Time-resolved spectroscopy of solvated electrons

The theory of time-dependent transient spectra of solvated electrons is developed. It is shown that the evolution of the spectral line centre reproduces the time dependence of the classical correlation function of the random process of electronic energy level fluctuations. The results of this investigation are compared with experiment.     

Time-resolved spectroscopy of Nan-cluster fragmentation

Using picosecond pump&probe technique followed by mass-selective detection, the dynamics of excited states of cold Na_n=3...8-clusters in a supersonic beam — excited at λ=422nm — have been studied in time-resolved two-photon-ionization (TPI)-experiments. With growing cluster size a monotonous decrease of the decay times is observed except in the case of Na₇ known to be a very symmetric cluster.     

Time-resolved laser-induced fluorescence spectroscopy for enhanced demarcation of human atherosclerotic plaques

We report on the enhanced demarcation between human atherosclerotic plaques and normal vessel wall obtained using time-resolved detection of laser-induced fluorescence rather than the customary time-integrated monitoring technique. A frequency-doubled mode-locked and cavity-dumped continuous wave dye laser was used for picosecond pulse generation at 320 nm, and photon-counting techniques were employed for the time-resolved signal monitoring from human aorta samples in vitro. Implications for imaging fluorescence angioscopy and spectroscopic guidance in laser ablation of plaque are indicated.     

Time-resolved photoacoustic spectroscopy in the picosecond regime

Photoacoustic detection has been applied, for the first time, to detect and study excited molecules and reactive intermediates on the picosecond timescale. Dilute and weakly-absorbing transients are readily observed under conditions which render them inaccessible to conventional methods.     

Time-resolved spectroscopy of attosecond quantum dynamics

The advent of attosecond pulsed radiation leads to a large unexplored scientific area in chemical physics: the direct time-resolved measurement of electronic quantum dynamics. Major scientific goals include spectroscopy of single- and multi-electron motion and dynamical electron correlations, relating to orbital interactions in valence and core electronic levels of atoms and molecules. The results of such studies address a wide array of scientific and technological applications. Here, the current state-of-the-art of attosecond-dynamics measurements is reviewed and several novel spectroscopic methods are discussed that are particularly important for applications in chemical physics: attosecond transient absorption/dispersion spectroscopy, laser-induced-dipole spectroscopy, and absolute-phase spectroscopy.     

Time-resolved spectroscopy of sulfur- and carboxy-substituted N-alkylphthalimides

The photophysical and photochemical properties of N-phthaloyl-methionine (1), S-methyl-N-phthaloyl-cysteine methyl ester (2) and N-phthaloyltranexamic acid (3) were studied by time-resolved UV/Vis spectroscopy, using laser pulses at 248 or 308 nm. The quantum yield of fluorescence is low (phi(f)< 10(-2)) for 1-3 in fluid and glassy media, whereas that of phosphorescence is large (0.3-0.5) in ethanol at - 196 degrees C. The triplet properties were examined in several solvents, at room temperature and below. The spectra and decay kinetics are similar, but the population of the pi(pi*) triplet state, as measured by T-T absorption, is much lower for 1 and 2 than for 3 or N-methyltrimellitimide (5') at ambient temperatures. The quantum yield (phi(delta)) of singlet molecular oxygen O2(1deltag) formation is substantial for 3 and 5' in several air- or oxygen-saturated solvents at room temperature, but small for 2 and 1. The quantum yield of decomposition is substantial (0.2-0.5) for 3 and small (<0.05) for 2 and 1. It is postulated that photoinduced charge separation in the spectroscopically undetectable 3n,pi* state may account for the cyclization products of 1 and 2. In aqueous solution, this also applies for 3, whereas in organic solvents cyclization involves mainly the lower lying 3pi,(pi*) state. Triplet acetone, acetophenone and xanthone are quenched by 1-3 in acetonitrile; the rate constant is close to the diffusion-controlled limit, but smaller for benzophenone. While the energy transfer from the triplet ketone occurs for 3, a major contribution of electron transfer to the N-phthalimide derivative is suggested for 1 and 2, where the radical anion of benzophenone or 4-carboxybenzophenone is observed in alkaline aqueous solution.     

Time-resolved TPI spectroscopy of Na3-clusters

The dynamics of excited states of cold Na₃-clusters in a supersonic beam have been studied in time-resolved two-photon-ionization (TPI)-experiments, using picosecond pump&probe technique followed by mass-selective detection. In case of the electronically excited B-state for the first time the pseudorotational motion could be recorded as a temporal sequence. The time constant of pseudorotation was found to be 3ps. For the D-state, known to predissociate very rapidly, it was possible to directly ionize this excited electronic state. Decay times of the TPI-signal were estimated to be in the order of 2.7ps providing direct information of the fragmentation probability. Simultaneously the temporal evolution of the fragment signal Na ₂ ⁺ was detected.     

Time-resolved vibrational spectroscopy of a molecular shuttle

Time-resolved vibrational spectroscopy is used to investigate the inter-component motion of an ultraviolet-triggered two-station molecular shuttle. The operation cycle of this molecular shuttle involves several intermediate species, which are observable in the amide I and amide II regions of the mid-IR spectrum. Using ab initio calculations on specific parts of the rotaxane, and by comparing the transient spectra of the normal rotaxane with that of the N-deuterated version, we can assign the observed vibrational modes of each species occurring during the shuttling cycle in an unambiguous way. The complete time- and frequency-dependent data set is analyzed using singular value decomposition (SVD). Using a kinetic model to describe the time-dependent concentrations of the transient species, we derive the absorption spectra associated with each stage in the operation cycle of the molecular shuttle, including the recombination of the charged species.     

Time-resolved spectroscopy at the picosecond laser-triggered electron accelerator ELYSE

ELYSE is a fast kinetics center created for pulse radiolysis with picosecond time-resolution. The facility is a 4–9 MeV electron accelerator using a subpicosecond laser pulse to produce an electron pulse from a Cs₂Te semiconductor photocathode and RF gun technology for the electron acceleration. The pulse duration is around 5 ps at low charge (<2 nC) and high energy (9 MeV), and is under routine conditions 10 ps at higher charge (5 nC) and >8 MeV. The dark current at the target is less than 1% of the pulse photocurrent.Time-resolved absorbance measurements in cells placed in front of the electron beam are achieved using pulsed laser diodes, or a xenon flash lamp as light sources, and photodiodes connected to a 3 GHz transient digitizer or a streak camera (250–800 nm range and 3.7 ps time resolution) as detection instruments. In addition, the synchronization between the laser beam and the electron beam is exploited to measure the absorbance by a pump-probe set-up, the pump being the electron pulse produced by the laser pulse, and the probe being part of the laser beam (120 fs–3 ps) delayed by a variable optical line.     

Time-resolved EPR spectroscopy in a Unix environment

A computer-aided time-resolved electron paramagnetic resonance (EPR) spectrometer implemented under version 2.9 BSD Unix was developed by interfacing a Varian E-9 EPR spectrometer and a Biomation 805 waveform recorder to a PDP-11/23A minicomputer having MINC A/D and D/A capabilities. Special problems with real-time data acquisition in a multiuser, multitasking Unix environment, addressing of computer main memory for the control of hardware devices, and limitation of computer main memory were resolved, and their solutions are presented. The time-resolved EPR system and the data acquisition and analysis programs, written entirely in C, are described. Furthermore, the benefits of utilizing the Unix operating system and the C language are discussed, and system performance is illustrated with time-resolved EPR spectra of the reaction center cation in photosystem 1 of green plant photosynthesis.     

Laser-induced breakdown spectroscopy: Time-resolved spectrochemical applications

We have added time resolution to laser-induced breakdown spectroscopy in two forms, by gating an optical multichannel analyzer (OMA) and by time-resolving the output of a photomultiplier with a boxcar amplifier. Spectra were obtained for temporal segments of 25 to 100 ns, from 25 ns to 50 µs after initiation of the breakdown. OMA spectra of oxygen illustrate the power of this technique for survey purposes. The photomultiplier-boxcar arrangement was used to detect phosphorus atoms from diisopropylmethyl phosphonate in air, and also to detect chlorine in air, both in real time. In the former experiments we detected 690 ppm (w/w) of phosphorus and project a limit of detection with our current apparatus of 15 ppm (w/w). For chlorine, we observed signal from 120 ppm (w/w) and project a limit of detection of 60 ppm (w/w).     

Time-resolved fluorescence spectroscopy for illuminating complex systems

Over the years, the emissive characteristics (spectral, temporal, and polarization) of fluorophores have been widely used to probe a wide variety of systems. Fluorescence lifetime and rotational reorientation time measurements, in particular, offer a means to elucidate key details about complex systems. Further, because fluorescence occurs on the nanosecond (10⁻⁹ s) timescale, competing or perturbing kinetic processes like collisional quenching, solvent relaxation, energy transfer, and rotational reorientation can affect the fluorescence and hence be quantified. Thus, a carefully chosen and “placed” fluorophore can serve as an reporter on a wide range of nanosecond or faster events. This contribution is divided into three sections. The Theory section discusses time-resolved anisotropy and intensity decay kinetics (time and frequency domains), pump–probe spectroscopy, and up-conversion. The second section describes time-correlated single photon counting (TCSPC) and multifrequency phase-modulation fluorescence instruments. The final section is divided into subsections on the use of time-resolved fluorescence: (1) to study solvation dynamics, biochemical systems, polymer photophysics, and organized media; (2) as a tool in the separation sciences, microscopy, and sensing; and (3) coupled with multiphoton excitation strategies.     

Time-resolved luminescence spectroscopy of photosensitizers of biomedical interest

Studying the fluorescence decay of chromophores, either used as fluorescent labels to stain specific biomolecules or as photosensitizers to produce irreversible chemical or physico-chemical modifications on biological substrates, is being demonstrated to be a valuable method of investigating the interactions underlying a variety of phenomena. In fact, all possible primary steps in a photosensitized biological system are phenomena that may occur during the chromophore S1 lifetime and act as quenching mechanisms of the S1 state. Thus they can be identified, and the relative importance of the corresponding transient species quantitatively determined, with suitable techniques of time-resolved fluorescence spectroscopy. The examples discussed in this paper concern both tumor photosensitizing drugs, such as anthracyclines and porphyrins, and skin sensitizers (e.g. furocoumarins).     

Time-resolved fluorescence spectroscopy of hematoporphyrin derivative in micelles

Time-resolved fluorescence spectroscopy was performed on hematoporphyrin derivative in buffer at different concentrations of surfactants. New molecular species were detected, with fluorescence decay time constants of ≈0.7 and ≈2.7 ns and emission peaks at 630 and 675 nm, respectively. Deaggregation effects were observed, mainly in the presence of cationic micelles.     

Time-resolved optical absorption spectroscopy of molten polyethylene blends

By means of pulse radiolysis and laser photolysis experiments with detection by optical absorption spectroscopy short-living radical transients in pure PE and in PE doped with additives (e.g. di->tert-butyl-p-cresol, diphenylamine, benzophenone and carbon tetrachloride) were characterized by their optical absorption spectra and kinetics. It was found that the additive radicals were formed in a very fast process probably via exciton migration and subsequent energy transfer and dissociation reactions.