Showing papers on "Excited state published in 2001"

Highly phosphorescent bis-cyclometalated iridium complexes: synthesis, photophysical characterization, and use in organic light emitting diodes.

Abstract: The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (C∧N) ligands and a single monoanionic, bidentate ancillary ligand (LX), i.e., C∧N2Ir(LX). The C∧N ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all β-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the C∧N2Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1−0.6), and microsecond lifetimes (e.g., 1−14 μs). The strongly allowed phosphorescence in these complexes is the result of significant spin−orbit coupling of the Ir center. The lowest energy (emissive) excited state in these C∧N2Ir(acac) complexes is a mixture of 3MLCT and 3(π−π*) states. By choosing the appropriate C∧N ligand, C∧N2Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the C∧N ligands, whic...

Endothermic energy transfer: A mechanism for generating very efficient high-energy phosphorescent emission in organic materials

Abstract: Intermolecular energy transfer processes typically involve an exothermic transfer of energy from a donor site to a molecule with a substantially lower-energy excited state (trap). Here, we demonstrate that an endothermic energy transfer from a molecular organic host (donor) to an organometallic phosphor (trap) can lead to highly efficient blue electroluminescence. This demonstration of endothermic transfer employs iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C2′)picolinate as the phosphor. Due to the comparable energy of the phosphor triplet state relative to that of the 4,4′-N,N′-dicarbazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic transfer of energy from phosphor to host, and subsequent slow endothermic transfer from host back to phosphor, is clearly observed. Using this unique triplet energy transfer process, we force emission from the higher-energy, blue triplet state of the phosphor (peak wavelength of 470 nm), obtaining a very high maximum organic light-emi...

Organometallic compounds and emission-shifting organic electrophosphorescence

Abstract: Emissive phosphorescent organometallic compounds are described that produce improved electroluminescence, particularly in the blue region of the visible spectrum. Organic light emitting devices employing such emissive phosphorescent organometallic compounds are also described. Also described is an organic light emitting layer including a host material having a lowest triplet excited state having a decay rate of less than about 1 per second; a guest material dispersed in the host material, the guest material having a lowest triplet excited state having a radiative decay rate of greater than about 1×10 5 or about 1×10 6 per second and wherein the energy level of the lowest triplet excited state of the host material is lower than the energy level of the lowest triplet excited state of the guest material.

High-Quality Manganese-Doped ZnSe Nanocrystals

Abstract: We demonstrate high-quality, highly fluorescent, ZnSe colloidal nanocrystals (or quantum dots) that are doped with paramagnetic Mn2+ impurities. We present luminescence, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) measurements to confirm that the Mn impurities are embedded inside the nanocrystal. Optical measurements show that by exciting the nanocrystal, efficient emission from Mn is obtained, with a quantum yield of 22% at 295 K and 75% below 50 K (relative to Stilbene 420). MCD spectra reveal an experimental Zeeman splitting in the first excited state that is large (28 meV at 2.5 T), depends on doping concentration, and saturates at modest fields. In the low field limit, the magnitude of the effective g factor is 430 times larger than in undoped nanocrystals. EPR experiments exhibit a six-line spectrum with a hyperfine splitting of 60.4 × 10-4 cm-1, consistent with Mn substituted at Zn sites in the cubic ZnSe lattice.

Ultrafast Electron Transfer Dynamics from Molecular Adsorbates to Semiconductor Nanocrystalline Thin Films

Abstract: Interfacial electron transfer (ET) between semiconductor nanomaterials and molecular adsorbates is an important fundamental process that is relevant to applications of these materials. Using femtosecond midinfrared spectroscopy, we have simultaneously measured the dynamics of injected electrons and adsorbates by directly monitoring the mid-IR absorption of electrons in the semiconductor and the change in adsorbate vibrational spectrum, respectively. We report on a series of studies designed to understand how the interfacial ET dynamics depends on the properties of the adsorbates, semiconductors, and their interaction. In Ru(dcbpy)2(SCN)2 (dcbpy = 2,2‘-bipyridine-4,4‘-dicarboxylate) sensitized TiO2 thin films, 400 nm excitation of the molecule promotes an electron to the metal-to-ligand charge transfer (MLCT) excited state, from which it is injected into TiO2. The injection process was characterized by a fast component, with a time constant of <100 fs, and a slower component that is sensitive to sample con...

Coherent Control of an Atomic Collision in a Cavity

Abstract: Following a recent proposal by S. B. Zheng and G. C. Guo [Phys. Rev. Lett. 85, 2392 (2000)], we report an experiment in which two Rydberg atoms crossing a nonresonant cavity are entangled by coherent energy exchange. The process, mediated by the virtual emission and absorption of a microwave photon, is characterized by a collision mixing angle 4 orders of magnitude larger than for atoms colliding in free space with the same impact parameter. The final entangled state is controlled by adjusting the atom-cavity detuning. This procedure, essentially insensitive to thermal fields and to photon decay, opens promising perspectives for complex entanglement manipulations.

Observation of a 6∧∧ He double hypernucleus

Abstract: A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified uniquely as the sequential decay of ${}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}^{6}\mathrm{He}$ emitted from a ${\ensuremath{\Xi}}^{\ensuremath{-}}$ hyperon nuclear capture at rest. The mass of ${}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}^{6}\mathrm{He}$ and the $\ensuremath{\Lambda}\ensuremath{-}\ensuremath{\Lambda}$ interaction energy $\ensuremath{\Delta}{B}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}$ have been measured for the first time devoid of the ambiguities due to the possibilities of excited states. The value of $\ensuremath{\Delta}{B}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}$ is $1.01\ifmmode\pm\else\textpm\fi{}{0.20}_{\ensuremath{-}0.11}^{+0.18}\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$. This demonstrates that the $\ensuremath{\Lambda}\ensuremath{-}\ensuremath{\Lambda}$ interaction is weakly attractive.

Exciton rabi oscillation in a single quantum dot

Abstract: A spectroscopic method, which enables characterization of a single isolated quantum dot and a quantum wave function interferometry, is applied to an exciton discrete excited state in an InGaAs quantum dot. Long coherence of zero-dimensional excitonic states made possible the observation of coherent population flopping in a 0D excitonic two-level system in a time-domain interferometric measurement. Corresponding energy splitting is also manifested in an energy-domain measurement.

On the density matrix based approach to time-dependent density functional response theory

Abstract: The formulation of time-dependent Kohn–Sham (TDKS) response theory based on the noninteracting one-particle density matrix is reanalyzed in detail. A transparent derivation starting from a von-Neumann-type equation of motion for the TDKS one-particle density matrix is presented. The resulting scheme has a simple structure and leads to compact expressions for frequency-dependent response properties. A systematic treatment of excited states is inferred from a pole analysis of the frequency-dependent density matrix response. A variational principle for excitation energies is established. Excited state properties are straightforward by analytical derivative techniques. The theory provides a particularly suitable starting point for linear scaling implementations. Magneto-optic properties such as rotatory strengths and the rotatory dispersion are accessible from the TDKS current-density response. The formalism is gauge-invariant. Various new sum rules within the adiabatic approximation (AA) are derived. It is shown that there is no “assignment problem” for excited states in the density matrix based formulation; the common density based approach is included as a special case. Merits and limitations of the AA are discussed.

Studying excited states of proteins by NMR spectroscopy

Abstract: Protein structure is inherently dynamic, with function often predicated on excursions from low to higher energy conformations. For example, X-ray studies of a cavity mutant of T4 lysozyme, L99A, show that the cavity is sterically inaccessible to ligand, yet the protein is able to bind substituted benzenes rapidly. We have used novel relaxation dispersion NMR techniques to kinetically and thermodynamically characterize a transition between a highly populated (97%, 25 degrees C) ground state conformation and an excited state that is 2.0 kcal mol(-1) higher in free energy. A temperature-dependent study of the rates of interconversion between ground and excited states allows the separation of the free energy change into enthalpic (Delta H = 7.1 kcal mol(-1)) and entropic (T Delta S = 5.1 kcal mol(-1), 25 degrees C) components. The residues involved cluster about the cavity, providing evidence that the excited state facilitates ligand entry.

Direct semiclassical simulation of photochemical processes with semiempirical wave functions

Abstract: We describe a new method for the simulation of excited state dynamics, based on classical trajectories and surface hopping, with direct semiempirical calculation of the electronic wave functions and potential energy surfaces ~DTSH method!. Semiempirical self-consistent-field molecular orbitals ~SCF MO’s! are computed with geometry-dependent occupation numbers, in order to ensure correct homolytic dissociation, fragment orbital degeneracy, and partial optimization of the lowest virtuals. Electronic wave functions are of the MO active space configuration interaction ~CI! type, for which analytic energy gradients have been implemented. The time-dependent electronic wave function is propagated by means of a local diabatization algorithm which is inherently stable also in the case of surface crossings. The method is tested for the problem of excited ethylene nonadiabatic dynamics, and the results are compared with recent quantum mechanical calculations. © 2001 American Institute of Physics. @DOI: 10.1063/1.1376633#

DNA Excited-State Dynamics: Ultrafast Internal Conversion and Vibrational Cooling in a Series of Nucleosides

Abstract: To better understand DNA photodamage, several nucleosides were studied by femtosecond transient absorption spectroscopy. A 263-nm, 150-fs ultraviolet pump pulse excited each nucleoside in aqueous solution, and the subsequent dynamics were followed by transient absorption of a femtosecond continuum pulse at wavelengths between 270 and 700 nm. A transient absorption band with maximum amplitude near 600 nm was detected in protonated guanosine at pH 2. This band decayed in 191 ± 4 ps in excellent agreement with the known fluorescence lifetime, indicating that it arises from absorption by the lowest excited singlet state. Excited state absorption for guanosine and the other nucleosides at pH 7 was observed in the same spectral region, but decayed on a subpicosecond time scale by internal conversion to the electronic ground state. The cross section for excited state absorption is very weak for all nucleosides studied, making some amount of two-photon ionization of the solvent unavoidable. The excited state life...

Surface Photovoltage Spectroscopy of Dye-Sensitized Solar Cells with TiO2, Nb2O5, and SrTiO3 Nanocrystalline Photoanodes: Indication for Electron Injection from Higher Excited Dye States

Abstract: The onset wavelengths of the surface photovoltage (SPV) in dye-sensitized solar cells (DSSCs) with different mesoporous, wide-band gap electron conductor anode materials, viz., TiO2 (anatase), Nb2O5 (amorphous and crystalline), and SrTiO3, using the same Ru bis-bipyridyl dye for all experiments, are different. We find a clear dependence of these onset wavelengths on the conduction band edge energies (ECB) of these oxides. This is manifested in a blue-shift for cells with Nb2O5 and SrTiO3 compared to those with TiO2. The ECB levels of Nb2O5 and SrTiO3 are known to be some 200−250 meV closer to the vacuum level than that of our anatase films, while there is no significant difference between the optical absorption spectra of the dye on the various films. We, therefore, suggest that the blue shift is due to electron injection from excited-state dye levels above the LUMO into Nb2O5 and SrTiO3. Such injection comes about because, in contrast to what is the case for anatase, the LUMO of the adsorbed dye in the s...

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

Abstract: Carotenoids are important biomolecules that are ubiquitous in nature and find widespread application in medicine In photosynthesis, they have a large role in light harvesting (LH) and photoprotection They exert their LH function by donating their excited singlet state to nearby (bacterio)chlorophyll molecules In photosynthetic bacteria, the efficiency of this energy transfer process can be as low as 30% Here, we present evidence that an unusual pathway of excited state relaxation in carotenoids underlies this poor LH function, by which carotenoid triplet states are generated directly from carotenoid singlet states This pathway, operative on a femtosecond and picosecond timescale, involves an intermediate state, which we identify as a new, hitherto uncharacterized carotenoid singlet excited state In LH complex-bound carotenoids, this state is the precursor on the reaction pathway to the triplet state, whereas in extracted carotenoids in solution, this state returns to the singlet ground state without forming any triplets We discuss the possible identity of this excited state and argue that fission of the singlet state into a pair of triplet states on individual carotenoid molecules constitutes the mechanism by which the triplets are generated This is, to our knowledge, the first ever direct observation of a singlet-to-triplet conversion process on an ultrafast timescale in a photosynthetic antenna

2-Aminopurine fluorescence quenching and lifetimes: Role of base stacking

Abstract: 2-Aminopurine (2AP) is a fluorescent analog of guanosine and adenosine and has been used to probe nucleic acid structure and dynamics. Its spectral features in nucleic acids have been interpreted phenomenologically, in the absence of a rigorous electronic description of the context-dependence of 2AP fluorescence. Now, by using time-dependent density functional theory, we describe the excited-state properties of 2AP in a B-form dinucleotide stacked with guanosine, adenosine, cytosine, or thymine. Calculations predict that 2AP fluorescence is quenched statically when stacked with purines, because of mixing of the molecular orbitals in the ground state. In contrast, quenching is predicted to be dynamic when 2AP is stacked with pyrimidines, because of formation of a low-lying dark excited state. The different quenching mechanisms will result in different experimentally measured fluorescence lifetimes and quantum yields.

Role of the energy transfer in the optical properties of undoped and Er-doped interacting Si nanocrystals

Abstract: In this article the luminescence properties of Si nanocrystals (nc) formed by plasma enhanced chemical vapor deposition and their interaction with Er ions introduced by ion implantation are investigated in detail. Si nc with different size distributions and densities were produced and all show quite intense room temperature luminescence (PL) in the range 700–1100 nm. It is shown that the time-decay of the luminescence follows a stretched exponential function whose shape tends towards a single exponential for almost isolated nc. This suggests that stretched exponential decays are related to the energy transfer from smaller towards larger nc. Indeed, by comparing samples with similar nc size distributions, but with very different nc densities, it is demonstrated that the PL has a quite strong redshift in the high density case, demonstrating a clear energy redistribution within the sample. Excitation cross sections have been measured in all samples yielding a value of ∼1.8×10−16 cm2 for isolated nc excited w...

Spin-Orbit Coupling and Intersystem Crossing in Conjugated Polymers: A Configuration Interaction Description

Abstract: Configuration−interaction calculations are performed to describe the singlet and triplet excited states of oligothiophene and oligo(phenylene ethynylene) conjugated chains. Intersystem crossing from the singlet to the triplet manifold is made possible by spin−orbit coupling, which leads to a mixing of the singlet (Sn) and triplet (Tn) wave functions. The electronic spin−orbit S1−Ti matrix elements, obtained from first-order perturbation theory, are used to compute the rates of intersystem crossing from the lowest singlet excited state, S1, into low-lying triplet states, Ti. On the basis of these results, a general mechanism is proposed to describe the intersystem crossing process in conjugated oligomers and polymers. The roles of chain length, heavy-atom derivatization, and ring twists are evaluated.

Nuclear reaction rates and energy in stellar plasmas: The effect of highly damped modes

Abstract: The effects of the highly damped modes in the energy and reaction rates in a plasma are discussed. These modes, with wave numbers k≫kD, even being only weakly excited, with less than kBT per mode, make a significant contribution to the energy and screening in a plasma. When the de Broglie wavelength is much less than the distance of closest approach of thermal electrons, a classical analysis of the plasma can be made. It is assumed, in the classical analysis, with ℏ→0, that the energy of the fluctuations ℏω≪kBT. Using the fluctuation-dissipation theorem, the spectra of fluctuations with ℏ≠0 is appreciably decreased. The decrease is mainly for the highly damped modes at high frequencies (∼0.5–3kBT). Reaction rates are enhanced in a plasma due to the screening of the reacting ions. This is taken into account by the Salpeter factor, which assumes slow motion for the ions. The implication of including the highly damped modes (with ℏ≠0) in the nuclear reaction rates in a plasma is discussed. Finally, the inves...

Capturing a photoexcited molecular structure through time-domain x-ray absorption fine structure.

Abstract: The determination of the structure of transient molecules, such as photoexcited states, in disordered media (such as in solution) usually requires methods with high temporal resolution. The transient molecular structure of a reaction intermediate produced by photoexcitation of NiTPP-L2 (NiTPP, nickeltetraphenylporphyrin; L, piperidine) in solution was determined by x-ray absorption fine structure (XAFS) data obtained on a 14-nanosecond time scale from a third-generation synchrotron source. The XAFS measurements confirm that photoexcitation leads to the rapid removal of both axial ligands to produce a transient square-planar intermediate, NiTPP, with a lifetime of 28 nanoseconds. The transient structure of the photodissociated intermediate is nearly identical to that of the ground state NiTPP, suggesting that the intermediate adopts the same structure as the ground state in a noncoordinating solvent before it recombines with two ligands to form the more stable octahedrally coordinated NiTPP-L2.

Chemically Induced Electronic Excitations at Metal Surfaces

Abstract: The energy released in low-energy chemisorption or physisorption of molecules on metal surfaces is usually expected to be dissipated by surface vibrations (phonons). Theoretical descriptions of competing electronic excitations are incomplete, and experimental observation of excited charge carriers has been difficult except at energies high enough to eject electrons from the surface. We observed reaction-induced electron excitations during gas interactions with polycrystalline silver for a variety of species with adsorption energies between 0.2 and 3.5 electron volts. The probability of exciting a detectable electron increases with increasing adsorption energy, and the measured time dependence of the electron current can be understood in terms of the strength and mechanism of adsorption.

Exciton dissociation mechanisms in the polymeric semiconductors poly(9,9-dioctylfluorene) and poly(9,9-dioctylfluorene-co-benzothiadiazole)

Abstract: We present femtosecond transient absorption measurements on the semiconductor conjugated polymers poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Detailed photophysical modeling reveals that, in F8, sequential excitation, first to the lowest singlet excited state, and then to a higher-energy state resonant with the pump photon energy, is predominantly responsible for the rapid $(l150\mathrm{fs})$ dissociation of photoinduced excitons. Resonant sequential excitation accesses high-energy states that can promptly evolve to charged or triplet states. In F8BT, however, we find that sequential excitation plays a lesser role in fast polaron-pair generation, and that exciton bimolecular annihilation can explain the charge population. We suggest that the electrophilic benzothiadiazole groups in F8BT facilitate charge formation by dissociation of the excited state formed by exciton-exciton annihilation. Modeling also reveals that exciton bimolecular annihilation can occur via two separate and competing processes. We find that in F8, the dominant mechanism involves exciton diffusion and collision. In F8BT, however, additional annihilation of spatially separated excitons occurs when they interact through the F\"orster transfer mechanism, where the critical distance for annihilation in F8BT is 4 nm.

Ground states in non-relativistic quantum electrodynamics

Abstract: The excited states of a charged particle interacting with the quantized electromagnetic field and an external potential all decay, but such a particle should have a true ground state — one that minimizes the energy and satisfies the Schrodinger equation. We prove quite generally that this state exists for all values of the fine-structure constant and the ultraviolet cutoff. We also show the same thing for a many-particle system under physically natural conditions.

Absolute calibration of atomic density measurements by laser-induced fluorescence spectroscopy with two-photon excitation

Abstract: The two-photon resonances of atomic hydrogen (λ = 2×205.1 nm), atomic nitrogen (λ = 2×206.6 nm) and atomic oxygen (λ = 2×225.6 nm) are investigated together with two selected transitions in krypton (λ = 2×204.2 nm) and xenon (λ = 2×225.5 nm). The natural lifetimes of the excited states, quenching coefficients for the most important collisions partners, and the relevant ratios of the two-photon excitation cross sections are measured. These data can be applied to provide a calibration for two-photon laser-induced fluorescence measurements based on comparisons with spectrally neighbouring noble gas resonances.

Experimental realization of a 2D fractional quantum spin liquid.

Abstract: The ground-state ordering and dynamics of the two-dimensional S = 1/2 frustrated Heisenberg antiferromagnet Cs(2)CuCl(4) are explored using neutron scattering in high magnetic fields. We find that the dynamic correlations show a highly dispersive continuum of excited states, characteristic of the resonating valence bond state, arising from pairs of S = 1/2 spinons. Quantum renormalization factors for the excitation energies (1.65) and incommensuration (0.56) are large.

Evidences of hot excited state electron injection from sensitizer molecules to TiO 2 nanocrystalline thin films

Abstract: Electron injection dynamics in dye sensitized TiO2 nanocrystalline thin films are studied with femtosecond mid-infrared spectroscopy. Three classes of sensitizer molecules, Ru(dcbpy)2(X)2 (X2 = 2SCN, 2CN, and dcbpy), Fe(dcbpy)2(CN)2, and ReCl(CO)3(dcbpy), are used to examine the dependence of injection rate and yield on the excited state redox potentials. We observed that electron injection occurred on the <100 fs time-scale and injection quantum yield depended on the redox potential for the series of Ru dyes. These results suggest that electron injection to TiO2 competes with electronic and vibrational relaxation within the sensitizer excited states and the branching ratio between these two processes determines the injection quantum yield for sensitizer molecules with an excited state redox potential below the conduction band-edge.

Investigation of the mechanism for rapid heating of nitrogen and air in gas discharges

Abstract: A rapid heating of nitrogen-oxygen mixtures excited by gas discharges is investigated numerically with allowance for the following main processes: the reactions of predissociation of highly excited electronic states of oxygen molecules (which are populated via electron impact or via the quenching of the excited states of N2 molecules), the reactions of quenching of the excited atoms O(1 D) by nitrogen molecules, the VT relaxation reactions, etc. The calculated results adequately describe available experimental data on the dynamics of air heating in gas-discharge plasmas. It is shown that, over a broad range of values of the reduced electric field E/N, gas heating is maintained by a fixed fraction of the discharge power that is expended on the excitation of the electronic degrees of freedom of molecules (for discharges in air, ηE⋍28%). The lower the oxygen content of the mixture, the smaller the quantity ηE. The question of a rapid heating of nitrogen with a small admixture of oxygen is discussed.

Excitation Energies from Time-Dependent Density Functional Theory for Linear Polyene Oligomers: Butadiene to Decapentaene

Abstract: Time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies of trans-1,3-butadiene, trans−trans-1,3,5-hexatriene, all-trans-1,3,5,7-octatetraene, and all-trans-1,3,5,7,9-decapentaene. Attachment and detachment densities for transitions in butadiene and decapentaene from the ground state to the 2 1Ag and 1 1Bu excited states are also calculated and analyzed. Based on comparisons with experimental results and high level ab initio calculations in the literature, significant improvement over configuration−interaction singles is observed for the 2 1Ag state of the polyenes, which has been known to have significant double excitation character. For the 1 1Bu state, TDDFT underestimates the excitation energy by 0.4−0.7 eV. In this case we have observed a significant difference between the results for TDDFT and TDDFT within the Tamm−Dancoff approximation, both in excitation energies and, at least for butadiene, in the character of the excited state.

Electron cotunneling in a semiconductor quantum dot.

Abstract: We report transport measurements on a semiconductor quantum dot with a small number of confined electrons. In the Coulomb blockade regime, conduction is dominated by cotunneling processes. These can be either elastic or inelastic, depending on whether they leave the dot in its ground state or drive it into an excited state, respectively. We are able to discriminate between these two contributions and show that inelastic events can occur only if the applied bias exceeds the lowest excitation energy. Implications to energy-level spectroscopy are discussed.

Cooling dynamics of an optically excited molecular probe in solution from femtosecond broadband transient absorption spectroscopy

Abstract: The cooling of p-nitroaniline (PNA), dimethylamino-p-nitroaniline (DPNA) and trans-stilbene (t-stilbene) in solution is studied experimentally and theoretically. Using the pump–supercontinuum probe (PSCP) technique we observed the complete spectral evolution of hot absorption induced by femtosecond optical pumping. In t-stilbene the hot S1 state results from Sn→S1 internal conversion with 50 fs characteristic time. The time constant of intramolecular thermalization or intramolecular vibrational redistribution (IVR) in S1 is estimated as τIVR≪100 fs. In PNA and DPNA the hot ground state is prepared by S1→S0 relaxation with characteristic time 0.3–1.0 ps. The initial molecular temperature is 1300 K for PNA and 860 K for t-stilbene. The subsequent cooling dynamics (vibrational cooling) is deduced from the transient spectra by assuming: (i) a Gaussian shape for the hot absorption band, (ii) a linear dependence of its peak frequency νm and width square Γ2 on molecular temperature T. Within this framework we de...