Pump probe transient absorption microscopy

Femtosecond Pump Probe Transient Absorption Microscopy (PPTAM)... 

The most daunting barrier to reaching higher time resolution in fluorescence microscopy is not of a technical nature, it is rather imposed by the photon bottleneck discussed above. The probability that an excellent fluorophore such as rhodamine 6G, will spontaneously emit a photon within a 10 fs window is smaller than 1 x 10 leading to a dauntingly large number of necessary excitations. Pump probe transient absorption microscopy (PPTAM) is not limited by the radiative rate and with carefully optimized all-reflective imaging components should be able to reach 20 fs or better time resolution. 

More recently, Gao et al reported transient absorption microscopy and spectroscopy measurements on chirality-assigned individual SWCNTs. Transient absorption spectra of individual SWCNTs shown in Figure 7.10 were obtained by recording transient pump probe images at different probe wavelengths and reveal different origins of photo-induced absorption. Population... 

Variation in exciton dynamics at the single particle level was also resolved in semiconducting nanowires. Recently Lo et al reported transient absorption microscopy of single CdTe nanowires and found that the time constants vary for different wires owing to differences in the energetics and density of surface trap sites. Mehl et al used pump probe microscopy of individual needle-shaped ZnO rods to characterize spatial differences in their response to photoexcitation. Dramatically different recombination dynamics was observed in the narrow tips compared to the interior owing to different recombination mechanisms. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

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