Picosecond time domain

Time-Correlated Single-Photon Counting. For the application of TCSPC in the picosecond time domain, lasers with pulses whose half-widths are 20 ps or less are used. For better time resolution, the combination of a microchan-nel plate photomultiplier tube (MCP-PMT) and a fast constant fraction discriminator (CFD) are used instead of a conventional photomultiplier tube (PMT). A TCSPC system with a time response as short as 40 ps has at its core a Nd YLF (neodymium yttrium lithium fluoride) laser generating 70-ps, 1053-nm pulses at... 

Diphenylethene (Stilbene). This molecule has been the subject of many photophysical and photochemical investigations and the subject of several reviews. It is the prototypical alkene for studies of photoisomerization. Transient spectroscopic measurements in the picosecond time domain have been performed on electronically excited tran -stilbene in a wide range of environments. Selections from these studies are described here. 

The magnesium porphyrin radical, Mg(tetraphenylporphyrin)C104 (136), has been used as a model for the structural and stereochemical consequences of loss of an electron in photosynthetic chromophores.521 The primary photosynthetic reaction in plants and bacteria consists of a transfer of an electron, in the picosecond time domain, from the chlorophyll phototrap to nearby acceptors yielding chlorophyll -cation radicals. The structure of (136) shows a five-coordinated Mg2+ cation which is not quite symmetrically sited in the porphyrin ring but has metal-ligand distances similar to those found in the previous structures. The perchlorate anion is tightly bound (Mg—O = 2.01 A) in a monodentate mode. It was concluded that the porphyrin can act as an electron sink and that no major effects are found in the bond lengths, or on the stereochemistry, of the macrocycle. 

Ultrafast vibrational spectroscopy offers a variety of techniques for unraveling the microsopic dynamics of hydrogen bonds occurring in the femto- to picosecond time domain. In particular, different vibrational couplings can be separated in nonlinear experiments by measuring vibrational dynamics in real-time. Both coherent vibrational polarizations and processes of population and energy relaxation have been studied for a number of hydrogen bonded systems in liquids [1],... 

The technique of up-conversion photoluminescence allows one to record the transient PL of a system at the temporal resolution of the laser pulse. It is used to study very fast processes below the picosecond time domain. A typical set-up for this experiment is shown in Fig. 3. The sample is excited at frequency uq by a femtosecond laser pulse and its PL at ujj- is mixed with that of an optically... 

The previous section clearly demonstrated the need for CD measurements on a time-resolved basis. Both stopped-flow and chromatographic applications require that CD measurements be obtained on the millisecond, or longer time scale. For this time domain, modification of conventional CD systems is appropriate and this approach has met with excellent success. However, there are a number of important applications where CD information is required in the microsecond to picosecond time domain. Certainly this time regime can not be accessed by simple modification of conventional CD detection strategies and new approaches have therefore been devised to extend CD measurements into the microsecond, and below, time domain. The crux of this section will be to consider CD detection strategies which are designed to improve the measurement SNR, and therefore allow extension into new domains such as time-resolved CD studies. 

The experimental configuration for acquiring CD information in the picosecond time domain is given in Fig. 14 below. 

Bitterling, K. Willig, F. Charge carrier dynamics in the picosecond time domain in photoelectrochemical cells, J. Electroanal. Chem. Interfacial Electrochem. 1986, 204, 211. 

Figure 16 shows the absorption spectrum obtained by additive-free polyethylene [67], At ambient temperature the absorption observed on nanosecond time-scale increased continuously from 500 to 200 nm without showing any maximum. The absorption in UV is similar to that obtained by y-irradiation. Considering the results obtained by liquid alkanes, the absorption seems to be comprised of several different free radicals. At 95 K additional absorption due to the trapped electron was observed at wavelengths longer than 600 nm the band was observable even at ambient temperature in the picosecond time-domain [96]. The electron decays presumably by the hole-electron recombination. The decay of the trapped electron was independent of the presence of carbon tetrachloride, suggesting that the additives reacted with a mobile electron but not with the trapped electron. On adding naphthalene, the radiation-induced spectrum showed the bands due to the first excited triplet state and the radical... 

Ultrafast techniques have really come into their own since the development over the past decade of reliable, relatively inexpensive femtosecond lasers, but in the 1980 s, ultrafast still meant picoseconds, and the chosen paper typifies work in this time-domain. The whole subject of course owes its inspiration to George Porter, who with Norrish pioneered flash photolysis, first in the milli-, then micro-, and ultimately nano- and picosecond time-domains. 

SSIP) is formed first this subsequently collapses into a contact ion pair (CIP). This mechanism was based on the observation of a blue shift of the absorption maximum of the ketyl radical anion from 715 to 690 nm which occurs with a half-life of 200 + 50 ps. The SSIP, being more solvated than the CIP, is expected to have its absorption spectrum red-shifted compared with that of CIP. Subsequent studies by Devadoss and Fessenden on the benzophenone-DABCO system indicated, however, that the spectrum of initial transient has an absorption maximum at 700 nm, which shifts to the red (720 nm) in the picosecond time domain [157, 158], These results seem to suggest that the initial species to be formed is the CIP which eventually separates to yield the SSIP and proton transfer occurs in the SSIP. 

Perhaps more challenging from both experimental and modeling perspectives is the -excitation case. Primary electron-hole pair separation occurs in the subnanosecond or picosecond time domain and light pulses with temporal resolution of 10 s (or better) are required. However, both nanosecond and picosecond light pulses have been employed [110 127] and although the analysis has been mostly confined to the slower (nanosecond) decay regime in the photocurrent (or photovoltage) transients, the rise-time domain has also been analyzed (see, e.g.. Ref. [256]). 

Subsequently, photoinduced interfacial electron transfer from the conduction band of colloidal Ti02 to dimeric viologen, an electron acceptor, was also examined by picosecond time-resolved methods (Serpone et at., 1987). Electron transfer takes place in the picosecond time domain = 2.5 x 10 s at pH 7.8. The reaction involves consecutive one-electron transfer to give the mono-reduced initially, followed by formation of DV ". In acidic aqueous media (pH 3.5), transient absorption spectra showed that electron transfer does not proceed beyond the formation of the mono-reduced species. 

Serpone N., Sharma D. K., Moser J. and Gratzel M. (1987), Reduction of acceptor relay species by conduction-band electrons of colloidal titanium dioxide. Light-induced charge separation in the picosecond time domain , Chem. Phys. Lett. 136, 47-51. 

Figure 15.7 (b) Comparison of the photoinduced absorption spectra for near steady state (millisecond) and ultrafast (picosecond) time domains for P30T/Cgo composite films. The picosecond photoinduced spectra are taken at 300 K at various delay times after a 2.01 eV 100 fs pump pulse for P3OT and PSOT/Cgo (reproduced by permission of World Scientific from Ref. [17]). 

In picosecond time-resolved Raman spectroscopy, the sample is pumped and probed by energetically well-defined optical pulses, producing a full vibrational spectrum over a 1000 2000 cm 1 window.207 One would expect vibrational spectroscopy to be restricted to the picosecond time domain and above by the Heisenberg uncertainty principle (Equation 2.1), because a 1 ps transform-limited pulse has an energy width of... 

Microcrystalline benzophenone [38] and benzil [16] were two of the first systems studied by nanosecond diffuse reflectance flash photolysis. Both samples gave transient absorptions which were positively identified as triplet-triplet absorptions. In the case of benzophenone an absorption, centred at 540 nm, was obsejrved which has, within experimental error, identical kinetics to the phosphorescence decay, which is predominantly second order. In the case of benzil a transient absorption of 60% at 510 nm was observed after 354 nm excitation. The assignment as triplet-triplet absorption was made on the basis of the absorption and phosphorescence kinetics being virtually identical, namely a mixture of first and second order kinetics. Ikeda et al [39] have also studied microcrystalline benzophenone on the picosecond time scale. Another microcrystalline sample studied is 1,5-diphenyl-3-styryl-2-pyrazoline, in which the triplet-triplet transient absorption was identified within the microsecond time domain [15] (see figure 7(b)). However, as mentioned above (see section 4 and figure 5), the transient absorption due to the excited singlet state has been observed on a picosecond time domain [17]. 

This unsatisfactory situation motivated Hesselink and Wiersma to attempt to generate and detect picosecond photon echos. Using a picosecond synchronously pumped dye laser system for excitation, and optical mixing as an echo-detection scheme they succeeded in measuring directly photon-echo relaxation times in the picosecond time domain. In Fig. 20 we show the results of such a picosecond photon-echo measurement on the... 

Laser hole-burning experiments in the nanosecond time domain can reveal the picosecond time history of the relaxation times in an n-level system of the molecule if those relaxation times are fast (picosecond) compared to the duration of the laser saturating pulse (nanosecond). Under these circumstances, the levels of the system can be treated as photostationary states and the kinetic equations solved exphcitly for the rate constant between any two specified levels. This leisurely approach to picosecond phenomena is equally valid in the picosecond time domain. 

Apart from being the group responsible for the development of the diffuse-reflectance laser fiash-photolysis technique in the temporal range from nanosecond up to seconds [10], Wilkinson et al. were also the first authors to publish transient absorption spectra of opaque materials in the picosecond time domain [11]. 

Fig. 3.50. Theoretical simulation of the variation of the pump pulse width (taken from [382]). (a) Results for variations in the femtosecond time domain, (b) results for the picosecond time domain...

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