Laser , quenching

P and a represent, respectively, the undercooled liquid, the b.c.c. solid solution and the amorphous phase. ( ) are results from enthalpy of crystallisation experiments. Horizontal bars represent amorphous phase (I) interdifliision reaction and (2) by laser-quenching. 

Figorc 11J8. Calculated critical cooling rates for suppression of various phases in (a) Ag-Si and (b) Au-Si from Kambli et al. (1985). Horizontal bars indicate experimental glass-forming range by laser quenching. 

Recent work on the carbon dioxide system shows another unusual high-pressure behaviour. Raman spectra of carbon dioxide show that CO2 molecules remain the basis of the phases to more than 40 GPa at temperatures below a few hundred Kelvin [52]. These results, however, do not mean that the molecular crystals are the stable phases indeed, recent studies of the combustion of carbon at high pressures by Yoo et al [53] reach another conclusion. They initiated combustion of a mixture of carbon and oxygen at pressures between 7 and 13 GPa by heating the carbon with a Nd YAG laser, quenching the products to ambient temperature under pressure and recording their Raman spectra. As well as features of unreacted O2 and CO2 in some samples,... 

M. O. Scully. Correlated Spontaneous-Emission Lasers Quenching of Quantum Fluctuations in the Relative Phase Angle. Physical Review Letters 1985 Dec 16 55(25) 2802-2805. 

The data obtained in tlie infrared-diode-laser-probe studies described above provides quenching infonnation at a given substrate donor energy E. By varying tlie laser excitation wavelengtli for production of vibrationally hot... 

The writing process, that is, the transition crystalline — amorphous, is caused by briefly (<50 100 ns) heating up the selected storage area (diameter (( )) ca 0.5—1 Hm) by a laser pulse to a temperature above the melting point of the memory layer (Eig. 15, Record), such that the film locally melts. When cooled faster than a critical quench rate (10 -10 ° K/s), the formation of crystalline nuclei is suppressed and the melted area sohdifies into the amorphous (glass-like) state. 

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

It has been shown in Chapter 5, the fluorescence quenching of the DPA moiety by MV2 + is very efficient in an alkaline solution [60]. On the other hand, Delaire et al. [124] showed that the quenching in an acidic solution (pH 1.5-3.0) was less effective (kq = 2.5 x 109 M 1 s 1) i.e., it was slower than the diffusion-controlled limit. They interpreted this finding as due to the reduced accessibility of the quencher to the DPA group located in the hydrophobic domain of protonated PMA at acidic pH. An important observation is that, in a basic medium, laser excitation of the PMAvDPA-MV2 + system yielded no transient absorption. This implies that a rapid back ET occurs after very efficient fluorescence quenching. 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Fig. 2. a) Transient absorption spectra of RF (35 pM) in N2-saturated MeOH-Water (1 1) solution observed after 60 ps of the laser pulse as a function of the concentration of GA. b) Effect of dissolved molecular oxygen ( 2) on the decay of the 3RF at 700 nm. Inset Stern-Volmer plot for the quenching of 3RF by 3O2. 

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]. 

FIG. 12 Simulation of fluorescent decays for dye species located in the aqueous phase following laser pulses in TIR from the water-DCE interface according to Eq. (38). A fast rate constant of excited state decay (10 s ) was assumed in (a). The results showed no difference between infinitely fast or slow kinetics of quenching. On the other hand, a much slower rate of decay can be observed for other sensitizers like Eu and porphyrin species. Under these conditions, heterogeneous quenching associated with the species Q can be readily observed as depicted in (b). (Reprinted with permission from Ref 127. Copyright 1997 American Chemical Society.)... 

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