Non-exponential fluorescence decay of gas phase acetaldehyde

Non-exponential fluorescence decay with “fast” and “slow” components was observed for acetaldehyde samples in the pressure range 25–400 m Torr using laser excitation at 320.0 nm. Both fluorescence lifetimes and quantum yields for the “slow” component were found to obey Stern-Volmer relationships. Above 5 Torr, lifetimes and yields become independent of pressure. These data are interpreted in terms of intermediate-type, singlet-triplet coupling and reversible intersystem crossing.     

Channel-three decay in benzene a picosecond fluorescence investigation

Fluorescence quantum yields, decay times and spectra have been measured following excitation of benzene vapour above the channel-three threshold. Experiments have been carried out on the static gas and on molecules jet-cooled to a vibrational temperature of 160 K. The excitation source was a recently developed high-power, picosecond, narrow-bandwidth laser continuously tunable from 224 to 252 nm. Channel-three decay was observed for all vibrational levels, even those not excited via line-broadened transitions. In addition it was observed that fluorescence decay curves resulting from excitation just above the channel-three threshold were non-exponential. It is proposed that the cause of absorption line broadening — intramolecular vibrational redistribution — is not directly related to channel-three, which is postulated to be coupling to a state X, which could be an isomeric form of benzene or a hidden singlet electronic state.     

Non-exponential phosphorescence decay of phenanthrene in biphenyl

Non-exponential phosphorescence decays of phenanthrene in biphenyl polycrystals have been observed. It is found that excitation with short duration quickens the decay while the decay is slower after excitation with weaker intensity. The origin of the non-exponentiality is ascribed to the distance-dependent interactions between guest molecules in the lowest excited triplet state.     

A study of the electronic structure of the hexafluorobenzene and pentafluorobenzene molecules by ultrasoft X-ray emission spectroscopy

The electronic structure of the hexafluorobenzene and pentafluorobenzene molecules was studied by ultrasoft X-ray emission spectroscopy. The FK _α and CK _α spectra of these compounds in the gas phase were obtained. The results of quantum-chemical calculations performed at the RHF/STO-6G//6-31G level were used to construct the theoretical spectra. The highest occupied molecular orbitals were found to consist largely of the 2p _π carbon atomic orbitals. The contribution of fluorine orbitals was small. π-Type interactions mainly involved deeper valence orbitals.     

Analysis of the decay of picosecond fluorescence in semiconductors: Criteria for the presumption of electroneutrality during the decay of an exponential electron-hole profile

When an exponential profile of electron-hole pairs is photogenerated (in a semiconductor) with a delta-function light pulse, unequal diffusion coefficients of holes and electrons (i.e. D_e≠ D_h) effect deviations from electroneutrality as electrons and holes diffuse into the bulk semiconductor. These deviations will in turn effect errors in the analysis of data (e.g. time resolved fluorescence) when using theory based on the presumption of electroneutrality. We deduce here the experimental conditions required for an effective electroneutrality to be maintained during the course of an experiment. Analyses were carried out using computer simulations without the presumption of electroneutrality and the analytic solution with the presumption of electroneutrality. The differences in the measured fluorescences predicted by the two computations are characterized as a function of a variety of experimental parameters and physical properties: intensity (of the excitation pulse), the absorption of the exciting and emitted light, the the ratio D_h/D_e, bulk dielectric constant of the semiconductor, bulk and surface recombination kinetics. It is shown that a conditon of adequate electroneutrality can be effectively attained when a well defined a minimum number of electron-hole pairs is generated; an upper limit of the number of e^?–k⁺ pairs is also established in order to avoid an intolerable temperature pulse.     

Non-exponential decay of the fluorescence from the 1Δg state in liquid oxygen at high excitation intensities

The ¹Δ_g state of liquid oxygen (natural O₂ and ¹⁸O₂) was excited by intense Nd: YAG laser pulses. The observed fluorescence decay is non-exponential and depends on pump laser intensity. Various decay channels of the ¹Δ_g state are discussed. The energy pooling reaction ¹Δ_g + ¹Δ_g → ¹Σ⁺_g + ³Σ^?_g is verified experimentally. Values f constant of liquid natural O₂ and ¹⁸O₂ are given.     

Time resolved fluorescence in the picosecond region

Time resolved fluorescence studies of four organic molecules in solution show that, contrary to an earlier report, the relaxed fluorescent S₁ states are formed within 10 ps of excitation even when excitation is to higher states. Fluorescence lifetimes for fluorescein and its halogen substituted derivatives vary over a factor of 40 owing to enhanced intersystem crossing. Measured radiative lifetimes agree with those calculated from integrated absorption intensities and are relatively constant.     

Nearly free induction decay observed in picosecond pulse propagation

Coherent reshaping of resonant pulses is investigated for moderately short samples. It is shown that a trailing pulse wing develops with decay time τ <T₂/2 (dephasing time T₂). This novel phenomenon is verified experimentally for HCl: Ar at 3 bar. Ultrafast dephasing and beating of the two HCl isotopes is directly observed on time-resolved infrared spectroscopy.     

Unimolecular decay of photoexcited tetramethyldioxetane probed by picosecond spectroscopy

The photolysis of tetramethyldioxetane at 264 nm was probed using picosecond spectroscopic techniques. Singlet excited acetone was one of the observed products. The rise time for acetone fluorescence was determined to be less than 10 ps. A mechanism consistent with these results is proposed.     

Long-lived fluorescence decay components in glyoxal

Long-lived components in glyoxal are considered to have been observed in the recent sensitized phosphorescence experiments reported by Ito and co-workers; this interpretation explains the apparent collision-free “intersystem crossing”, which is inconsistent with the accepted idea that glyoxal behaves according to the weakly coupled small-molecule limit. Mode selectivity as well as rovibronic level selectivity is also interpreted.     

Synchronous cultures of the green alga Scenedesmus obliquus: Comparison of picosecond decay associated emission spectra and fluorescence induction kinetics

Decay-associated emission spectra of synchronized cultures of Scenedesmusobliquus have been studied at two stages of their life cycle corresponding to the maximum and minimum of photosynthetic capacity. These decay-associated spectra comprise three kinetic components. The two components which are assigned to photosystem II show variations in their relative amplitudes depending on the life cycle of the cells. From the correlations observed in the decay-associated fluorescence spectra on the one hand and the fluorescence induction parameters on the other hand we obtained further evidence that the two photosystem II fluorescence components are directly related to the two fluorescence induction phases. This correlation supports our previous assignment of the two photosystem II fluorescence decay components of about 0.3 ns and about 0.6 ns lifetimes at the F₀ level (open photosystem II reaction centres to photosystem II α units and photosystem II β units respectively. The most pronounced difference between cells at the 8th hour of the life cycle and those at the 16th hour consists in the size of the photosystem II β units which are about 30% larger for the latter. In agreement with previous studies it was found that at these two stages the photosystem I units do not differ in size.     

Luminescent decay kinetics of malachite green on a picosecond time scale

Luminescence measurements after laser pulse excitation are reported for the dye malachite green in solutions of different viscosity with a time resolution of 10 ps.     

Characterization of hemicyanine Langmuir-Blodgett films by picosecond time-resolved fluorescence

Hemicyanine Langmuir-Blodgett films have been elaborated and characterized using stationary and time-resolved spectroscopic techniques. Depending on the experimental conditions, especially the pH of the water subphase, the absorption spectra of the films indicate the presence of non-fluorescent H-aggregates in the monolayer. Time-resolved fluorescence measurements revealed three mono-exponential decay times: a very short one (20-23 ps) attributed to an excited intramolecular charge transfer state and two longer ones (100-120 ps and 400-590 ps) attributed to the photoisomerization of the chromophores.     

Timing gases for picosecond chemical timing of molecular fluorescence spectra

The generality of the chemical timing or collisional timing technique for imposing picosecond time resolution on fluorescence from molecular gases is demonstrated by comparing S₁ → S₀p-difluorobenzene fluorescence spectra quenched by NO, CS₂, CF₃NO, and CO₂ with spectra quenched by O₂, the timing gas used in all previous studies. Additions of NO, CS₂ and CF₃NO reproduce the spectral changes previously observed with added O₂. Thus the changes in fluorescence spectra at high gas additions truly appear to be the consequence of fluorescence timing. All of these gases can be used to impose time resolution on molecular fluorescence spectra, and the desirability of each as a timing gas is discussed. CO₂ is not effective because collision-induced vibrational relaxation dominates the collisional interaction. The rate constant for quenching of the S₁ state of naphthalene (zero-point level) is reported for each of fourteen gases.     

Extracting decay curves of the correlated fluorescence photons measured in fluorescence correlation spectroscopy

We have developed a new method to extract the decay curves of the correlated fluorescence photons from the data of fluorescence correlation spectroscopy using time-correlated single photon counting. In this method, a two-dimensional correlation map of photon pairs is generated at an absolute delay time with reference to the excitation-emission delay of each photon. Using a dye-labeled DNA as an example, we have demonstrated that the decay curve of the correlated fluorescence photons is separated from the uncorrelated background signals simply by subtracting a two-dimensional correlation map at sufficiently long delay time without additional prior information.     

Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles

Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles (polyacrylamide, PAAm) was reported. Ultrafast fluorescence dynamics of polymer/water solution was monitored using a fluorescent probe molecule (C153). In the femtosecond time-resolved fluorescence measurement at 480 nm, slowly decay components having lifetimes of tau(1) approximately 53 ps and tau(2) approximately 5 ns were observed in addition to rapid fluorescence decay. Picosecond time-resolved fluorescence spectra of C153/PAAm/H2O solution were also measured. In the time-resolved fluorescence spectra of C153/PAAm/H2O, a peak shift from 490 to 515 nm was measured, which can be assigned to the solvation dynamics of polymer fine particles. The fluorescence peak shift was related to the solvation response function and two time constants were determined (tau(3) approximately 50 ps and tau(4) approximately 467 ps). Therefore, the tau(1) component observed in the femtosecond time-resolved fluorescence measurement was assigned to the solvation dynamics that was observed only in the presence of polymer fine particles. Rotational diffusion measurements were also carried out on the basis of the picosecond time-resolved fluorescence spectra. In the C153/PAAm/H2O solution, anisotropy decay having two different time constants was also derived (tau(6) approximately 76 ps and tau(7) approximately 676 ps), indicating the presence of two different microscopic molecular environments around the polymer surface. Using the Stokes-Einstein-Debye (SED) equation, microscopic viscosity around the polymer surface was evaluated. For the area that gave a rotational diffusion time of tau(6) approximately 76 ps, the calculated viscosity is approximately 1.1 cP and for tau(7) approximately 676 ps, it is approximately 10 cP. The calculated viscosity values clearly revealed that there are two different molecular environments around the polyacrylamide fine particles.     

Excitation and decay processes in helium clusters studied by fluorescence spectroscopy

Excitation and decay processes of helium clusters are investigated with fluorescence methods. The results differ remarkably from that obtained for the heavier rare gas clusters. They are discussed in view of the unusual structural and electronic properties of helium.     

Direct measurement of blue fluorescence lifetimes in polymethine dyes using a picosecond laser

The lifetimes of the blue fluorescence from a higher excited state are directly measured to be 80 ± 30 ps for cryptocyanine and 35 ± 15 ps for DTTC in methanolic solution. The relaxation processes associated with electronic excited states are discussed in the case of DTTC.