Excited State Manifold Research Articles

Ab initio and quantum-defect calculations for the Rydberg states of ArH

Potential energy curves were evaluated for the ground and thirteen low-lying excited electronic states of the ArH molecule over a wide range of internuclear distances by the multi-reference averaged quadratic coupled cluster method. The ab initio energy differences and transition dipole moments were used to estimate Einstein emission coefficients, absorption oscillator strengths and radiative lifetimes. Diagonal and off-diagonal quantum defects, as functions of internuclear distance, were extracted from ab initio potentials of the lowest Rydberg states of the neutral ArH molecule by taking account of configuration interaction between Rydberg series converging to the ground and two electronic excited states of the ArH(+) cation. The derived quantum-defect functions were used to generate manifolds of higher excited Rydberg states. The agreement between experimental and calculated energies and radiative transition probabilities was found to be as good as or better than that obtained by earlier calculations.

Ultrashort pulse propagation in multilevel systems

Here, I discuss the propagation of an ultrashort pulse through a collection of harmonic multilevel systems. In the limit of weak excitation and a large number of excited states, I show that the amplitude of the input driving pulse decays exponentially with propagation distance. The absorption coefficient associated with this decay is determined by the characteristic time of the manifold of excited states, instead of the polarization decay time as in the conventional absorption coefficient of a two-level atom. The input ultrashort pulse creates in the excited states a wave packet, which oscillates emitting secondary pulses in the process. Analytic solutions are obtained that describe the propagation of individual wave-packet re-emission pulses, and it is shown that their phase depends on the detuning of the input pulse.

Tuning Energy Transfer in the Peridinin–chlorophyll Complex by Reconstitution with Different Chlorophylls

In vitro studies of the carotenoid peridinin, which is the primary pigment from the peridinin chlorophyll-a protein (PCP) light harvesting complex, showed a strong dependence on the lifetime of the peridinin lowest singlet excited state on solvent polarity. This dependence was attributed to the presence of an intramolecular charge transfer (ICT) state in the peridinin excited state manifold. The ICT state was also suggested to be a crucial factor in efficient peridinin to Chl-a energy transfer in the PCP complex. Here we extend our studies of peridinin dynamics to reconstituted PCP complexes, in which Chl-a was replaced by different chlorophyll species (Chl-b, acetyl Chl-a, Chl-d and BChl-a). Reconstitution of PCP with different Chl species maintains the energy transfer pathways within the complex, but the efficiency depends on the chlorophyll species. In the native PCP complex, the peridinin S1/ICT state has a lifetime of 2.7 ps, whereas in reconstituted PCP complexes it is 5.9 ps (Chl-b) 2.9 ps (Chl-a), 2.2 ps (acetyl Chl-a), 1.9 ps (Chl-d), and 0.45 ps (BChl-a). Calculation of energy transfer rates using the Förster equation explains the differences in energy transfer efficiency in terms of changing spectral overlap between the peridinin emission and the absorption spectrum of the acceptor. It is proposed that the lowest excited state of peridinin is a strongly coupled S1/ICT state, which is the energy donor for the major energy transfer channel. The significant ICT character of the S1/ICT state in PCP enhances the transition dipole moment of the S1/ICT state, facilitating energy transfer to chlorophyll via the Förster mechanism. In addition to energy transfer via the S1/ICT, there is also energy transfer via the S2 and hot S1/ICT states to chlorophyll in all reconstituted PCP complexes.

Crystal growth and spectroscopic characterization of Yb 3+:LiTaO 3

Spectroscopic properties are presented for Yb 3+ incorporated into single crystals of LiTaO 3 grown by the top-seeded solution growth method. From an analysis of the absorption and fluorescence spectra, we are able to determine the Stark-level components of the 2F 7/2 (the ground-state multiplet manifold) and the 2F 5/2 (the excited-state multiplet manifold of Yb 3+ (4f 13)). The room-temperature fluorescence lifetime of 2F 5/2 is 678 μs as measured on a thin sample to reduce possibilities for reabsorption. Spectral comparisons of Yb 3+-doped LiTaO 3 and LiNbO 3 are drawn. The crystal-field splitting of Yb 3+(4f 13) in both crystal hosts is modeled using a set of crystal-field splitting parameters, B nm , determined from a recent spectroscopic analysis of Er 3+(4f 11) in LiNbO 3. Without adjustment of the B nm parameters, the model predicts the Stark-level energy and the symmetry label for each level in reasonable agreement with the experimental values. Less photorefractive than its niobate cousin, LiTaO 3 has potential for use in numerous integrated electro-optical circuits and devices.

The excited-state dynamics of magnesium octaethylporphyrin studied by femtosecond time-resolved four-wave-mixing

Femtosecond four-wave-mixing spectroscopy was used to study the initial molecular relaxation processes of magnesium octaethylporphyrin, an effective modelling compound for porphyrin and porphyrin-like compounds in nature. By choosing the excitation- and probe-wavelengths to be resonant with the S 1 and S 2 excited states, a detailed mapping of the relaxation dynamics within the excited-state manifold is achieved. This data allows the placing of an upper limit of ∼100 fs for the Q 01 → Q 00 electronic relaxation and due to the lower time resolution in experiments with excitation at 400 nm, of less than 350 fs for the Soret → Q 00 population flow. After thermal equilibration, the S 1 excited Q 00-state is depopulated on a time scale much longer than that accessible to our measurements. However, within the Q 00-band an intrastate relaxation is observed, which proceeds on the time scale of a few hundred femtoseconds depending on whether lower or higher excited vibrational levels are involved in the respective relaxational process.

Laser cooling of vibrational degrees of freedom of a molecular system

We consider the cooling of vibrational degrees of freedom in a photoinduced excited electronic state of a model molecular system. For the various parameters of the potential surfaces of the ground and excited electronic states and depending on the excitation frequency of a single-mode laser light, the average energy or average vibrational temperature of the excited state passes through a minimum. The amount of cooling is quantified in terms of the overlap integral between the ground and excited electronic states of the molecule. We have given an approach to calculate the Franck-Condon factor for a multimode displaced-distorted-rotated oscillator surface of the molecular system. This is subsequently used to study the effect of displacement, distortion, and Duschinsky rotation on the vibrational cooling in the excited state. The absorption spectra and also the average energy or the effective temperature of the excited electronic state are studied for the above model molecular system. Considering the non-Condon effect for the symmetry-forbidden transitions, we have discussed the absorption spectra and average temperature in the excited-state vibrational manifold.

A priori evaluation of the solvent contribution to the reorganization energy accompanying intramolecular electron transfer: Predicting the nature of the Creutz–Taube ion

The solvent contribution to the reorganization energy is one of the key physical properties controlling electron-transfer processes. A priori computational schemes have been developed that can predict realistic values for all other properties controlling charge transfer such as electronic couplings, vibronic couplings for intramolecular modes, the effects of asymmetry, and the effects of symmetric modes, but robust schemes for treating solvation remain elusive. In recent years, excited-state quantum chemical electronic-structure packages have gained the ability to calculate non-equilibrium solvation effects without the need to treat solvent explicitly, establishing the core of the required technology. While quantum chemical schemes involving excited-states represented as perturbed ground states can readily be applied in such calculations, they in general fail to include sufficient inactive-electron correlation to properly describe the excited-state manifold of charge transfer states, states for which only multi-reference procedures may robustly be applied. We develop computational procedures by which complete-active-space self-consistent-field (CASSCF) and related multi-reference calculations may be applied to electron-transfer systems whose energetics are controlled by the solvent–solute interaction. This is tested through the evaluation of the reorganization energy for electron-transfer in the bis-pentaammineruthenium complexes of α,ω-dipyridyl trans-polyenes in aqueous solution. Predictions are then made for the Creutz–Taube ion that place it clearly in the region of maximum breakdown of the Born–Oppenheimer approximation for which both the localized and delocalized diabatic descriptions of its electronic structure become inappropriate. This result is in qualitative agreement with conclusions drawn over the last four years based on a number of sources.

Angular momentum partitioning and the subshell multipole moments in impulsively excited argon ions

˛ s 1 Dd4p 2 F5/2 s463.7 nmd, s 1 Pd3d 2 D5/2˛ s 1 Dd4p 2 D5/2 s448.2 nmd, and s 1 Dd4s 2 D3/2˛ s 1 Dd4p 2 P3/2 s423.7 nmd. We develop the angular momentum algebra necessary to extract from these data, starting from the overall atomic J multipoles, the partitioning of orbital angular momentum into the 1 D core electric quadrupole and hexadecapole moments, and the outer 4p electric quadrupole moment. The magnetic dipole of the outer electron is also determined. This procedure requires the assumption of good LS coupling for these states, which is justified. We recouple these individual core- and outer-electron moments to calculate the initial electric quadrupoles, hexadecapoles, and hexacontatetrapoles of the initial excited-state manifold. The detailed time structure of the electron-atom collision is considered, as well as the time evolution of the excited ionic state. The Rubin-Bederson hypothesis is thus shown to hold for the initial ionic L and S terms. The consequences of the breakdown of LS coupling are considered. From the circular polarization data, estimates of the relative importance of direct and exchange excitation cross section are made. We discuss experimental issues related to background contributions, Hanle depolarization of the fluorescence signal, and cascade contributions. Nonlinearity of the equations relating the Stokes parameters to the subshell multipole moments complicates the data analysis. Details of the Monte Carlo terrain-search algorithm used to extract multipole data is discussed, and the implications of correlation between the various subshell multipole moments is analyzed. The physical significance of the higher-order multipole moments is discussed, and graphical representations of the effects of these multipoles on the excited ionic charge clouds is presented.

Ultrafast transient lens spectroscopy of various C40 carotenoids: lycopene, β-carotene, (3R,3′R)-zeaxanthin, (3R,3′R,6′R)-lutein, echinenone, canthaxanthin, and astaxanthin

The ultrafast internal conversion (IC) dynamics of seven C(40) carotenoids have been investigated at room temperature in a variety of solvents using two-color transient lens (TL) pump-probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and-for the first time-echinenone, as well as new data for lycopene, beta-carotene, (3R,3'R)-zeaxanthin and (3R,3'R,6'R)-lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S2 band at 400 nm. For the S2 lifetime solvent-independent upper limits on the order of 100-200 fs are estimated for all carotenoids studied. The S1 lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthin/echinenone/beta-carotene (13/12/11 double bonds) one finds tau1 approximately 5, 7.7 and 9 ps for the S1-->S0 IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on tau1, as observed for canthaxanthin/astaxanthin (tau1 approximately 5 ps in both cases). For all carotenoids studied, tau1 is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e.g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C40 carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured tau1.

Resolving the Excited State Equilibrium of Peridinin in Solution

The carotenoid peridinin is abundant in the biosphere, as it is the main pigment bound by the light-harvesting complexes of dinoflagellates, where it collects blue and green sunlight and transfers energy to chlorophyll a with high efficiency. Its molecular structure is particularly complex, giving rise to an intricate excited state manifold, which includes a state with charge-transfer character. To disentangle the excited states of peridinin and understand their function in vivo, we applied dispersed pump-probe and pump-dump-probe spectroscopy. The preferential depletion of population from the intramolecular charge transfer state by the dump pulse demonstrates that the S(1) and this charge transfer state are distinct entities. The ensuing dump-induced dynamics illustrates the equilibration of the two states which occurs on the time scale of a few picoseconds. Additionally, the dump pulse populates a short-lived ground state intermediate, which is suggestive of a complex relaxation pathway, probably including structural reorientation or solvation of the ground state. These findings indicate that the unique intramolecular charge transfer state of peridinin is an efficient energy donor to chlorophyll a in the peridinin-chlorophyll-protein complex and thus plays a significant role in global light harvesting.

The Long-Wavelength Chlorophyll States of Plant LHCI at Room Temperature: A Comparison with PSI-LHCI

The red antenna states of the external antenna complexes of higher plant photosystem I, known as LHCI, have been analyzed by measurement of their preequilibrium fluorescence upon direct excitation at 280 K. In addition to the previously detected F735 state, a hitherto undetected low-energy state with emission maximum around 713 nm was observed. The 280 K bandwidths (FWHM) are 55 nm for the F735 state and ∼27 nm for the F713-nm state, much greater than for non-red-shifted antenna chlorophylls. The origin absorption band for the F735-nm state was directly detected by determination of its excitation (action) spectrum and lies at 709–710 nm. The absorption spectrum for F735, calculated using the Stepanov expression, closely overlaps the excitation spectrum, indicating that the very large Stokes shift (25 nm) is due to vibrational relaxation within the excited-state manifold and solvent effects can be excluded. Fluorescence anisotropy measurements, with direct excitation of F735, indicate that the transition dipoles of the two red states are parallel. Similar experiments performed in the long-wavelength absorbing tail of PSI-LHCI indicate the presence of emission state(s) that are red-shifted with respect to F735 of isolated LHCI. It is suggested that these are brought about by interactions between the complexes in PSI-LHCI, which occur in some yet undefined way, and which are broken upon solubilization of the component parts.

Resonant charge exchange and relevant transport cross sections for excited states of oxygen and nitrogen atoms

Resonant charge-exchange cross sections and the relevant transport (diffusion) cross sections for excited states of nitrogen and oxygen atoms have been calculated. The calculation is performed using the asymptotic approach, based on the single-electron asymptotic representation of the electron wave function. The ground-state cross sections are in a good agreement with those calculated via comprehensive quantum chemical approach. The results of calculations demonstrate a reasonable accuracy and a high convenience of this approach in determination of cross sections for the manifold of excited states of atoms.

Excited-state relaxation of Ag8 clusters embedded in helium droplets.

Neutral silver clusters Ag(N) are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for Ag8, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of (6.89+/-0.09) eV for the vertical ionization potential of Ag8. The influence of the helium matrix on photoemission is discussed.

A Quantum Chemical Study of Intramolecular Charge Transfer in a Closely-Spaced, Donor−Acceptor Molecule

This investigation explores the use of contemporary quantum chemistry to mimic the light-induced, intramolecular charge-transfer processes that occur in 2-methyl-2,3-dihydrobenz[d,e]isoquinoline (DHBIQ) in polar solvents. Thus, the computed excited-state manifold, comprising two locally excited π,π* singlets, a locally excited π,π* triplet, and a charge-transfer (CT) state, is in excellent agreement with the experimental findings. It is shown that, whereas the energies of the various locally excited states are insensitive to molecular geometry and environment, the energy of the CT state depends markedly on structure and solvent polarity. The most favorable charge-transfer interactions occur within a distorted geometry that is midway between the axial and equatorial conformers identified for the ground state. The calculated nuclear and solvent reorganization energies are in good agreement with prior experimental work. Molecular dynamics simulations were employed to estimate the change in Gibbs free energy ...

Properties of low-lying excited manifolds inMn12acetate

Most experimental data on the single-molecule magnet Mn$_{12}$ acetate have been successfully explained by the assumption that the Mn$_{12}$ acetate has an effective ground-state spin of S=10. However, the effect of the low-lying excited manifolds caused by interactions between Mn spins has not been well understood. To investigate the features of the low-lying excited manifolds, the intramolecular exchange interactions are calculated using density-functional theory (DFT). With the calculated exchange parameters, the energy gap between the S=10 ground-state and the first excited-state manifold is calculated by diagonalization of the Heisenberg Hamiltonian. The upper limit on the energy gap is about 40.5 K which is likely to be overestimated due to incomplete treatment of the Coulomb potential within DFT. It is found that there are several S=9 low-energy excited-state manifolds above the S=10 ground-state manifold. The magnetic anisotropy barriers for the low-lying spin excitations are calculated using DFT. Based on the calculations, it is found that the anisotropy barriers for the low-lying excited manifolds are approximately the same as that for the ground-state manifold, which is applicable to other single-molecule magnets such as Mn$_4$.

Sequential nonadiabatic excitation of large molecules and ions driven by strong laser fields

Electronic processes leading to dissociative ionization of polyatomic molecules in strong laser fields are investigated experimentally, theoretically, and numerically. Using time-of-flight ion mass spectroscopy, we study the dependence of fragmentation on laser intensity for a series of related molecules and report regular trends in this dependence on the size, symmetry, and electronic structure of a molecule. Based on these data, we develop a model of dissociative ionization of polyatomic molecules in intense laser fields. The model is built on three elements: (i) nonadiabatic population transfer from the ground electronic state to the excited-state manifold via a doorway (charge-transfer) transition; (ii) exponential enhancement of this transition by collective dynamic polarization of all electrons, and (iii) sequential energy deposition in both neutral molecules and resulting molecular ions. The sequential nonadiabatic excitation is accelerated by a counterintuitive increase of a large molecule's polarizability following its ionization. The generic theory of sequential nonadiabatic excitation forms a basis for quantitative description of various nonlinear processes in polyatomic molecules and ions in strong laser fields.

Computational methods to study radiation effects in oxide materials

In this work, we briefly review recent theoretical studies of irradiation effects in oxide materials. Research based on classical mechanics and electronic structure methods to study ballistic and electronic excitation processes, respectively, in oxide materials is presented. Oxide materials have both ionic and covalent bonding interactions that make them more complex than metals and semiconductors. Hence, the modern strategy is to use electronic structure methods to help parameterize classical potential models, thus reducing the ambiguity of the potential model. Oxide materials can also support long-lived electronic excitations either as self-trapped holes and excitons or as in the formation of higher energy states, such as in a color center. We have learned that the electronic structure, in particular the low-lying conduction band states of insulators, is strongly coupled to the ionic forces, and so the excited state manifold must be mapped. In carrying out these tasks, many pitfalls are found, some discoveries are made, and many digressions are required to understand how molecular dynamics simulations can eventually couple the ballistic and electronic excitations that occur in irradiation processes in oxide materials.

Optical transitions and absorption intensities of Dy 3+ (4f 9) in YSGG laser host

Spectroscopic and laser properties of Dy 3+-doped in Y 3Sc 2Ga 3O 12 (YSGG) are characterized by employing the Judd–Ofelt theory. The Judd–Ofelt model has been applied to the room temperature absorption intensities of Dy 3+ (4f 9) transitions to obtain the three phenomenological parameters (referred to as Judd–Ofelt intensity parameters), from which the spectroscopic quality factor for Dy 3+ in YSGG is determined to be 0.68. The intensity parameters are used to determine the radiative decay rates (emission probabilities), radiative lifetimes, and branching ratios of the Dy 3+ transitions from the excited state manifolds to the corresponding lower-lying multiplet manifolds. Using the radiative decay rates for the Dy 3+ transitions between the excited states and the lower-lying manifold states in YSGG, radiative lifetimes of the excited states are determined. The room temperature fluorescence lifetimes of the 4F 9/2→ 6H 13/2 and 4F 9/2→ 6H 15/2 transitions of Dy 3+ in YSGG are measured to approximately 1.5 ms. We have calculated the radiative lifetime of 2.36 ms for the 4F 9/2 manifold state using the Judd–Ofelt model. Therefore, the quantum efficiency of Dy 3+ in YSGG is determined to be approximately 59%.

X-ray structures, photophysical characterization, and computational analysis of geometrically constrained copper(I)--phenanthroline complexes.

A series of three geometrically constrained C(2)-symmetric Cu(I) mono-phenanthroline complexes were characterized by X-ray structural analysis, and their photophysical properties were investigated by absorption and emission spectroscopy. Visible light excitation yielded metal-to-ligand charge-transfer (MLCT) excited states with luminescence lifetimes up to 155 ns. Ultrafast transient absorption spectroscopy provided further insights into the excited-state dynamics and suggests for all three complexes the formation of a phenanthroline radical anion. In agreement with electrochemical measurements, the data further indicate that coordinative rearrangements are involved in nonradiative deactivation of the excited states. According to time-dependent density functional theory calculations (B3LYP/6-31G), the major MLCT transitions are polarized along the C(2) axis of the complex and originate predominantly from the copper d(xz) orbital. The computational analysis identifies an excited-state manifold with a number of close-lying, potentially emissive triplet states and is in agreement with the multiexponential decay kinetics of the MLCT luminescence. The relationship between structural and photophysical data of the studied Cu(I) mono-phenanthroline complexes agrees well with current models describing the photophysics of the related Cu(I) bis-diimine complexes.

On the inversion of the 1 Bu and 2 Ag electronic states in α,ω-diphenylpolyenes

An alternative model to that of the inversion of the states 1 Bu and 2 Ag is proposed for interpreting the photophysics of the α,ω-diphenylpolyenes. This model is based upon the existence of two chemical structures with Bu symmetry, which may be ascribed to the same excited electronic state 1 Bu. One of the two chemical structures corresponds to the Franck–Condon structure with conjugated single and double bonds for the polyene chain, and another consists of a nearly equivalent series of partial double bonds along the polyene chain. The latter relaxed structure is consistent with the observation of high torsional energy barriers and low photoisomerization quantum yields for diphenylhexatriene in the singlet excited state manifold. Interestingly, such a simple quantum model as that of the particle in a one-dimensional box provides quite an accurate description of the absorption spectroscopic properties of these major compounds. This is partly the result of the most stable structures for these compounds being of the all-trans type; such structures increase in length as additional ethylene units are added, which makes them very similar to a one-dimensional box becoming increasingly longer.