Femto-picosecond relaxation of triazole-bridged bis(zinc porphyrin)

The femtosecond dynamics of excitation relaxation has been revealed for the zinc porphyrin dimer using the pump-probe technique. The data obtained have been analyzed with the use of quantum-chemical calculations. The excitation relaxation dynamics shows that systems of this kind hold promise as models for investigation of photosystems and development of artificial analogues of natural photosynthetic centers. A model has been proposed to explain the found coherent dynamics of exciton bands.     

Excited-state processes in the carotenoid zeaxanthin after excess energy excitation

Aiming for better understanding of the large complexity of excited-state processes in carotenoids, we have studied the excitation wavelength dependence of the relaxation dynamics in the carotenoid zeaxanthin. Excitation into the lowest vibrational band of the S2 state at 485 nm, into the 0-3 vibrational band of the S2 state at 400 nm, and into the 2B(u)+ state at 266 nm resulted in different relaxation patterns. While excitation at 485 nm produces the known four-state scheme (S2 --> hot S1 --> S1 --> S0), excess energy excitation led to additional dynamics occurring with a time constant of 2.8 ps (400 nm excitation) and 4.9 ps (266 nm excitation), respectively. This process is ascribed to a conformational relaxation of conformers generated by the excess energy excitation. The zeaxanthin S state was observed regardless of the excitation wavelength, but its population increased after 400 and 266 nm excitation, suggesting that conformers generated by the excess energy excitation are important for directing the population toward the S state. The S2-S1 internal conversion time was shortened from 135 to 70 fs when going from 485 to 400 nm excitation, as a result of competition between the S2-S1 internal conversion from the vibrationally hot S2 state and S2 vibrational relaxation. The S1 lifetime of zeaxanthin was within experimental error the same for all excitation wavelengths, yielding approximately 9 ps. No long-lived species have been observed after excitation by femtosecond pulses regardless of the excitation wavelength, but excitation by nanosecond pulses at 266 nm generated both zeaxanthin triplet state and cation radical.     

State dynamics of acetylene excited to individual rotational level of the V1(2)K1(0,1,2) subbands

The dynamics of the IR emission induced by excitation of the acetylene molecule at the 3(2) Ka2, A1Au<--4(1) la1, X1Sigmag+ transition was investigated. Vibrationally resolved IR emission spectra were recorded at different delay times after the laser excitation pulse. The observed IR emission was assigned to transitions between vibrational levels of the acetylene molecule in the ground state. Values of the relaxation parameters of different vibrational levels of the ground state were obtained. The Ti-->Tj transition was detected by cavity ring-down spectroscopy in the 455 nm spectral range after excitation of the acetylene molecule at the same transition. Rotationally resolved spectra of the respective transition were obtained and analyzed at different delay times after the laser excitation pulse. The dynamics of the S1-->Tx-->T1-->S0 transitions was investigated, and the relaxation parameter values were estimated for the T1 state.     

On the nature of OH-stretching vibrations in hydrogen-bonded chains: pump frequency dependent vibrational lifetime

Two-dimensional infrared spectroscopy was carried out on stereoselectively synthesized polyalcohols. Depending upon the stereochemical orientation of their hydroxyl groups, the polyols can either feature linear chains of hydrogen bonds that are stable for extended periods of time or they can display ultrafast dynamics of hydrogen-bond breakage and formation. In the former case, the OH-stretching vibrations and their transition dipoles are substantially coupled, hence prior to vibrational relaxation, the initial OH-stretching excitation is rapidly redistributed among the set of hydroxyl-groups constituting the hydrogen-bonded chain. This redistribution is responsible for an ultrafast loss of memory regarding the frequency of initial excitation and as a result, a pump-frequency independent vibrational lifetime is observed. In contrast, in the latter case, the coupling of the OH-groups and their transition dipoles is much weaker. Therefore, the OH-stretching excitation remains localized on the initially excited oscillator for the time scale of vibrational energy relaxation. As a result inhomogeneous relaxation dynamics with a pump-frequency-dependent lifetime are observed.     

Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals

Electron-hole excitation and relaxation in the bulk, at interfaces, and surfaces of solid state materials play a key role in a variety of physical and chemical phenomena that are important for surface photochemistry, particle-surface interactions, and device physics. Information on charge carrier relaxation in metals can be obtained through analysis of linewidths measured by photoemission and related techniques, which give an estimate of the upper limit for electron and hole relaxation; however, many factors can contribute to spectral broadening, thus it is difficult to extract specific information on electronic relaxation processes. With femtosecond lasers it is possible to probe directly in a time-resolved fashion the charge carrier dynamics in metals by a variety of linear and nonlinear optical techniques. Femtosecond time-resolved two-photon photoemission has attracted particularly strong interest because it incorporates many of the surface analytical capabilities of photoemission and inverse photoemission — the traditional probes for surface and bulk band structures of solid state materials — with time-resolution that is approaching the fundamental response of electrons to optical excitation. Advances in the direct measurements of electron-hole excitation, charge carrier relaxation, and dynamics of intrinsic and adsorbate induced surface states are reviewed. With femtosecond lasers it also is possible to probe a variety of coherent phenomena, and even to control the charge carrier dynamics in metals through the optical phase of the excitation light. Pioneering experiments in this new field also are discussed.     

Slow relaxation processes in nematic liquid crystals at weak surface anchoring

We present new results of experimental investigations of azimuthal director reorientation dynamics for a nematic liquid crystal on solid substrates. Two types of substrate with weak anchoring were studied: glass/polystyrene and glass/UV-activated dye. Slow and fast relaxation processes were observed in both cases under the action of a strong 'in-plane' electric field. The slow surface reorientation and memory effects were controlled by two parameters: the electric voltage and the excitation time. It was established that the increase of the excitation time results in a slowing of the relaxation of the system to the initial state after turning off the electric field. A phenomenological model of a gliding of easy axes is proposed to explain the slow relaxation process.     

Shedding new light on the role of the Rydberg state in the photochemistry of aniline

Efficient electronic relaxation following the absorption of ultraviolet light is crucial for the photostability of biological chromophores, so understanding the microscopic details of the decay pathways is of considerable interest. Here, we employ femtosecond time-resolved photoelectron imaging to investigate the ultrafast intramolecular dynamics of aniline, a prototypical aromatic amine, following excitation just below the second absorption maximum. We find that both the second ππ* state and the Rydberg state are populated during the excitation process. Surprisingly, the dominant non-radiative decay pathway is an ultrafast relaxation mechanism that transfers population straight back to the electronic ground-state. The vibrational energy resolution and photoelectron angular distributions obtained in our experiments reveal an interesting bifurcation of the Rydberg population to two non-radiative decay channels. The existence of these competing non-radiative relaxation channels in aniline illustrates how its photostability arises from a subtle balance between dynamics on different electronically excited states and importantly between Rydberg and valence states.     

The effect of replacing a benzene ring with a saturated six-membered ring on the mesomorphic properties of 4,4′-disubstituted diphenyldiacetylenes

We present new results of experimental investigations of azimuthal director reorientation dynamics for a nematic liquid crystal on solid substrates. Two types of substrate with weak anchoring were studied: glass/polystyrene and glass/UV‐activated dye. Slow and fast relaxation processes were observed in both cases under the action of a strong ‘in‐plane’ electric field. The slow surface reorientation and memory effects were controlled by two parameters: the electric voltage and the excitation time. It was established that the increase of the excitation time results in a slowing of the relaxation of the system to the initial state after turning off the electric field. A phenomenological model of a gliding of easy axes is proposed to explain the slow relaxation process.     

Time-resolved polaron dynamics in molten solutions of cesium-doped cesium iodide

Temperature-dependent investigations of excess electrons in molten solutions of cesium-doped cesium iodide (Cs-CsI) (mole fraction of Cs approximately 0.003) were performed applying femtosecond pump-probe absorption spectroscopy. The pulse-limited induced bleach observed at probe wavelengths from 600 to 1240 nm was attributed to the excitation of equilibrated excess electrons which were initially formed by melting a Cs-CsI mixture. The interpretation of the relaxation process is based on strongly localized polarons that constitute the majority of defect states in this melt. As expected, the bipolaron contribution was insignificant. The time constants (tau1) were found to be temperature dependent confirming our earlier findings in Na-NaI melts that ionic diffusion almost exclusively controls the dynamics of excess electrons in high temperature ionic liquids. Apart from this temperature dependence, the relaxation dynamics of excess electrons do not differ irrespective of the excitation regime (blue or red part of the respective stationary spectra).     

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.     

Interface effects and relaxation processes in nanocomposites based on CdSe/ZnS semiconductor quantum dots and porphyrin molecules

Controllable self-assembly and properties of nanocomposites based on CdSe/ZnS semiconductor quantum dots (QDs) and tetrapyridylporphyrin molecules (H₂P) as well as the dynamics of relaxation processes in these systems were studied for solutions and single nanoobjects in the temperature range of 77–295 K. It was proved that the formation of surface states of different nature is crucial to nonradiative relaxation of exciton excitation in QDs. The efficiency of QD→Н₂Р energy transfer was shown to be at most 10–15%. Regularities of photoluminescence (PL) quenching for QDs in nanocomposites in solutions of different polarity correlate with the dependences of PL blinking for single QDs. A scheme was proposed of excited states and main relaxation channels of exciton excitation energy in semiconductor QDs and QD–Н₂Р nanocomposites.     

Singlet fission in rubrene single crystal: direct observation by femtosecond pump-probe spectroscopy

The excited state dynamics of rubrene in solution and in the single crystal were studied by femtosecond pump-probe spectroscopy under various excitation conditions. Singlet fission was demonstrated to play a predominant role in the excited state relaxation of the rubrene crystal in contrast to rubrene in solution. Upon 500 nm excitation, triplet excitons form on the picosecond time scale via fission from the lowest excited singlet state. Upon 250 nm excitation, fission from upper excited singlet states is observed within 200 fs.     

Restoration of coherence effects in high-power excitation of an intramolecular quasicontinuum

Three theoretical models were advanced for the dynamics of molecular multiphoton excitation: (i) The zero-order optically active mode connected by intramolecular random anharmonic couplings to a background manifold. (ii) Molecular eigenstates coupled by random radiative transition dipole moments. (iii) The kinetic master equation approach. It is demonstrated that in the Markoffian limit, as long as the intramolecular vibrational relaxation width is small relative to the Rabi frequency, these three approaches are equivalent. In the case of high-field excitation, coherent quantum effects are exhibited even in a randomly coupled system. Resurrection of the quantum oscillations and coherent pumping can be exhibited in intense field excitation on the time scale of intramolecular vibrational relaxation.     

Ultrafast Heme Dynamics of Ferric Cytochrome c in Different Environments: Electronic,Vibrational, and Conformational Relaxation

The excited‐state dynamics of ferric cytochrome c (Cyt c), an important electron‐transfer heme protein, in acidic to alkaline medium and in its unfolded form are investigated by using femtosecond pump–probe spectroscopy, exciting the heme and Tryptophan (Trp) to understand the electronic, vibrational, and conformational relaxation of the heme. At 390 nm excitation, the electronic relaxation of heme is found to be ≈150 fs at different pH values, increasing to 480 fs in the unfolded form. Multistep vibrational relaxation dynamics of the heme, including fast and slow processes, are observed at pH 7. However, in the unfolded form and at pH 2 and 11, fast phases of vibrational relaxation dominate, revealing the energy dissipation occurring through the covalent bond interaction between the heme and the nearest amino acids. A significant shortening of the excited‐state lifetime of Trp is observed at various pH values at 280 nm excitation due to resonance energy transfer to the heme. The longer time constant (25 ps) observed in the unfolded form is attributed to a complete global conformational relaxation of Cyt c.     

Electron-hole dynamics in CdTe tetrapods

We present transient absorption studies with femtosecond time resolution on the electron-hole dynamics in CdTe tetrapod nanostructures. Electron-hole pairs are generated by optical excitation in the visible spectral range, and an immediate bleach and induced absorption signal are observed. The relaxation dynamics to the lowest excitonic state is completed in about 6 ps. Experiments with polarized excitation pulses give information about the localization of the excited-state wave functions. The influence of the nanocrystal shape on the optical properties of CdTe nanoparticles is discussed.     

Time-resolved intraband electronic relaxation dynamics of Hg(n) (-) clusters (n=7-13,15,18) excited at 1.0 eV

Time-resolved photoelectron imaging has been used to study the relaxation dynamics of small Hg(n) (-) clusters (n=7-13,15,18) following intraband electronic excitation at 1250 nm (1.0 eV). This study furthers our previous investigation of single electron, intraband relaxation dynamics in Hg(n) (-) clusters at 790 nm by exploring the dynamics of smaller clusters (n=7-10), as well as those of larger clusters (n=11-13,15,18) at a lower excitation energy. We measure relaxation time scales of 2-9 ps, two to three times faster than seen previously after 790 nm excitation of Hg(n) (-), n=11-18. These results, along with size-dependent trends in the absorption cross-section and photoelectron angular distribution anisotropy, suggest significant evolution of the cluster anion electronic structure in the size range studied here. Furthermore, the smallest clusters studied here exhibit 35-45 cm(-1) oscillations in pump-probe signal at earliest temporal delays that are interpreted as early coherent nuclear motion on the excited potential energy surfaces of these clusters. Evidence for evaporation of one or two Hg atoms is seen on a time scale of tens of picoseconds.     

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright (1)L(a) and (1)L(b) electronic states of (1)ππ? character and spectroscopically dark and dissociative (1)πσ? states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited (1)L(a) state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived (1)L(b) state or the (1)πσ? state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the (1)πσ? state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.     

Nonradiative deexcitation dynamics of 9H-adenine: an OM2 surface hopping study

The nonradiative relaxation of 9H-adenine was studied at the semiempirical OM2/MR-CI level using the surface-hopping approach. Geometry optimizations of energy minima and conical intersections as well as single-point calculations of excitation energies at critical points were performed to characterize the relevant potential energy surfaces of 9H-adenine and to assess the accuracy of the OM2 results. Surface-hopping calculations were performed to describe the nonradiative dynamics of 9H-adenine after vertical excitation into the optically active state. They showed that the deexcitation process is mainly governed by a two-step relaxation consisting of an ultrashort component and a longer component. These findings compare well with experimental results from time-resolved photoelectron spectroscopy.     

Effect of conformational heterogeneity on excitation energy transfer efficiency in directly meso-meso linked Zn(II) porphyrin arrays

We have investigated the overall excitation energy relaxation dynamics in linear porphyrin arrays as well as the energy transport phenomena by attaching an energy acceptor to one end of a linear porphyrin array by using steady state and time-resolved spectroscopic measurements. We have revealed that the solvation dynamics as well as the conformational dynamics contributes significantly to the energy relaxation processes of linear porphyrin arrays. Consequently, long porphyrin arrays no longer serve as good energy transmission elements in donor-acceptor linked systems due to conformational heterogeneities which provide the non-radiative deactivation channels as energy quenchers.     

Ultrafast dynamics of UV-excited imidazole

The ultrafast dynamics of UV-excited imidazole in the gas phase is investigated by theoretical nonadiabatic dynamics simulations and experimental time-resolved photoelectron spectroscopy. The results show that different electronic excited-state relaxation mechanisms occur, depending on the pump wavelength. When imidazole is excited at 239.6 nm, deactivation through the NH-dissociation conical intersection is observed on the sub-50 fs timescale. After 200.8 nm excitation, competition between NH-dissociation and NH-puckering conical intersections is observed. The NH-dissociation to NH-puckering branching ratio is predicted to be 21:4, and the total relaxation time is elongated by a factor of eight. A procedure for simulation of photoelectron spectra based on dynamics results is developed and employed to assign different features in the experimental spectra.