Excited-state Population Decay Research Articles

Influence of Mode-Specific Excitation on the Nonadiabatic Dynamics of Methyl Nitrate (CH3ONO2).

The impact of mode-specific vibrational excitations on initial-preparation conditions was studied by examining the excited-state population decay rates in the nonadiabatic dynamics of methyl nitrate (CH3ONO2). In particular, exciting a few specific modes by adding a single quantum of energy clearly decelerated the nonadiabatic dynamics population decay rates. The underlying reason for this slower population decay was explained by analyzing the profiles of the excited-state potential energy surfaces in the Franck-Condon regions and the topology of the S1/S0 conical intersection. This study not only provides physical insights into the key mechanisms controlling nonadiabatic dynamics but also shows the possibility of controlling nonadiabatic dynamics via mode-specific vibrational excitations.

Structural or population dynamics: what is revealed by the time-resolved photoelectron spectroscopy of 1,3-cyclohexadiene? A study with an ensemble density functional theory method.

Time-resolved photoelectron spectra during the photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) are modeled by an ensemble density functional theory (eDFT) method. The computational methodology employed in this work is capable of correctly describing the multi-reference effects arising in the ground and excited electronic states of molecules, which is important for the correct description of the ring-opening reaction of CHD. The geometries of molecular species along the non-adiabatic molecular dynamics (NAMD) trajectories reported in a previous study of the CHD photochemical ring-opening were used in this work to calculate the ionization energies and the respective Dyson orbitals for all possible ionization channels. The obtained theoretical time-resolved spectra display decay characteristics in a reasonable agreement with the experimental observations; i.e., the decay (and rise) of the most mechanistically significant signals occurs on the timescale of 100-150 fs. This is very different from the excited state population decay characteristics (τS1 = 234 ± 8 fs) obtained in the previous NAMD study. The difference between the population decay and the decay of the photoelectron signal intensity is traced back to the geometric transformation that the molecule undergoes during the photoreaction. This demonstrates the importance of including the geometric information in interpretation of the experimental observations.

Quantum dynamics on a three-sheeted six-dimensional ab initio potential-energy surface of the phosphine cation: Simulation of the photoelectron spectrum and the ultrafast radiationless decay dynamics.

A diabatic three-sheeted six-dimensional potential-energy surface has been constructed for the ground state and the lowest excited state of the PH3 (+) cation. Coupling terms of Jahn-Teller and pseudo-Jahn-Teller origin up to eighth order had to be included to describe the pronounced anharmonicity of the surface due to multiple conical intersections. The parameters of the diabatic Hamiltonian have been optimized by fitting the eigenvalues of the potential-energy matrix to ab initio data calculated at the CASSCF/MRCI level employing the correlation-consistent triple-ζ basis. The theoretical photoelectron spectrum of phosphine and the non-adiabatic nuclear dynamics of the phosphine cation have been computed by propagating nuclear wave packets with the multiconfiguration time-dependent Hartree method. The theoretical photoelectron bands obtained by Fourier transformation of the autocorrelation function agree well with the experimental results. It is shown that the ultrafast non-radiative decay dynamics of the first excited state of PH3 (+) is dominated by the exceptionally strong Jahn-Teller coupling of the asymmetric bending vibrational mode together with a hyperline of conical intersections with the electronic ground state induced by the umbrella mode. Time-dependent population probabilities have been computed for the three adiabatic electronic states. The non-adiabatic Jahn-Teller dynamics within the excited state takes place within ≈5 fs. Almost 80% of the excited-state population decay to the ground state within about 10 fs. The wave packets become highly complex and delocalized after 20 fs and no further significant transfer of electronic population seems to occur up to 100 fs propagation time.

Decoherence in models for hard-core bosons coupled to optical phonons

Understanding coherent dynamics of excitons, spins, or hard-core bosons (HCBs) has tremendous scientific and technological implications for light harvesting and quantum computation. Here, we study decay of excited-state population and decoherence in two models for HCBs, namely: an infinite-range HCB model governed by Markovian dynamics and a two-site HCB model with site-dependent strong potentials and subject to non-Markovian dynamics. Both models are investigated in the regimes of antiadiabaticity and strong HCB-phonon coupling with each site providing a different local optical phonon environment; furthermore, the HCB systems in both models are taken to be initially uncorrelated with the environment in the polaronic frame of reference. For the infinite-range model, we derive an effective many-body Hamiltonian that commutes with the long-range system Hamiltonian and thus has the same set of eigenstates; consequently, a quantum-master-equation approach shows that the quantum states of the system do not decohere. In the case of the two-site HCB model, we show clearly that the degree of decoherence and decay of excited state are enhanced by the proximity of the site-energy difference to the eigenenergy of phonons and are most pronounced when the site-energy difference is at resonance with twice the polaronic energy; additionally, the decoherence and the decay effects are reduced when the strength of HCB-phonon coupling is increased. Even for a multimode situation, the degree of decoherence and decay are again dictated by the nearness of the energy difference to the allowed phonon mode eigenenergies.

Theoretical study of constant current scanning tunneling spectroscopy in Pb overlayers

We present a theoretical study of the constant current scanning tunneling spectroscopy of quantum well states localized in Pb(111) overlayers on Cu(111) surfaces. The distance-voltage characteristic of the tunneling junction is obtained with a mixed approach. The wave packet propagation technique is applied to describe electron tunneling from the tip into the sample, and the density functional calculations provide the necessary inputs for the one-dimensional model potential representation of the system. The excited-state population decay mechanisms via inelastic electron-electron and electron-phonon interactions are taken into account with a bias-dependent absorbing potential introduced in the metal. Our results are in good agreement with recent experimental studies [Phys. Rev. Lett. 102, 196102 (2009), Phys. Rev. B 81, 205438 (2010)] over the energy range where the free-electron description of the Pb overlayer used here applies. We find that at high bias the quantum well states experience a Stark energy shift and partially acquire a character of field emission resonances. The present model study also sheds light at the experimentally observed departure of the energies of the quantum well states from the particle-in-a-box prediction for the bias above 4 eV. The measured trend can be consistently explained as due to the departure of the realistic Pb band structure in the $\ensuremath{\Gamma}\ensuremath{-}\text{L}$ direction from free-electron parabola when the electron momentum approaches the $\ensuremath{\Gamma}$ point.

Origin of the Red Sites and Energy Transfer Rates in Single MEH‐PPV Chains at Low Temperature

Single poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) molecules dispersed in thin poly(methylmethacrylate) films have been investigated by fluorescence emission, excitation and time-resolved spectroscopy at 1.2 K. For the molecular weight studied (∼200 kDa) a bimodal distribution of emission maxima is observed. Based on a comparison of the spectroscopic properties of blue and red sites and on polarisation-resolved measurements, we argue in agreement with recent quantum-chemical calculations that the red subpopulation most probably does not arise from interchromophoric excitation delocalisation but is to be attributed to longer chromophoric units originating from ordered regions of a polymer chain, where due to constraints on the chain conformation larger conjugation lengths can be realised. In excitation spectra within the red spectral region we can identify multiple chromophoric units, among them chromophores without correspondence in the emission spectrum-donors of the intramolecular energy transfer. Zero-phonon lines of donor chromophores proved to be significantly broadened, indicating fast excited-state population decay due to energy transfer. Thus, a distribution of energy transfer times within MEH-PPV chains could be determined from donor zero-phonon line widths, with an average value of 3.9 ps. Our study represents the first direct measurement of energy transfer times in conjugated polymers, parameters that are crucial for the performance of many technical applications based on this class of material.

Global Analysis of Time-Resolved Anisotropy from Multiple Probes in a Rigid Globular Protein

Rotational diffusion of a rigid globular protein is described by a second-rank tensor (a 3x3 matrix). In the reference frame aligned with the principal axes only the three diagonal elements are non-zero. In the general case one must assume that all three principal diffusion coefficients are different. The knowledge of the three coefficients makes it possible to estimate the protein shape. The aim of this work is determining the three principal diffusion coefficients and the approximate shape of a protein from the time-resolved fluorescence anisotropy data. We developed from basic principles the theory of polarized fluorescence for naturally occurring fluorophores within rigid proteins. Our theory does not assume that the excited-state population decay is monoexponential or that the exciting wavelength is in resonance with a single electronic transition. Thus, our model is applicable to tryptophan residues. In the special case of monoexponential fluorescence and a single electronic transition our model becomes equivalent to that of Chuang and Eisenthal J. Chem. Phys. 57, 5094 (1972). For an arbitrary-shape rigid protein the time-resolved anisotropy is a linear combination of five exponentials that cannot be resolved if the rate constants are treated as free fitting parameters. In our model the five rate constants are functions of only three principal diffusion coefficients, which play the roles of the global fitting parameters. Furthermore, we simultaneously analyze data sets obtained using two exciting wavelengths (varying 1La and 1Lb excitation) and multiple single-tryptophan-containing variants of the same protein. The preexponential amplitudes vary as a function of the directions of 1La and 1Lb transitions for each variant. In principle, this makes it possible to find both the shape of the protein and the probe orientations.

Effect of High-Frequency Modes and Hot Transitions on Free Energy Gap Dependence of Charge Recombination Rate

The charge recombination (CR) dynamics of geminate ion pairs formed by excitation of the ground-state donor-acceptor complexes in polar solvent have been investigated within the framework of stochastic approach. It is shown that for low exergonic reactions these dynamics critically depend on the reorganization energy of intramolecular high-frequency mode. Even moderate reorganization energies (0.1-0.2 eV) significantly accelerate the excited-state population decay making it nearly exponential. In the solvent-controlled regime, the majority of the excited donor-acceptor complexes recombine at nonthermal (hot) stage when the nonequilibrium initial wave packet passes through a number of term crossings corresponding to the transitions with creation of several vibrational quanta. Analysis of this mechanism allows to conclude (i) the CR in viscous solvents proceeds much faster than the diffusive relaxation of solvent, (ii) under certain conditions, the CR rate becomes practically independent of the diffusive component of solvent relaxation which is determined by solvent viscosity, (iii) in contrast to predictions of Marcus theory, the CR rate decreases monotonically with the rise of reaction exergonicity even at small free energy gaps, in accordance with experimental results. Two semiquantitative approaches providing rather simple analytical expressions for the hot charge recombination dynamics are suggested. These approximations give a good reproduction of the excited-state decay in the wide area of model parameters.

Nonadiabatic Trajectory Studies of NaI(H2O)n Photodissociation Dynamics

We have investigated the photodissociation dynamics of NaI(H(2)O)(n) [n = 1-4] clusters using the molecular dynamics with quantum transitions method and a quantum mechanics/molecular mechanics description of NaI(H(2)O)(n), which involves a semiempirical valence-bond approach to describe the NaI electronic structure and classical solvent-solvent and solute-solvent interaction potentials. Our simulation results show that the NaI(H(2)O)(n) excited-state population decay upon reaching the NaI curve-crossing region increases with cluster size due to the stabilization of the ionic branch of the NaI excited state by the surrounding water molecules, and the resulting increase in nonadiabatic transition probability. After reaching the curve-crossing region for the first time, however, the excited-state population decay resembles that of bare NaI because of rapid evaporation of 99% and 95% of the water molecules for NaI(H(2)O) and NaI(H(2)O)(n) [n = 2-4], respectively. This extensive evaporation is due to the reversed NaI polarity in the Franck-Condon region of the NaI first excited state, which causes strong repulsive NaI-H(2)O forces and induces rapid nonstatistical water evaporation, where product water molecules are formed more rotationally than translationally hot. A few water molecules (5% or less) remain transiently or permanently bound to NaI, forming long-lived clusters, when NaI remains predominantly ionic, i.e., remains in the excited state, after reaching the curve-crossing region. To connect simulation results with experiment, we have simulated femtosecond probe signals resulting from two-photon and one-photon excitation to the X and I NaI(+) probe states. In agreement with experimental findings, the probe signals resulting from the two-photon probe scheme, where excitation occurs from the covalent branch of the excited state, decay exponentially over the NaI first excited-state vibrational period, with very little evidence of long-time dynamics. The one-photon probe scheme (not used for experimental cluster studies) is shown to be less sensitive to solvation, in that excitation energies will remain similar over a range of cluster sizes, as the ionic branch of the excited state and the NaI(+) probe states are stabilized to the same extent by the presence of water molecules. The resulting probe signals are also more revealing of the NaI(H(2)O)(n) photodissociation dynamics than the two-photon probe signals, as they may allow monitoring of solvation effects on the NaI nonadiabatic dynamics and of successive evaporation of water molecules. Time-resolved photoelectron spectra provide limited additional information regarding the NaI(H(2)O)(n) photodissociation dynamics. A key consequence of the rapid water evaporation demonstrated here is that experimentally observed signals may arise from the photodissociation of much larger NaI(H(2)O)(n) parent clusters.

Observation of Coherent Recurrence Motion of Excitons in Anthracene Dimers

Abstract Intramolecular electronic excitation transfer in anthracene dimers in solution at room temperature has been investigated by probing the fluorescence anisotropy decay with a femtosecond up-conversion method. Two types of anthracene dimers were used: anthracenophane (DTA) in which two anthracene rings are stacked parallel, but with nearly orthogonal orientation, and two dianthrylbenzenes (m- and o-DAB) in which two anthracene rings are linked to a benzene ring at meta- and ortho-positions. There appeared damped oscillations of apparent periods of 0.6–1.0 ps and damping time constants of 0.7–1.2 ps. The ordinary fluorescence decay (excited-state population decay) obtained with the magic angle excitation exhibits no oscillation in all the cases. It has been found from a theoretical analysis that the oscillatory behaviors on fluorescence anisotropy are consistent with the coherent recurrence motion of an exciton between two anthracene moieties. The magnitudes of the dipole–dipole energy transfer interaction are estimated to be 10–80 cm−1, which are considerably smaller than the experimental value (30–100 cm−1) deduced from the angular frequencies of oscillation. This means that some interchromophore interaction other than the dipole–dipole resonance interaction is involved in the energy transfer interaction in strongly coupled dimers. The longer dephasing time can be explained as a consequence of rigid dimeric conformations in anthracene dimers.

Population shelved all-optical modulation

An all-optical modulation method is proposed and experimentally demonstrated to overcome the modulation bandwidth limitations, imposed by the excited-state population decay rate, in a conventional resonant optical modulator. In the experimental demonstration, we have observed an all-optical modulation bandwidth \ensuremath{\sim}200 times wider than the population decay rate. The enhancement of the modulation bandwidth is due to the fast spin dephasing caused by spin inhomogeneous broadening.

A simple model for the calculation of nonlinear optical response functions and femtosecond time-resolved spectra

A simple model is developed for the calculation of optical response functions for electronically nonadiabatic systems coupled to a heat bath. A phenomenological dissipation ansatz is suggested which describes the major bath-induced relaxation processes, namely excited-state population decay, optical dephasing, and vibrational relaxation. The model is constructed in such a way that it allows one to express the nonlinear response functions for a dissipative system in terms of those for the corresponding bath-free system. The explicit formulas for the third order response functions are given. Several special cases are considered for which the evaluation of the response functions simplifies considerably. To illustrate the performance and validity of the theory, the response functions have been used for the calculation of time and frequency gated spontaneous emission spectra. The time-dependent spectra derived from the model have been compared with those calculated (i) for the standard damped harmonic oscillator model and (ii) for a model nonadiabatic electron-transfer system with Redfield theory. It is concluded that the present model provides qualitatively correct response functions and, therefore, may be useful for the interpretation of observed time-resolved spectra.

Exciton Annihilation in J-Aggregates Probed by Femtosecond Fluorescence Upconversion

Femtosecond fluorescence upconversion was used to study the excited-state dynamics of two-dimensional J-aggregates of a thiacyanine dye. At high intensities the excited-state population decay is faster than exponential and the decay is intensity dependent. A singlet−singlet exciton annihilation decay mechanism is consistent with the observed behavior. A value of 4 × 1012 molecule s-1 is calculated for the exciton annihilation constant γ. Exciton annihilation measurements reported in the literature for J-aggregates of several cyanine dyes are recast into the same units for comparison. The physical state of these J-aggregates is elucidated to explore the relationship between the magnitude of the exciton annihilation constants and the liquid-crystalline nature of these aqueous colloidal systems. Attempts to time-resolve spectral features related to intraband relaxation in the excited excitonic states of the thiacyanine J-aggregate prior to annihilation indicate that these relaxation processes are faster than 100 fs at room temperature.

Energy transfer in a conjugated polymer with reduced inter-chain coupling

Energy transfer in spin-coated films and solutions of poly(dioctylphenylthiophene) with bulky side chains has been studied by femtosecond pump-probe spectroscopy, polarisation anisotropy and excitation annihilation. A red shift of stimulated emission spectra with time and a gradual decay of anisotropy are observed in film and solution and attributed to excitation migration towards lower-energy sites. At high-excitation intensities singlet-singlet annihilation with time-independent rate dominates the decay of excited-state population in the film for short times. It is concluded that energy migration in film, which results in effective excitation trapping and nonradiative decay, is a three-dimensional process, while in solution a one-dimensional intra-chain energy migration is much more limited.

Nonlinear polarization description of phase-locked pulse-pair spectroscopy

The recently demonstrated technique of optically phase-locked pulse-pair (PLPP) excited spontaneous emission is described by a third-order perturbative density matrix approach. A nonlinear polarization description shows how PLPP spectroscopy depends on all the relevant material dephasing time scales. The time and frequency integrated resonance spontaneous emission consists entirely of resonance fluorescence, and is derived exclusively from excited-state population decay terms, i.e., diagonal second-order density-matrix elements. These third-order polarization results are proportional to the previously derived linear polarization expressions found to describe the observed PLPP I2 vapor emission. The nonlinear treatment allows a comparison of this technique to other forms of ultrafast pump–probe spectroscopies such as transient absorption and photon echo techniques. The role of impulsively prepared coherences is clearly described by this analysis. The effect of pulse duration, relative to material dephasing times, is explored for relaxation given by the optical Bloch equations. The most significant differences between a linear and nonlinear polarization treatment occur for pulse durations greater than the optical dephasing time or when excited state population and coherence decays are of the order of rovibrational periods. A fluorescence line narrowing effect, due to short pulse excitation, is predicted.

Theory of transient hole-burning spectrum for molecules in solution

Theory based on a configuration coordinate model is presented to clarify solute–solvent relaxation effect on the line shape of a time-resolved hole-burning spectrum. The time-resolved hole-burning spectrum is affected by both a hole created in a ground-state population and an excess excited-state population. The solute–solvent relaxation causes thermalization in both populations and then the shift and broadening of the spectrum is expected observable. The line shapes of three typical cases are mainly considered: (1) The excited state is assumed to have an infinitely long lifetime; (2) the excited state has a finite lifetime, the population being assumed to relax toward an electronic state other than the ground state, and (3) the population created in the excited state relaxes to the ground state, where a hole-filling effect is taken into account. Results obtained by numerical calculations show that a time evolution of the ground-state hole is clearly observed when the low-energy tail of the absorption band is excited under the condition that the population decay to the other electronic state is very fast. Further, it is predicted that the hole-filling effect causes a considerable deformation of the spectral shape when the solvent relaxation rate is comparable to the excited-state population decay rate.