Time-resolved picosecond absorption

Fig. 3 Electronic absorption spectra of (A) the EDA complex of 9-cyanoanthracene (CNA) and tetracyanoethylene (TCNE) in comparison with the uncomplexed CNA and TCNE (B) the donor cation CNA+- and acceptor anion TCNE -generated spectroelectrochemically and (C) time-resolved picosecond absorption spectra taken at 25, 50, 75 and lOOps following the charge-transfer excitation at 532 nm of [CNA, TCNE] with a 25-ps laser pulse.

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

A transient absorption band, with a profile similar to that of the dimethylani-line radical cation, was observed in polar solvents by time-resolved picosecond absorption spectroscopy. 

Figure 1. (A) Apparatus used for time-resolved picosecond absorption spectroscopy which utilizes the TPF technique. A set ofphotodiodes and oscilloscopes (PI, P2, P3, and P4)are used to measure the intensity of the laser pulse before and after each pass through the cell. (Reproduced with permission from Ref 15. Copyright 1969, North-Holland...

A diporphyrin cyclophane (Fig. 23) has been synthesized in which the macrocycle planes are cofacial and separated by about 0.4 nm. It is possible to preferentially insert one or two metal atoms into these dimers. Thus it is possible to study three separate cyclophanes, containing two, one, or no magnesium atoms. The fluorescence emission of the dimers is much lower than that of the monomers with the dimetal compound having an emission three times larger than the other two. For the dimetal and the nonmetallated dimer the estimated energy of the ion pair is above the energy of the relaxed excited singlet state. " The time-resolved picosecond absorption spectrum of these compounds decay slowly from what appears... 

The fast transfer and trapping of excitation energy in photosynthetic bacteria have been intensively studied in recent years. For reviews we refer to (1). Steady-state (2) and time-resolved fluorescence (3) and time-resolved picosecond absorption recovery measurements have led to the formation of the following scheme for energy transfer in Rhodobacter (Rb.) sphaeroides. (4,5,6)... 

Pairwise EET rates cannot be directly measured in antenna systems. The closest approach to direct determination is offered on the one hand by time resolved picosecond and sub-picosecond absorption and fluorescence measurements and on the other hand by hole burning spectroscopies. Time resolved techniques do not detect transfer between isoenergetic sites. A somewhat more indirect approach to determining pairwise rates is that of analysing excited state lifetime data in terms of a particular antenna and an EET model. 

Due to low, dark current and rapid readout characteristics, their large dynamic range (>18 bit for the CCD) and the two-dimensional array feature, the CCD and LCI-CCD are more versatile detectors. These detectors and the MCP-SPD array detector are particularly useful for picosecond, time-resolved emission, absorption, and Raman spectroscopy, and for imaging applications where signal averaging is required. 

This problem is related to the question of appropriate electronic degeneracy factors in chemical kinetics. Whereas the general belief is that, at very low gas pressures, only the electronic ground state participates in atom recombination and that, in the liquid phase, at least most of the accessible states are coupled somewhere far out on the reaction coordinate, the transition between these two limits as a function of solvent density is by no means understood. Direct evidence for the participation of different electronic states in iodine geminate recombination in the liquid phase comes from picosecond time-resolved transient absorption experiments in solution [40, 44] that demonstrate the participation of the low-lying, weakly bound iodine A and A states, which is also taken into account in recent mixed classical-quantum molecular d5mamics simulations [42. 43]. 

The power of time-resolved IR (TRIR) methods to solve mechanistic quandaries is superbly demonstrated in a study of the photochemical fac-mer isomerization of the Mn(Br)(CO)3(/Pr-DAB) complex (/Pr-DAB =N,N -di-rPr-1,4-diazabutadiene) (Scheme 6). Time-resolved visible absorption showed an initial species with Amax at 605 run formed within 400fs following excitation. This species decayed with an lips lifetime accompanied by the appearance of the product with identical kinetics. Based on this evidence alone, it was not possible to differentiate between two mechanistic interpretations (i) the 605 nm transient is an excited state and the 11 ps process is concerted CO loss and Br movement, or (ii) the 605 nm species is the CO loss primary photoproduct and the 11 ps process is the axial — equatorial Br movement. Subsequent experiments with picosecond TRIR showed that the IR bands of the 605 nm species were shifted to lower frequencies relative to the starting complex, a result interpreted as consistent only with the assignment of the species to the CO loss species and not the excited state. Note that a slower 22 ps process was attributed to coordination of a pyridine solvent molecule to the Mn(Br)(CO)2(/Pr-DAB) intermediate. 

As a summary of our experiments it is reasonable to draw the general conclusion that Triton X-100 disturbs the interaction between LHC-II and the PS 2 core particle. The validity of this conclusion is based upon observations made in steady state fluorescence and time resolved picosecond fluorescence experiments. Both these methods reveal a disturbed energy transfer from the LHC-II to the PS 2 core particle. Based upon the results obtained in the steady state experiments and the absorption of the sample, we estimated the number of Triton X-100 molecules, sufficient to interrupt the energy transfer to be 46 molecules per 250 chlorophyll a. Integral proteins are known to interact extensively with the hydrocarbon chains of membrane lipids and so they can be released only by agents that compete for this nonpolar interaction. (4). Anderson and Andersson reports of a picture of PS 2 with the core particle surrounded by LHC-II protein complexes.(3). According to this picture the LHC-II is divided into two sub populations an inner LHC-II pool which is tightly bound to the PS 2 core particle and an peripheral pool which surrounds the inner pool and the PS 2 core. Within this model the interpretation must be that Triton X-100 affects the interaction between the inner LHC-II pool. 

Laser-induced fluorescence data provide a wide variety of detailed information about physical and chemical reactions. Laser-based time-resolved (picosecond) fluorescence spectroscopic techniques have been used to investigate the mechanism of photo-stabilisation by UVAs such as benzophenones, ben-zotriazoles and polymer-bound UV stabilisers [117]. Such ultrafast spectroscopic measurements can provide insight into the dynamics of the primary energy dissipation processes in polymers and polymer additives following light absorption. Excimer LIF spectra of plasticised PVC showed two distinct regions... 

Picosecond to Femtosecond Time-Resolved Infrared Absorption Measurements... 

Picosecond to femtosecond time-resolved infrared absorption measurements were initiated in the middle of the 1980s. In 1984, Heilweil et al. [17] studied the dynamics of vibrational relaxation by using picosecond infrared pulses obtained from an OPA (LiNb03) excited by a mode-locked Nd YAG laser. 

Sakamoto, A., Nakamura, O., Yoshimoto, G. and Tasumi, M. (2000) Picosecond time-resolved infrared absorption studies on the photoexcited states of poly(p-phenylenevinylene). J. Phys. Chem. A, 104,4198-4202. 

Sakamoto, A. and Takezawa, M. (2009) Picosecond time-resolved infrared absorption study on photoexcited dynamics of regioregular poly(3-hexylthiophene). Synth. Met., 159, 809-812. 

Rao reported measurement of third-order optical non-linearity in the nanosecond and picosecond domains for phosphorus tetratolyl porphyrins bearing two hydroxyl groups in apical position [89]. Strong nonlinear absorption was found at both 532 nm and 600 nm. The high value of nonlinearity for nanosecond pulses is attributed to higher exited singlet and triplet states. Time resolved studies indicate an ultra-fast temporal evolution of the nonlinearity in this compound. 

However, time-resolved X-ray diffraction remains a young science. It is still impossible, or is at least very difficult, to attain time scales below to a picosecond. General characteristics of subpicosecond X-ray diffraction and absorption are hardly understood. To progress in this direction, free electron laser X-ray sources are actually under construction subject to heavy financial constraints. Nevertheless, this field is exceptionally promising. Working therein is a challenge for everybody ... 

Yamakata, A., Uchida, T., Kubota, J. and Osawa, M. (2006) Laser-induced potential jump at the electrochemical interface probed by picosecond time-resolved surface-enhanced infrared absorption spectroscopy./. Phys. Chem. B, 110, 6423-6427. 

Figure 3.30. (a) UV-vis absorption spectra of the HPAA product (solid line) and the HPDP substrate (dash line) in a H20/MeCN (1 1) mixed solvent, (b) Picosecond time-resolved resonance Raman (ps-TR ) spectra of HPDP obtained with a 267 nm pump and 200 nm prohe wavelengths in a HjO/MeCN (1 1) mixed solvent. Resonance Raman spectrum of an authentic sample of HPAA recorded with 200 nm excitation is displayed at the top. (Reprinted with permission from reference [49]. Copyright (2006) American Chemical Society.)... 

From the mechanism and values of the rate constants, the formation of B occurs very rapidly within a few hundred picoseconds and AB is formed on the microsecond time scale. These species exhibit characteristic absorption bands in the 550 to 600 nm region of the spectrum. At very long times, i.e. several seconds of steady state irradiation, the red shift in the absorption band is complete and presumably due to AnB as suggested by Krongauz (1 —2) Thus far, it has not been possible to clearly time resolve the formation of aggregates from AB dimers, although subtleties in the transient absorption indicate this is occurring. For instance, the time resolved buildup in absorbance at the red end of 600 nm band seems to be slower than it is 10 or 20 nm further to the blue. This may indicate a process such as ... 

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