Time-resolved differential absorption

The excited state dynamics that are probed are expressed in terms of differential absorbance (AA), that is the difference between the absorbances of the excited (Ap) and unexcited (A) sample. The experiments provide the spectra of the probe beams which have travelled respectively through the sample (Is) and reference (Jr) cell in the presence the pump pulse or without. The time resolved differential absorption across the explored spectral range is ... 

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. 

Bimolecular photoinduced electron transfer between an electron donor and an electron acceptor in a polar solvent may result in the formation of free ions (FI). Weller and coworkers [1] have invoked several types of intermediates for describing this process (Fig.la) exciplex or contact ion pair (CIP), loose ion pair (LIP), also called solvent separated ion pair. The knowledge of the structures of these intermediates is fundamental for understanding the details of bimolecular reactions in solution. However, up to now, no spectroscopic technique has been able to differentiate them. The UV-Vis absorption spectra of the ion pairs and the free ions are very similar [2], Furthermore, previous time resolved resonant Raman investigations [3] have shown that these species exhibit essentially the same high frequency vibrational spectrum. 

Figure 10-7. (a) Absorption spectrum of LPPP. The arrow indicates the spectral position of the excitation pulse in the time-resolved measurements, (b) PL spectrum for LPPP for low excitation pulse energies. (c) Differential transmission spectrum observed in LPPP after photoexcitation with a femtosecond pulse having a pulse energy of 80 nJ at a wavelength of 400 nm. The arrow indicates the spectral position of the probe pulses used for a more detailed investigation of the gain dynamics. 

Circular dichroism (CD), the differential absorption of left versus right cireularly polarized light, is the polarization spectroseopy perhaps best suited to detecting the presence of asymmetry in the structure or environment of moleeular ehromophores. Various time-resolved CD (TRCD) methods have been developed to take advantage of this sensitivity and obtain more detailed structural information about kinetic processes than is found from ordinary time-resolved absorption measurements [33]. Some examples of the processes studied with TRCD methods are the effeets of electronie excitation on the structure of chiral inorganic complexes, the ehanges in a-helical secondary... 

In this section, the focus is on the surface-active properties of aqueous solutions of MEGA and MELA surfactants. Their surface properties and micelle formation have been studied by light scattering [25,53], spectro-fluorimetry, ultrasonic absorption and time resolved fluorescence quenching [30], differential scanning calorimetry (DSC) [54], and measurements of aqueous solution densities [27] and surface tensions [24,49,55]. The results can be summarized as follows ... 

Cr=crystal Sm=smectic CrSmB = crystal smectic B N=nematic Ch=cholesteric I=isotropic fluorescence = steady state fluorescence SPC = time-resolved single photon counting CPF=circularly polarized fluorescence UV-vis = UV-visible absorption spectrophotometry DSC=differential scanning calorimetry OM = optical microscopy XRD = X-ray diffraction EPR=electron paramagnetic resonance NMR=nuclear magnetic resonance. 

The differential pulse and square wave techniques are among the most sensitive means for the direct evaluation of concentrations, and they find wide use for trace analysis. When they can be applied, they are often far more sensitive than molecular or atomic absorption spectroscopy or most chromatographic approaches. In addition, they can provide information about the chemical form in which an analyte appears. Oxidation states can be defined, complexation can often be detected, and acid-base chemistry can be characterized. This information is frequently overlooked in competing methods. The chief weakness of pulse analysis, common to most electroanalytical techniques, is a limited ability to resolve complex systems. Moreover, analysis time can be fairly long, particularly if deaeration is required. 

---