Excited-state dynamics of carotenoids in light-harvesting complexes. 1. Exploring the relationship between the S1 and S* states

Dispersed transient absorption spectra collected at variable excitation intensities in combination with time-resolved signals were used to explore the underlying connectivity of the electronic excited-state manifold of the carotenoid rhodopin glucoside in the light-harvesting 2 complex isolated from Rhodopseudomonas acidophila. We find that the S state, which was recently identified as an excited state in carotenoids bound in bacterial light-harvesting complexes, exhibits a different response to the increase of excitation intensity than the S(1) state, which suggests that the models used so far to describe the excited states of carotenoids are incomplete. We propose two new models that can describe both the time-resolved and the intensity-dependent data; the first postulates that S(1) and S* are not populated in parallel after the decay of the initially excited S(2) state but instead result from the excitation of distinct ground-state subpopulations. The second model introduces a resonantly enhanced light-induced transition during excitation, which promotes population to higher-lying excited states that favors the formation of S* over S(1). Multiwavelength target analysis of the time-resolved and excitation-intensity dependence measurements were used to characterize the involved states and their responses. We show that both proposed models adequately fit the measured data, although it is not possible to determine which model is most apt. The physical origins and implications of both models are explored.     

X-ray absorption and resonant Auger spectroscopy of O2 in the vicinity of the O 1s-->sigma* resonance: experiment and theory

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s-->sigma(*) excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.     

High-efficiency carrier multiplication and ultrafast charge separation in semiconductor nanocrystals studied via time-resolved photoluminescence

We demonstrate novel methods for the study of multiple exciton generation from a single photon absorption event (carrier multiplication) in semiconductor nanocrystals (or nanocrystal quantum dots) that are complementary to our previously reported transient absorption method. By monitoring the time dependence of photoluminescence (PL) from CdSe nanocrystals via time-correlated single photon counting, we find that carrier multiplication is observable due to the Auger decay of biexcitons. We compare these data with similar studies using transient absorption and find that the two methods give comparable results. In addition to the observation of dynamical signatures of carrier multiplication due to the Auger decay, we observe spectral signatures of multiple excitons produced from the absorption of a single photon. PL spectra at short times following excitation with high-energy photons are red-shifted compared to the single-exciton emission band, which is consistent with previous observations of significant exciton-exciton interactions in nanocrystals. We then show using a combination of transient absorption and time-resolved PL studies that charge transfer between a nanocrystal and a Ru-based catalyst model compound takes place on a time scale that is faster than Auger recombination time constants, which points toward a possible design of donor-acceptor assemblies that can be utilized to take advantage of the carrier multiplication process.     

Fundamental limits of the dispersion of the two-photon absorption cross section

We rigorously apply the sum rules to the sum-over-states expression to calculate the fundamental limits of the dispersion of the two-photon absorption cross section. A comparison of the theory with the data suggests that the truncated sum rules in the three-level model give a reasonable fundamental limit. Furthermore, we posit that the two-photon absorption cross section near the limit must have only three dominant states, so by default, the three-level model is appropriate. This ansatz is supported by a rigorous analytical calculation that the resonant term gets smaller as more states are added. We also find that the contributions of the nonexplicitly resonant terms cannot be neglected when analyzing real molecules with many excited states, even near resonance. However, puzzling as it may be, extrapolating an off-resonant result to resonance using only the resonant term of the three-level model is shown to be consistent with the exact result. In addition, the off-resonant approximation is shown to scale logarithmically when compared with the full three-level model. This scaling can be used to simplify the analysis of measurements. We find that existing molecules are still far from the fundamental limit; so, there is room for improvement. But, reaching the fundamental limit would require precise control of the energy-level spacing, independently of the transition dipole moments-a task that does not appear possible using today's synthetic approaches. So, we present alternative methods that can still lead to substantial improvements which only require the control of the transition moment to the first excited state. While it is best to normalize measured two-photon absorption cross sections to the fundamental limits when comparing molecules, we show that simply dividing by the square of the number of electrons per molecule yields a good metric for comparison.     

Evidence of ultra-fast dissociation in ammonia observed by resonant Auger electron spectroscopy

We present evidence for ultra-fast dissociation of molecular ammonia when photo-excited to the N1s→4a₁ core-hole state. This finding is based on resonant Auger spectroscopical results as well as qualitative arguments concerning the photon energy dependence of the Auger structures. Calculations of the excited state potential based on the Z+1 approximation were performed. Both the calculations and the measurements indicate that the most likely fragmentation pathway for the core excited ammonia molecules leads to NH₂^* and H fragments.     

A one-electron model for the aqueous electron that includes many-body electron-water polarization: Bulk equilibrium structure, vertical electron binding energy, and optical absorption spectrum

Previously, we reported an electron-water pseudopotential designed to be used in conjunction with a polarizable water model, in order to describe the hydrated electron [L. D. Jacobson et al., J. Chem. Phys. 130, 124115 (2009)]. Subsequently, we found this model to be inadequate for the aqueous electron in bulk water, and here we report a reparametrization of the model. Unlike the previous model, the current version is not fit directly to any observables; rather, we use an ab initio exchange-correlation potential, along with a repulsive potential that is fit to reproduce the density maximum of the excess electron's wave function within the static-exchange approximation. The new parametrization performs at least as well as the previous model, as compared to ab initio benchmarks for (H(2)O)(n) (-) clusters, and also predicts reasonable values for the diffusion coefficient, radius of gyration, and absorption maximum of the bulk species. The new model predicts a vertical electron binding energy of 3.7 eV in bulk water, which is 1.4 eV smaller than the value obtained using nonpolarizable models; the difference represents the solvent's electronic reorganization energy following electron detachment. We find that the electron's first solvation shell is quite loose, which may be responsible for the electron's large, positive entropy of hydration. Many-body polarization alters the electronic absorption line shape in a qualitative way, giving rise to a high-energy tail that is observed experimentally but is absent in previous simulations. In our model, this feature arises from spatially diffuse excited states that are bound only by electronic reorganization (i.e., solvent polarization) following electronic excitation.     

Auger decay calculations with core-hole excited-state molecular-dynamics simulations of water

We report a new theoretical procedure for calculating Auger decay transition rates including effects of core-hole excited-state dynamics. Our procedure was applied to the normal and first resonant Auger processes of gas-phase water and compared to high-resolution experiments. In the normal Auger decay, calculated Auger spectra were found to be insensitive to the dynamics, while the repulsive character of the first resonant core-excited state makes the first resonantly excited Auger decay spectra depend strongly on the dynamics. The ultrafast dissociation of water upon O(1s)-->4a(1) excitation was analyzed and found to be very sensitive to initial vibrational distortions in the ground state which furthermore affect the excitation energy. Our calculated spectra reproduce the experimental Auger spectra except for the Franck-Condon vibrational structure which is not included in the procedure. We found that the Auger decay of OH and O fragments contributes to the total intensity, and that the contribution from these fragments increases with increasing excitation energy.     

Phase control of the competition between electronic transitions in a solvated laser dye

Excited state population can be manipulated by resonant chirped laser pulses through pump–dump processes. We investigate these processes in the laser dye LD690 as a function of wavelength by monitoring the saturated absorption of chirped ultrafast pulses. The resulting nonlinear absorption spectrum becomes increasingly complex as the pulse is tuned to shorter wavelengths. However, fluorescence measurements indicate that the excited state population depends weakly on chirp when the pump wavelength is far from the lowest order electronic transition. Using a learning algorithm and closed-loop control, we find nonlinear chirp parameters that optimize features in the transmission spectrum. The results are discussed in terms of competition between excited state absorption and stimulated resonant Raman scattering.     

Molecular modeling and simulation of conjugated polymer oligomers: ground and excited state chain dynamics of PPV in the gas phase

The ground and excited state dynamics of poly(p-phenylenevinylene) (PPV) chains is studied through an implementation of mixed quantum/classical molecular dynamics simulation. The model used in the simulations combines the semiempirical Pariser-Parr-Pople (PPP) Hamiltonian to treat the pi molecular electronic structure with a mechanical force field capturing all other aspects. Nuclear degrees of freedom are treated classically. We first validate the model by simulating PPV chains of various length, and evaluate the absorption spectra. The thermal disorder contribution to the breadth of the first absorption band is estimated to be 0.2 eV at T = 300 K. To investigate the relationship between the emission and chain conformation, we simulate an isolated ten unit chain of PPV in the ground and the lowest excited state. The emission spectrum, red-shifted with respect to absorption of about 0.2 eV as found in experiments, shows a structured line shape that we relate to the photoinduced CC bond distortions. In accord with earlier studies, the exciton self-traps in the middle of the chain. We introduce two collective variables that reflect geometrical distortion, and find these to be effective in describing the contribution of chain conformation to the emission spectrum. The collective variables are also shown to be effective in describing the bond relaxation dynamics after photoexcitation. Such a relaxation is found to occur in approximately 100 fs and is guided by a compensatory release of energy between the double and single bonds in the vinylene junctions and p-phenyl rings. Finally, we find that the chain has a very slight preference for a more planar conformation in the excited state, compared to the ground state. However, the thermal motions induce the chain to explore out-of-plane conformations in both the ground and the excited states with an amplitude significantly greater than this difference.     

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.     

Mass transport following impulsive optical excitation

Transition from reverse-saturable absorption to saturable absorption of the chloroaluminum phthalocyanine solution excited by a giant laser pulse is ascribed not just to the saturation of excited state absorption, but also to the outward migration of the solute molecules at the laser beam center. While the saturation of excited state absorption occurs within a single picosecond laser pulse, the beam center population decrease is sustained much longer than the pulse duration. We distinguish these two mechanisms with the Z-scan technique, utilizing picosecond pulses with pulse-to-pulse separations ranging from 0.1 to 5.0 s.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

A transient absorption study of allophycocyanin

Transient dynamics of allophycocyanin trimers and monomers are observed by using the pump-probe, transient absorption technique. The origin of spectral components of the transient absorption spectra is discussed in terms of both kinetics and spectroscopy. We find that the energy gap between the ground and excited states of the unexcited subunit of allophycocyanin monomer decreases via an interaction with another excited subunit. For allophycocyanin trimer, we find that the fast dynamics results from the fast internal conversion and the first excited state is the only one electronic state which can trap the final population.     

Excited state two photon absorption of a charge transfer radical dimer in the near infrared

Nonlinear transmission measurements of a solution of radical dimers of tetramethyl-tetrathiafulvalene, (TMTTF+)2, recorded with 9 ns laser pulses at 1064 nm are reported and interpreted on the basis of a multiphoton absorption process. One finds that the process can be interpreted with a sequence of three photon absorption, the first being a one photon absorption related to the intermolecular charge transfer process characteristic of the dimers and the second a two photon absorption from the excited state created with the first process. A model calculation allows one to obtain the value of the two photon absorption cross section which is found to be several orders of magnitude larger than those usually found for two photon absorbing systems excited from the ground state. These results show the importance of an excited-state population for obtaining large nonlinear optical responses.     

Absorption spectra of blue-light-emitting oligoquinolines from time-dependent density functional theory

Recently, it has been discovered that a series of four conjugated oligomers, oligoquinolines, exhibits many desirable properties of organic materials for developing high-performance light-emitting diodes: good blue color purity, high brightness, high efficiency, and high glass-transition temperatures. In this work, we investigate the optical absorption of oligoquinolines in the gas phase and chloroform (CHCl3) solution, respectively, using time-dependent density functional theory with the adiabatic approximation for the dynamical exchange-correlation potential. Our calculations show that the first peak of optical absorption corresponds to the lowest singlet excited state, whereas several quasi-degenerate excited states contribute to the experimentally observed higher-frequency peak. We find that, compared with the gas phase, there is a moderate red shift in excitation energy in solution due to the solute-solvent interaction simulated using the polarizable continuum model. Our results show that the lowest singlet excitation energies of oligoquinolines in chloroform solution calculated with the adiabatic hybrid functional PBE0 are in a good agreement with experiments. Our simulated optical absorption agrees well with the experimental data. Finally, analysis of the natural transition orbitals corresponding to the excited states in question underscores the underlying electronic delocalization properties.     

Observation of interband two-photon absorption saturation in CdS nanocrystals

We report the observation of interband two-photon absorption (TPA) saturation in cadmium sulfide nanocrystals (CdS NCs) under intense femtosecond laser excitation with 1.6 eV photon energy. The observation has been compared to interband TPA saturation in bulk CdS under the same experimental conditions. By using both Z-scan techniques and transient absorption measurements, the saturation intensity has been determined to be 190 GW/cm2 for CdS NCs of 4-nm diameter, which is 2 orders of magnitude greater than that for CdS bulk crystal. The results are in agreement with an inhomogeneously broadened, saturated TPA model.     

Ultrafast excited state dynamics of modified phthalocyanines: p-HPcZn and p-HPcCo

Two modified metallophthalocyanines (MPcs) containing sulfonic naphthoxy substituents were synthesized. The measurements of transient absorption and time-resolved photoluminescence were used to study the ultrafast response and excited state dynamics of two MPcs in dimethyl sulfoxide (DMSO) solution, which were predominantly in the monomeric form. Under excitation at 400 nm, these molecules experience vibrational relaxation to the bottom of the first excited state and then the excitation rapidly converts to the low-lying charge-transfer (CT) state and finally reaches the triplet states. Under excitation at 800 nm, they show a two-photon absorption character, and their excited state dynamics exhibit strong dependence on the probe wavelength. The main results with 400 nm pumping are similar to the results with 800 nm pumping. For p-HPcZn, weak two-photon photoluminescence was also observed with a lifetime of 52 +/- 2 ps. A four-level model was used to illustrate the excited state dynamics of p-HPcZn, while a five-level model was suggested for p-HPcCo molecule.     

Photobleaching pathways in single-molecule FRET experiments

To acquire accurate structural and dynamical information on complex biomolecular machines using single-molecule fluorescence resonance energy transfer (sm-FRET), a large flux of donor and acceptor photons is needed. To achieve such fluxes, one may use higher laser excitation intensity; however, this induces increased rates of photobleaching. Anti-oxidant additives have been extensively used for reducing acceptor's photobleaching. Here we focus on deciphering the initial step along the photobleaching pathway. Utilizing an array of recently developed single-molecule and ensemble spectroscopies and doubly labeled Acyl-CoA binding protein and double-stranded DNA as model systems, we study these photobleaching pathways, which place fundamental limitations on sm-FRET experiments. We find that: (i) acceptor photobleaching scales with FRET efficiency, (ii) acceptor photobleaching is enhanced under picosecond-pulsed (vs continuous-wave) excitation, and (iii) acceptor photobleaching scales with the intensity of only the short wavelength (donor) excitation laser. We infer from these findings that the main pathway for acceptor's photobleaching is through absorption of a short wavelength photon from the acceptor's first excited singlet state and that donor's photobleaching is usually not a concern. We conclude by suggesting the use of short pulses for donor excitation, among other possible remedies, for reducing acceptor's photobleaching in sm-FRET measurements.     

Localized versus delocalized excitations just above the 3d threshold in krypton clusters studied by Auger electron spectroscopy

We present Auger spectroscopy studies of large krypton clusters excited by soft x-ray photons with energies on and just above the 3d(52) ionization threshold. The deexcitation spectra contain new features as compared to the spectra measured both below and far above threshold. Possible origins of these extra features, which stay at constant kinetic energies, are discussed: (1) normal Auger process with a postcollision interaction induced energy shift, (2) recapture of photoelectrons into high Rydberg orbitals after Auger decay, and (3) excitation into the conduction band (or "internal" ionization) followed by Auger decay. The first two schemes are ruled out, hence internal ionization remains the most probable explanation.     

Probing the valence character of O 1s-->Rydberg excited O2 by participator Auger decay measurements and partial ion yield spectroscopy following x-ray absorption

The valence character of O 1s-->Rydberg excited O2 is investigated by means of participator Auger decay spectroscopy, performed at selected photon energies across the K-shell resonance region, and by means of partial ion yield x-ray absorption spectroscopy. For several of the excitation energies studied, the authors find substantial sigma*(4Sigmau-, 2Sigmau-) valence character being mixed with nssigma and npsigma (4Sigmau-, 2Sigmau-) Rydberg states. An experimental indication of a coupling between the channels associated with quartet and doublet ion cores is considered and discussed. New spectroscopic constants are derived for the singly ionized X 2Pig state of O2 based on the observation of at least 20 vibrational sublevels.