Picosecond laser light

In the present system adsorbed dye molecules are excited by picosecond laser light and electron transfer takes place from the valence band of the substrate molecular crystals to the excited dye and concomitantly quenches the fluorescence (6,7). We are able to determine the electron transfer rate by measuring the fluorescence decay dynamics of the adsorbed dye. Figure 2 shows the energy-gap (a) and temperature dependence (b) of the rate constant of electron transfer in the adsorption systems calculated according to eq.[1], which is Sarai s three-mode-variant (38) of Jortner s original equation (43) ... 

Even with a picosecond laser light source and an MCP PMT, the measuremem of short decay times rnnains chal-lenging. This is illustrated by the intensity decay of erythrosin B in water, shown in Bgure 4.43. The width of the instrument response function is seen to be near 100... 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Nonlinear light-matter interactions have been successfully applied to create new visualization contrast mechanisms for optical microscopy. Nonlinear optical microscopy employs femtosecond and picosecond lasers to achieve a high photon flux density by focusing the beam onto a sample with a high numerical aperture (NA) microscope... 

On the other hand, a liquid/liquid interface is fluctuating thermally in the time scale of tens of picoseconds, and this induces thermal capillary waves as discussed above. Thermal capillary waves at a flat liquid/liquid interface have been studied by laser light-scattering methods, and the wavelength of the capillary wave (2) has been reported to be A 102 /im [29-31], A large... 

In CARS two ultrashort pulses of laser light (from femtoseconds to picoseconds in duration) arrive simultaneously at the sample of interest (Mukamel, 2000 Fourkas, 2001 and references herein). The difference between the frequencies (W) - w2) matches the frequency of a Raman active vibrational mode in the sample. A probe pulse (w3) emits a signal pulse of frequency Wj - w2 + w3 in a unique special direction. By scanning the delay time between the pump and probe pulses, the delay of the vibrational coherence can be measured. The distinct advantage of CARS is that it is a background free technique, since the signal propagates in a unique direction. 

Further improvement is achieved using a picosecond-pulsed laser light source in contrast to a continuous one. Time resolution thus is determined by the lifetime of the probe beam instead of the response time of the detector in analogy to picosecond time resolved experiments in the visible spectral range (for a recent review see Stoutland et al., 1992). 

Figure 1. Transient absorption spectra of liquid benzene excited with a picosecond 266nm laser light.

The fluorescence detection system employed is shown in Figure 29.3. Both the IR and visible light ( 3 ps, 15 cm-1) generated by the picosecond laser system were introduced into a home-made laser fluorescence microscope [29, 30]. For the measurement of both solutions and fluorescent beads, both beams were adjusted onto a co-linear path by a beam-combiner and focused into the sample by an objective... 

Fluorescence can be resolved overtime. The use of very short pulsed light sources (picosecond lasers and laser diodes) has rendered accessible graphs of fluorescence decay as a function of the time. New applications based upon a greater knowledge of the lifetime are under development, although they are still not used much in chemical analysis. 

---