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1.
Anestopoulos D Fakis M Polyzos I Tsigaridas G Persephonis P Giannetas V 《The journal of physical chemistry. B》2005,109(19):9476-9481
The excited-state dynamics of two oligothiophenes, 5,5'-dicarboxyhaldehyde 2,2',5',2' '-terthiophene and 5-carboxyhaldehyde 2,2',5',2' '-terthiophene, were studied by time-resolved fluorescence spectroscopy, in the femtosecond regime. The isotropic and anisotropic parameters of their fluorescence were calculated. The angle (alpha) between the absorption and emission molecular dipoles was estimated from the initial fluorescence anisotropy. The effect of the chemical substituents, at the ends of the main chain of the molecule, on the temporal behavior of the fluorescence was investigated. Particularly, the nonsymmetric oligothiophene molecule (containing one aldehyde group) exhibits shorter excited-state isotropic decay time than the symmetric one (containing two aldehyde groups). This is due to the higher value of the emission dipole moment of the nonsymmetric oligothiophene in comparison with that of the symmetric one. Additionally, the two materials have almost the same anisotropic fluorescence parameters, and this is attributed to the same rotational motions in the excited state due to their similar molecular structures. 相似文献
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Mohammed OF Banerji N Lang B Nibbering ET Vauthey E 《The journal of physical chemistry. A》2006,110(51):13676-13680
Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions. 相似文献
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Rouzée A Boudon V Lavorel B Faucher O Raballand W 《The Journal of chemical physics》2005,123(15):154309
Femtosecond Raman-induced polarization spectroscopy (RIPS) was conducted at low pressure (250 mb at 295 K and 400 mb at 373 K) in ethylene. The temporal signal, resulting from the beating between pure rotational coherences, was measured with a heterodyne detection. The temporal traces were converted to the frequency domain using a Fourier transformation and then analyzed thanks to the D2hTDS software (http://www.u-bourgogne.fr/LPUB/shTDS.html) dedicated to X2Y4 molecules with D2h symmetry. The effective Hamiltonian was expanded up to order 2, allowing the determination of five parameters with an rms of 0.017 cm(-1). Special care was taken in the precise modeling of intensities, taking into account all instrumental effects. Relative intensities were fitted (with an rms of 7.2%) and two polarizability operators were determined. 相似文献
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Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH(2)-COOH) and doubly deprotonated ((-)S-CH(2)-COO(-)) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semiconducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S(1) and S(2) excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the π(CO) (*)←n(O) transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an α-thiol-substituted acyl radical (α-TAR, S-CH(2)-CO(●)). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 ± 5 fs; subsequent unimolecular decay took 400 ± 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 ± 10 fs radical formation; 420 ± 40 fs decay). The production of the α-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (≤100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems. 相似文献
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Okabe C Nakabayashi T Inokuchi Y Nishi N Sekiya H 《The Journal of chemical physics》2004,121(19):9436-9442
Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S(1)(n,pi( *)) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S(1)(pi,pi( *)) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to the S(1)(n,pi( *)) state, occur on a time scale less than a few hundred femtoseconds from the S(1)(pi,pi( *)) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of the S(1)(pi,pi( *)) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5-1.6 ps for the excitation at 365-320 nm. The decrease in the decay time has been attributed to the opening of an efficient nonradiative channel; an IC from S(1)(pi,pi( *)) to S(1)(n,pi( *)) of the cis-keto form promotes the production of the trans-keto form as the final photochromic products. The two IC processes may provide opposite effect on the quantum yield of photochromic products: IC in the enol form may substantially reduce the quantum yield, but IC in the cis-keto form increase it. 相似文献
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Fernandes MM Schmidt M Stumpf T Walther C Bosbach D Klenze R Fanghänel T 《Journal of colloid and interface science》2008,321(2):323-331
Three samples of calcite homogeneously doped with Eu(3+) were synthesized in a mixed-flow reactor. By means of selective excitation of the 5D0-->7F0 transition at low temperatures (T<20 K), three different Eu(3+) species (species A, B, and C, respectively) could be discriminated. For each one, the emission spectrum and lifetime were obtained after selective excitation of the single species. On the basis of these data, species C could be identified as Eu(3+) incorporated into the calcite lattice on the (nearly) octahedral Ca(2+) site. Species B was also identified as Eu(3+) incorporated into the calcite lattice, but the ligand field shows a much weaker symmetry. Species A, however, is not incorporated into the crystal's bulk, having 1-2 H(2)O ligands left in its first coordination sphere and showing very little symmetry, and is considered as Eu(3+) adsorbed onto the calcite surface. The emission spectra of species C for Eu:calcite grown in the presence of Na(+) were found to differ from those of Eu:calcite synthesized in the presence of K(+). The latter revealed a strong distortion in site symmetry, which was not observed in the samples grown in Na(+) solutions. This finding provides spectroscopic evidence in favor of an incorporation mechanism based on the charge-balanced coupled substitution of Na(+)+Eu(3+)<-->2Ca(2+). 相似文献
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Electronic relaxation pathways in photoexcited nucleobases have received much theoretical and experimental attention due to their underlying importance to the UV photostability of these biomolecules. Multiple mechanisms with different energetic onsets have been proposed by ab initio calculations yet the majority of experiments to date have only probed the photophysics at a few selected excitation energies. We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA base adenine in a molecular beam at multiple excitation energies between 4.7-6.2 eV. The two-dimensional TRPES data is fit globally to extract lifetimes and decay associated spectra for unambiguous identification of states participating in the relaxation. Furthermore, the corresponding amplitude ratios are indicative of the relative importance of competing pathways. We adopt the following mechanism for the electronic relaxation of isolated adenine; initially the S(2)(ππ*) state is populated by all excitation wavelengths and decays quickly within 100 fs. For excitation energies below ~5.2 eV, the S(2)(ππ*)→S(1)(nπ*)→S(0) pathway dominates the deactivation process. The S(1)(nπ*)→S(0) lifetime (1032-700 fs) displays a trend toward shorter time constants with increasing excitation energy. On the basis of relative amplitude ratios, an additional relaxation channel is identified at excitation energies above 5.2 eV. 相似文献
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The excited electronic state dynamics of N(6),N(6)-dimethyladenine (DMAde), a molecule known to emit dual fluorescence, has been studied in aqueous solution using femtosecond fluorescence up-conversion spectroscopy. Time profiles of the fluorescence of DMAde excited at lambda= 258 nm were measured at a series of wavelengths in the range 320 nm or= 500 nm), which appeared slightly delayed compared to the UV fluorescence, the long-lived fluorescence component (tau(3)) dominated, the second component (tau(2)) disappeared. The results are consistent with the assumption that DMAde is primarily excited to a short-lived local excited (LE) electronic state that fluoresces mostly in the UV and decays rapidly, on a approximately 0.5 ps timescale, to an intramolecular charge transfer (ICT) state that emits only at longer wavelengths in the visible spectrum. The fluorescence-time profiles and transient fluorescence spectra reconstructed from the time profiles provided further information on secondary relaxation processes within and between the excited states and their non-radiative relaxation to the electronic ground state. 相似文献
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Time-resolved molecular fluorescence spectroscopy is presented as a sensitive and selective method for the characterization of mixtures of polynuclear aromatic hydrocarbons in the collision-free vapor phase at elevated temperature, in the condensed phase at room temperature, and at 77 K in a Shpol'skii matrix. This technique is applicable for all three conditions and the quantitative analysis for each component in mixtures has a fairly small error. Also the advantages and disadvantages are discussed for these different environments. Anthracene, pyrone and fluoranthene are used as model compounds. 相似文献
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《Chemical physics letters》1987,140(4):394-400
Analysis of time-resolved fluorescence anisotropy measurements on DPH and TMA-DPH in POPC vesicles with and without cholesterol in terms of the rotational diffusion model shows two distinct χr2 minima which are statistically equivalent. This is explained by the fact that the anisotropy decay function is given by a sum of three correlation functions which cannot be uniquely separated into individual contributions. The two solutions yield contradictory results for the effect of cholesterol on the probe dynamics. It is shown that the Maier-Saupe potential and the “wobble-in-cone” model do not give an adequate picture of the orientational order and the reorientational dynamics. 相似文献
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Yang Y Jones D von Haimberger T Linke M Wagnert L Berg A Levanon H Zacarias A Mahammed A Gross Z Heyne K 《The journal of physical chemistry. A》2012,116(3):1023-1029
We combine femtosecond polarization resolved VIS-pump IR-probe spectroscopy with DFT and TD-DFT calculations to identify and assign absorption bands to electronic transitions for corroles. These macrocycles and their corresponding metal complexes are receiving great attention because of their utility in many fields, while many of their spectroscopic features have not yet been fully described. Analysis of the perturbed free induction decay provides information about the bleaching signal at time zero and allows for determination of overlapping excited state and bleaching signal amplitudes. The S(0) → S(1) and S(0) → S(2) transitions in the Q-band of the hexacoordinated Al(tpfc)(py)(2) and Br(8)Al(tpfc)(py)(2) absorption spectra are explicitly assigned. Angles between these electronic transition dipole moments (tdms) with a single vibrational transition dipole moment of (53 ± 2)° and (34 ± 2)° when excited at 580 and 620 nm for hexacoordinated Al(tpfc)(py)(2) and (51 ± 2)° and (43 ± 2)° when excited at 590 and 640 nm for hexacoordinated Br(8)Al(tpfc)(py)(2) were determined. The relative angles between the two lowest electronic tdms are (90 ± 8)° and (94 ± 3)° for Al(tpfc)(py)(2) and Br(8)Al(tpfc)(py)(2), respectively. Angles are determined before time zero by polarization resolved perturbed free induction decay and after time zero by polarization resolved transients. Comparison of corrole's wave functions with those of porphine show that the reduced symmetry in the corrole molecules results in lifting of Q-band degeneracy and major reorientation of the electronic transition dipole moments within the molecular scaffold. This information is necessary in designing optimal corrole-based electron and energy transfer complexes. 相似文献
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We present a quantum mechanical wave packet treatment of time-resolved femtosecond stimulated Raman spectroscopy (FSRS), or two-dimensional (2D) FSRS, where a vibrational coherence is initiated with an impulsive Raman pump which is subsequently probed by FSRS. It complements the recent classical treatment by Mehlenbacher et al. [J. Chem. Phys. 131, 244512 (2009)]. In this 2D-FSRS, two processes can occur concurrently but with different intensities: a direct fifth-order process taking place on one molecule, and a cascade process comprising two third-order processes on two different molecules. The cascade process comprises a parallel and a sequential cascade. The theory is applied to the 2D-FSRS of CDCl(3) where calculations showed that: (a) the cascade process is stronger than the direct fifth-order process by one order of magnitude, (b) the sidebands assigned to C-Cl E and A(1) bends, observed on both sides of the Stokes C-D stretch frequency, are not due to anharmonic coupling between the C-D stretch and the C-Cl bends, but are instead due to the coherent anti-Stokes Raman spectroscopy (CARS) and coherent Stokes Raman spectroscopy (CSRS) fields produced in the first step of the cascade process, (c) for each delay time between the femtosecond impulsive pump and FSRS probe pulses, the line shape of the sidebands shows an inversion symmetry about the C-D stretch frequency, and this is due to the 180(°) phase difference between the CARS and CSRS fields that produced the left and right sidebands, and (d) for each sideband, the line shape changes from positive Lorentzian to dispersive to negative Lorentzian, then to negative dispersive and back to positive Lorentzian with the period of the bending vibration, and it is correlated with the momentum of the wave packet prepared on the ground-state surface by the impulsive pump along the sideband normal coordinate. 相似文献
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Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X. 相似文献
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Birch DJ 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2001,57(11):2313-2336
Recent work on the emerging application of multiphoton excitation to fluorescence studies of biomolecular dynamics and structure is reviewed. The fundamental principles and experimental techniques of multiphoton excitation are outlined, fluorescence lifetimes, anisotropy and spectra in membranes, proteins, hydrocarbons, skin, tissue and metabolites are featured, and future opportunities are highlighted. 相似文献
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It is shown that the decay rate γ2 of the excited state of a molecule can be determined by measuring the decrease of the resonance Raman intensity on a time scale much longer than γ?12, provided that the time needed for repopulation of the ground state, following rapid intramolecular radiationless transitions from the excited state to a molecular quasicontinuum, is itself much longer than γ?12. Thus, it should be possible to avoid the use of ultrashort pulses in the study of very fast intramolecular decay process. 相似文献
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The analysis of various varnishes from different origins, which are commonly found on historical musical instruments was carried out for the first time with both fluorescence excitation emission spectroscopy and laser-induced time-resolved fluorescence spectroscopy. Samples studied include varnishes prepared using shellac, and selected diterpenoid and triterpenoid resins from plants, and mixtures of these materials. Fluorescence excitation emission spectra have been collected from films of naturally aged varnishes. In parallel, time-resolved fluorescence spectroscopy of varnishes provides means for discriminating between short- (less than 2.0 ns) and long-lived (greater than 7.5 ns) fluorescence emissions in each of these complex materials. Results suggest that complementary use of the two non destructive techniques allows a better understanding of the main fluorophores responsible for the emission in shellac, and further provides means for distinguishing the main classes of other varnishes based on differences in fluorescence lifetime behaviour. Spectrofluorimetric data and time resolved spectra presented here may form the basis for the interpretation of results from future in situ fluorescence examination and time resolved fluorescence imaging of varnished musical instruments. 相似文献
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Ressler T 《Analytical and bioanalytical chemistry》2003,376(5):584-593
Time-resolved X-ray absorption spectroscopy (TR-XAS) possesses excellent capabilities to reveal quantitative phase composition and average valence together with the evolution of the local structure of a system under dynamic reaction conditions. The work discussed here focused on time-resolved in-situ XAS investigations aiming, first, at understanding structural evolution under dynamic conditions and, second, at revealing properties of the system studied not available from investigations under stationary conditions. Hence, not only was the local structure of a material studied under reaction conditions, but characteristic properties of the reaction, such as reaction intermediates or the kinetics of the reaction, were also elucidated. The solid–gas reactions presented here clearly demonstrate the potential of TR-XAS investigations to extend the suitability of XAS for in-situ studies in solid-state chemistry to investigations under dynamic conditions. 相似文献