Time-resolved Differential Absorption Measurements

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  

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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 ... 

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