Fluorescent molecules photon emission

The electronic excited state is inherently unstable and can decay back to the ground state in various ways, some of which involve (re-)emission of a photon, which leads to luminescence phenomena (fluorescence, phosphorescence, and chemiluminescence) (22). Some biologic molecules are naturally fluorescent, and phosphorescence is a common property of many marine and other organisms. (Fluorescence is photon emission caused by an electronic transition to ground state from an excited singlet state and is usually quite rapid. Phosphorescence is a much longer-lived process that involves formally forbidden transitions from electronic triplet states of a molecule.) Fluorescence measurement techniques can be extremely sensitive, and the use of fluorescent probes or dyes is now widespread in biomolecular analysis. For example, the large increase in fluorescence... 

While a fluorescent molecule transits in a focused laser beam (during a few ms), it undergoes cycles of photon absorption and emission so that its presence is signaled by a burst of emitted photons, which allows us to distinguish the signal from... 

Tunnelling electrons from a STM have also been used to excite photon emission from individual molecules, as has been demonstrated for Zn(II)-etioporphyrin I, adsorbed on an ultrathin alumina film (about 0.5 nm thick) grown on a NiAl(l 10) surface (Qiu et al, 2003). Such experiments have demonstrated the feasibility of fluorescence spectroscopy with submolecular precision, since hght emission is very sensitive to tip position inside the molecule. As mentioned before the oxide spacer serves to reduce the interaction between the molecule and the metal. The weakness of the molecule-substrate interaction is essential for the observation of STM-excited molecular fluorescence. 

The details of the scintillation process are complicated and depend very much on the molecular structure of the scintillator. In organic crystals, the molecules of the organic solid are excited from their ground states to their electronic excited states (see Fig. 18.18). The decay of these states by the emission of photons occurs in about 10-8 s (fluorescence). Some of the initial energy absorbed by the molecule is dissipated as lattice vibrations before or after the decay by photon emission. As a result, the crystal will generally transmit its own fluorescent radiation without absorption. 

A liquid scintillation counter is actually two photon counters connected in coincidence for measuring the shower or pulse of electrons resulting from the relaxation of fluorescent molecules excited by b-particle emission. In the out-of-coincidence mode, the instrument is a single photon counter, i.e., it counts single photon events. 

As shown in Figure 27-1, fluorescence is one of several mechanisms by which a molecule returns to the ground state after it has been excited by absorption of radiation. All absorbing molecules have the potential to fluoresce, but most compounds do not because their structure provides radiationless pathways for relaxation to occur at a greater rate than fluorescence emission. The quantum yield of molecular fluorescence is simply the ratio of the number of molecules that fluoresce to the total number of excited molecules, or the ratio of photons emitted to photons absorbed. Highly fluorescent molecules, such as fluorescein, have quantum efficiencies that approach unity under some conditions. Nonfluorescent species have efficiencies that are essentially zero. 

Fig. 10 Fluorescence effect (a) molecule is excited by light, (b) Schematic explanation of excitation and emission processes, (1) excitation from the ground state, (2) decay to the nearest energy level and (3) photon emission via returning to the ground state...

---