that relax via sequential kinetics through an intermediate state. Based on this model, we conclude that the coherent oscillations result from superpositions of Fermi-resonance-coupled states involving formic acid overtone and combination states.     

Laser field induced charge transfer: Para-nitroaniline coupled to a quantum mechanical radiation field

Summary We present a preliminary model for describing a solvated intramolecular charge transfer reaction coupled to a quantum mechanical radiation field. Actual calculations of energies and couplings were performed with a recently developed self-consistent reaction field response method. The representation of dressed molecular states is used for calculating state populations for various laser fields. The state populations are sensitive to the properties of the laser field.     

Algebraic modifications to second quantization for non‐Hermitian complex scaled hamiltonians with application to a quadratically convergent multiconfigurational self‐consistent field method

The algebraic structure for creation and annihilation operators defined on orthogonal orbitals is generalized to permit easy development of bound‐state techniques involving the use of non‐Hermitian Hamiltonians arising from the use of complex‐scaling or complex‐absorbing potentials in the treatment of electron scattering resonances. These extensions are made possible by an orthogonal transformation of complex biorthogonal orbitals and states as opposed to the customary unitary transformation of real orthogonal orbitals and states and preserve all other formal and numerical simplicities of existing bound‐state methods. The ease of application is demonstrated by deriving the modified equations for implementation of a quadratically convergent multiconfigurational self‐consistent field (MCSCF) method for complex‐scaled Hamiltonians but the generalizations are equally applicable for the extension of other techniques such as single and multireference coupled cluster (CC) and many‐body perturbation theory (MBPT) methods for their use in the treatment of resonances. This extends the domain of applicability of MCSCF, CC, MBPT, and methods based on MCSCF states to an accurate treatment of resonances while still using L² real basis sets. Modification of all other bound‐state methods and codes should be similarly straightforward. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Stereocontrol of attosecond time-scale electron dynamics in ABCU using ultrafast laser pulses: a computational study

The attosecond time-scale electronic dynamics induced by an ultrashort laser pulse is computed using a multi configuration time dependent approach in ABCU (C(10)H(19)N), a medium size polyatomic molecule with a rigid cage geometry. The coupling between the electronic states induced by the strong pulse is included in the many electron Hamiltonian used to compute the electron dynamics. We show that it is possible to implement control of the electron density stereodynamics in this medium size molecule by varying the characteristics of the laser pulse, for example by polarizing the electric field either along the N-C axis of the cage, or in the plane perpendicular to it. The excitation produces an oscillatory, non-stationary, electronic state that exhibits localization of the electron density in different parts of the molecule both during and after the pulse. The coherent oscillations of the non-stationary electronic state are also demonstrated through the alternation of the dipole moment of the molecule.     

Design of UV laser pulses for the preparation of matrix isolated homonuclear diatomic molecules in selective vibrational superposition states

The preparation of matrix isolated homonuclear diatomic molecules in a vibrational superposition state c0Phie=1,v=0+cjPhie=1,v=j, with large (|c0|2 approximately 1) plus small contributions (|cj|2<1) of the ground v=0 and specific v=j low excited vibrational eigenstates, respectively, in the electronic ground (e=1) state, and without any net population transfer to electronic excited (e>1) states, is an important challenge; it serves as a prerequisite for coherent spin control. For this purpose, the authors investigate two scenarios of laser pulse control, involving sequential or intrapulse pump- and dump-type transitions via excited vibronic states Phiex,k with a dominant singlet or triplet character. The mechanisms are demonstrated by means of quantum simulations for representative nuclear wave packets on coupled potential energy surfaces, using as an example a one-dimensional model for Cl2 in an Ar matrix. A simple three-state model (including Phi1,0, Phi1,j and Phiex,k) allows illuminating analyses and efficient determinations of the parameters of the laser pulses based on the values of the transition energies and dipole couplings of the transient state which are derived from the absorption spectra.