Equations ( 5.301 ) and ( 5.302 ) only consider the intramolecular decay processes of the excited-state mol- ecule in the absence of external reactants. Because electronically excited molecules are highly energetic, they are susceptible to intermolecular reactions in which M ∗ physically or chemically interacts with species in its environment (called quenchers Q) [5. 564 ] to again return to ground state M or to another molecule P (Fig. 5.150 b). When quenching processes are present, ( 5.301 ) and ( 5.302 ) must be modified by adding k q [ Q ] to their denominators, I ∼ Φ e = k r k r + k nr + k q [ Q ] , (5.303) τ = 1 k r + k nr + k q [ Q ] , (5.304) where k q is the quenching rate constant and the con- centration of the reacting partner [Q] accounts for the bimolecular nature of the quenching process. It fol- lows from this formalism that quenching pathways are dissipative and their presence will diminish the lumi- nescence intensity and shorten the excited-state lifetime. The Stern–Volmer relation quantitatively defines the at- tenuation in luminescence lifetime and intensity under quenching conditions as I 0 I = τ 0 τ = 1 + τ 0 k q [ Q ] , (5.305) where I 0 , I , and τ 0 , τ are the luminescence intensity and lifetime in the absence and presence of Q, respec- tively. Owing to the short lifetimes of singlet excited Part B 5.4
Velocity, Vorticity, and Mach Number 5.4 Molecular Tagging Velocimetry ( MTV ) 365 states, significant concentrations (typically 0 . 01 molar or greater) of Q are required to quench fluorescence. This is not the case for phosphorescence. The long lifetimes of phosphorescent excited states makes them especially susceptible to quenching at extremely small concentrations of Q. Most of the imaging techniques based on molecu- lar tagging may be understood in the context of the simple relations defined by ( 5.301 – 5.305 ). Below is a description of the chemistry behind these techniques. The Different Mechanisms of MTV The four basic mechanisms that encompass current MTV techniques are shown in Fig. 5.151 . Sometimes referred to as laser-induced photochemical anemometry ( LIPA ), mechanism A describes measurements based on the image produced by a photochromic dye and is the only MTV technique that relies on measuring ab- sorbance. Light excitation produces a high-energy form of the dye, which is usually more strongly absorbing than the ground-state molecule M (usually from trans- parent to opaque); the dye molecules in the flow are therefore tagged by the darkened image. Mechanism B describes the Raman excitation plus laser-induced elec- tronic fluorescence ( RELIEF ) technique. In a RELIEF experiment, a high-energy form of the molecule, M (specifically vibrationally excited oxygen), is also re- sponsible for creating the image. But unlike in LIPA , M emits photons upon subsequent irradiation. The RELIEF image is therefore revealed by detecting luminescence rather than absorbance. Mechanism C relies on the pro- duction of a luminescent molecule P upon excitation of a ground-state molecule M. For the technique with the moniker photoactivated non-intrusive tracing of mo- lecular motion ( PHANTOMM ), a laser dye is produced.
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