Process fluorescence

Poly(aryl ether) branches of generation 1 to 3 have been appended to a pho-totautomerizable quinoHne core to investigate the effect of dendritic architecture on the excited state intramolecular proton transfer [45]. The changes observed in the absorption and emission spectra on increasing dendrimer generation indicate that the dendritic branches affect the planarity of the core and therefore the efficiency of the excited state intramolecular proton transfer and of the related fluorescence processes. 

Numerous substances that produced fluorescence were examined by Stokes plant extracts (e.g., chestnut rind, chlorophyll in water), glass, paper, animal material, uranium compounds, etc., and he pointed out that the rays produced by the fluorescence process were much more refrangible than the rays initiating them. ... 

Other influences of the structure and the environment are manifest in the rates of processes competing with fluorescence for deactivation of the lowest excited singlet state. These are the processes and properties that influence the fluorescence process and will be discussed briefly here. 

We have tacitly assumed that the photoemission event occurs sufficiently slowly to ensure that the escaping electron feels the relaxation of the core-ionized atom. This is what we call the adiabatic limit. All relaxation effects on the energetic ground state of the core-ionized atom are accounted for in the kinetic energy of the photoelectron (but not the decay via Auger or fluorescence processes to a ground state ion, which occurs on a slower time scale). At the other extreme, the sudden limit , the photoelectron is emitted immediately after the absorption of the photon before the core-ionized atom relaxes. This is often accompanied by shake-up, shake-off and plasmon loss processes, which give additional peaks in the spectrum. 

The fluorescence process used in some x-ray sources, as described in Section 10.1, can also be used as an analytical tool. One can direct either high-energy electron beams or x-rays at an unknown sample and perform qualitative and quantitative analysis by making measurements on the lower-energy x-ray emissions that occur. Let us first briefly review what we have discussed to this point concerning the concept of fluorescence. 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

Time-of-flight (TOF) dispersion. This effect creates significant tailing in the decay curves of fast fluorescence processes. For details see Section 8.4.4. 

Carl Zeiss, Inc. also describes a spectrofluorometer system for process monitoring," but it does not currently appear as a standard marketed product on their web site. HORIBA Jobin Yvon also markets a fluorescent process analyzer, but it is a laser-induced time-domain based measurement system tailored for uranium or equivalent analysis." Finally, while numerous miniature spectrofluorometers are also available (Carl Zeiss, StellarNet Inc., Ocean Optics and Avantes), they are not packaged and configured for process applications. Although there is an established need and continued growing interest in realtime process spectrofluorometry, relative to conventional process spectroscopic instruments such as NIR, UV-vis and Raman, commercial process spectrofluorometers are currently available on a very limited basis. 

This complex was luminescent in solution and in the solid state. In acetonitrile solution, it displayed an emission at 345 nm, by excitation at 284 nm, which is similar to that of the ligand precursor and is attributed to a fluorescent process. In the solid state, it emitted at 515 nm and the ligand precursor, [H(py)2im]BF4, at 514 nm. Consequently, the remarkable similarity between the emissions of the ligand and the metal salt in both the solid state and solution suggested a ligand-centered emission. 

In our discussion above, it was pointed out that a molecule in the excited state can return to lower energy levels by collisional transfer or by light emission. Since these two processes are competitive, the fluorescence intensity of a fluorescing system depends on the relative importance of each process. The fluorescence intensity is often defined in terms of quantum yield, represented by (X This describes the efficiency or probability of the fluorescence process. By definition, XL is the ratio of the number of photons emitted to the number of photons absorbed (Equation 5.6). 

Energy-level diagram outlining the fluorescence process. 

The non-monochromaticity of the x-ray photons results firstly from the finite width of the Is- and 2p-levels involved in the fluorescence process and then from the overlap of the Koj-Koj-spin doublet. Not considering the monochromatic UV-light 1S), Magnesium and Aluminium with a line width of 0.7 eV, resp. 0.9 eV proved to be most suitable as anticathode-material. 

On the other hand, by Eq. (16.12) there is the quantum efficiency of the fluorescence process (< )) given by the ratio between intensities of fluorescence and absorbed light therefore,... 

Both the initial absorption event, and the subsequent fluorescence emission are directional processes. The strength of a particular fluorescence process will then be propor-... 

The mobile phase plays an important part in the fluorescence of a molecule. Unless chosen with care, the mobile phase can quench the fluorescence of the molecule of interest. Most of the non-halogen-containing solvents used in HPLC can be used with fluorescence detection. However, dissolved oxygen or other impurities in the eluent can cause quenching. The solvent polarity and the pH of the mobile phase can also affect the fluorescent process if they influence the charge status of the chromophore. For example, aniline fluoresces at pH 7 and at pH 12, but at pH 2, where it is cationic, it does not fluoresce. Table 3.5 shows wavelength selections for some common LC-fluorescence applications.30... 

Relaxation of the neutral excited complex towards the ground state with emission of fluorescence (process IV). The red shift in the absorption (excitation) spectra of the AH-B complex with respect to the absorption of the bare AH molecule will only measure the increase of the binding energy in the excited neutral form AH - B of the cluster. Then, the emission spectrum will be similar to the fluorescence of the free molecule. 

The fluorescence technique, like other methods based on scatter (elastic or inelastic), has been shown by us - and others to be a reliable unperturbing method of measuring spatial/ temporal flame temperatures and species concentrations. To avoid the dependency of the fluorescence signal on the environment of the emitting species, it has been shown by several workers that optical saturation of the fluorescence process (i.e., the condition occurring when the photoinduced rates of absorption and emission dominate over the spontaneous emission and colli sional quenching rates) is necessary. Pulsed dye lasers have sufficient spectral irradiances to saturate many transitions. Our work has so far been concerned with atomic transitions of probes (such as In, Pb, or T1) asoirated into combustion flames and plasmas. 

Spatial density profiles of atomic (and molecular) species can also be made via saturation fluorescence approaches. For a "2-level" atom, like Sr, a plot of 1/Bp vs 1/E (Bp is the fluorescence radiance, in J s 1m-2sn l, and Ex is the excitation spectral irradiance, in J s nT nm ) allows estimation of the quantum efficiency, Y of the fluorescence process (and thsu estimation of "radiationless" rate constants) and the total number density nj, of the species of interest by means of... 

Let us consider the effect of an external magnetic field on the angular momenta distribution at a level populated in the fluorescence process see Section 3.4, Fig. 3.14. In the presence of an external magnetic field the following polarization moments are created on the lower level J" via spontaneous transitions at weak excitation, x — 0 ... 

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