Fluorescence experiments

The fluorescence signal is linearly proportional to the fraction/of molecules excited. The absorption rate and the stimulated emission rate 1 2 are proportional to the laser power. In the limit of low laser power,/is proportional to the laser power, while this is no longer true at high powers 1 2 <42 j). Care must thus be taken in a laser fluorescence experiment to be sure that one is operating in the linear regime, or that proper account of saturation effects is taken, since transitions with different strengdis reach saturation at different laser powers. 

The blue satellite peak associated with resonance line of rubidium (Rb) saturated with a noble gas was closely examined by Lepoint-Mullie et al. [10] They observed SL from RbCl aqueous solution and from a 1-octanol solution of rubidium 1-octanolate saturated with argon or krypton at a frequency of 20 kHz. Figure 13.4 shows the comparison of the SL spectra of the satellite peaks of Rb-Ar and Rb-Kr in water (Fig. 13.4b) and in 1-octanol (Fig. 13.4c) with the gas-phase fluorescence spectra (Fig. 13.4a) associated with the B —> X transition of Rb-Ar and Rb-Kr van der Waals molecules. The positions of the blue satellite peaks obtained in SL experiments, as indicated by arrows, exactly correspond to those obtained in the gas-phase fluorescence experiments. Lepoint-Mullie et al. attributed the blue satellites to B — X transitions of alkali-metal/rare-gas van der Waals species, which suggested that alkali-metal atom emission occurs inside cavitating bubbles. They estimated the intracavity relative density to be 18 from the shift of the resonance line by a similar procedure to that adopted by Sehgal et al. [14],... 

Fig. 13.4 Rubidium line spectra obtained by gas-phase fluorescence (a). Sonoluminescence spectra from RbCl aqueous solution saturated with Ar and Kr (b) and from 1-octanol solution of rubidium 1-octanolate (c). The numbers indicated in the figures correspond to the difference in wave number k in gas-phase fluorescence experiments, where k0 is the reference attributed to the maximum intensity of the blue line [10] (Reprinted from Elsevior. With permission)...

All steady state fluorescence experiments were conducted with the sample placed in a thermostated cell with temperature maintained at 30°C. The concentrations of anthracene and initiator used were 0.000505 and 0.00608 moles per liter, respectively. The relative quantities of solvents (n-propanol and glycerol) were adjusted from 0 to 100% to achieve solutions of different viscosities, while maintaining the same molar concentration of the reactive solutes. 

A series of steady-state fluorescence experiments were performed in mixtures of propanol and glycerol to investigate the effect of viscosity on the effective second order photosensitization rate constant, k2. Figure 3 illustrates that the effective rate constant decreases as the viscosity of the system is increased. For example, as the reaction solvent is changed from pure propanol to pure glycerol, the viscosity of the system rises by three orders of magnitude, while the effective reaction rate coefficient, k2, decreases by approximately one order of magnitude. 

The dynamic range of the fluorescence experiment is related to a number of factors but it can be orders of magnitude. It is possible, for example, to determine quinine in water from nanomolar to millimolar concentration by direct measurement. Quinine fluorescence is familiar to most people that have noticed the blue glow of quinine tonic water in sunlight. 

The photophysics of fluorophores undergoing photoinduced charge transfer and/or internal rotation(s) is often complex. Time-resolved fluorescence experiments, transient absorption spectroscopy measurements, quantum chemical calculations, and comparison with model molecules are helpful in understanding their complex photophysical behavior. 

Time-resolved fluorescence experiments carried out with 1,2-benzanthracene quenched by CBr4 in propane-1,2-diol show a better fit with the Collins-Kimball equation than with the Smoluchowski equation. 

As in the case of intermolecular excimer formation, it should be recalled that difficulties may arise from the possible temperature dependence of the excimer lifetime, when effects of temperature on fluidity are investigated. It is then recommended that time-resolved fluorescence experiments are performed. The relevant equations established in Chapter 4 (Eqs 4.43-4.47) must be used after replacing ki[M] by k. ... 

The flexibility of the sugar linkages in glycoproteins results in multiple conformations that can be detected by time-resolved fluorescence experiments. Figure B9.1.1 shows an example of a triantennary glycopeptide labeled with a... 

As outlined in Section 9.2.1, orientational heterogeneity may affect the determination of a distance distribution, especially in the case of static orientation. Time-resolved fluorescence experiments provide an apparent average distance and an apparent distance distribution containing contributions from both distance and orientation (Wu and Brand, 1992). 

The broad tunability of the Ti sapphire laser accounts for its ability to produce extremely short pulses when modelocked several companies produce models with specified femtosecond pulsewidths. While shorter pulses are, to a first approximation, better, there are some additional tradeoffs to be borne in mind. First, with very few exceptions all common time-resolved fluorescence experiments may be carried out... 

Thus, at present, fluorescence spectroscopy is capable of providing direct information on molecular dynamics on the nanosecond time scale and can estimate the results of dynamics occurring beyond this range. The present-day multiparametric fluorescence experiment gives new opportunities for interpretation of these data and construction of improved dynamic models. A further development of the theory which would provide an improved description of the dynamics in quantitative terms with allowance for the structural inhomogeneity of protein molecules and the hierarchy of their internal motions is required. 

The experimental arrangement for Raman spectroscopy is similar to that used for fluorescence experiments (see Figure 1.8), although excitation is always performed by laser sources and the detection system is more sophisticated in regard to both the spectral resolution (lager monochromators) and the detection limits (using photon counting techniques see Section 3.5). 

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