Luminescence excited radiation

Fluorescence and phosphorescence are both forms of luminescence [3]. If the emission of radiation has decayed within 10 s after the exciting radiation is cut off it is known as fluorescence [4], if the decay phase lasts longer (because the electrons return to the ground state from a forbidden triplet state (Fig. 5), then the phenomenon is known as phosphorescence. A distinction is also made between... 

Stokes law spect The wavelength of luminescence excited by radiation is always greater than that of the exciting radiation. stoks, 16 )... 

Luminescence spectrophotometry consists of fluorescence, phosphorescence and low-temperature total luminescence. Fluorescence is generally measured at room temperature. Phosphorescence is generally observed at liquid nitrogen temperature (77K) with the aid of a chopper to interrupt the exciting radiation. Total luminescence is the combined fluorescence and phosphorescence obtained at low temperature (77K). Luminescence spectrophotometry is generally much more sensitive and specific than absorption spectrophotometry. 

If the loss of luminescent species after the exciting radiation is shut off is uni-molecular or pseudo-first-order, then we may define mean lifetimes for the decay of emission as the inverse of the sum of all the effective first-order rate... 

Kasha-Vavilov rule The quantum yield of luminescence is independent of the wavelength of exciting radiation. There are exceptions to this rule. 

Luminescence spectra were recorded on a double Czerny-Turner scanning monochromator Model 1902 Fluorolog Spex spectrof1uoro-meter. Variations in the excitation radiation are automatically corrected by a reference detector equipped with a Rhodamine B quantum counter. Emission spectra were recorded with the right angle mode. Further details are available elsewhere (31). 

Many lanthanide ions exhibit luminescence, emitting radiation from an excited electronic state, the emitted light having sharp lines characteristic of f-f transitions of a Ln + ion. As will be seen later, this can be enhanced considerably by attaching a suitable organic ligand (e.g. a /3-diketonate, phenanthroline, crown ether, etc.) to the lanthanide. 

Excitation spectrum variation of the luminescence intensity at a given wavelength as a function of the wavelength of the excitation radiation... 

More recently (1965) Ryskin, Tkachuk and Tolstoi (30) measured the relaxation time t of a large number of platinocyanides and found t to be of the order of 10 to 10 sec. They also noted that the independence of the luminescent spectrum with regard to the exciting radiation shows that the redistribution of the electrons on the excited levels responsible... 

Depending on the excitation method used, luminescence techniques are divided into photoluminescence excited by photons, cathodoluminescence generated under the action of cathode rays, X-ray luminescence excited by X-rays, candoluminescence generated under the action of heat, and sonoluminescence excited by ultrasound. Emission generated under the action of a stream of ions from alkali metals in vaccum is called ionoluminescence radiation which atoms emit on optical excitation in plasma is known as atomic fluorescence chemiluminescence is the emission of radiation generated by the energy of chemical reactions, it does not require an external excitation source. The excitation source needed in each particular case is chosen on the basis of this classification. 

Photoluminescence spectra have b een measured at room temperature using a semiconductor laser as an excitation light source (10 W/cm at 980 run). Luminescence light has been collected by a lens condenser and then dispersed with a grating monochromator (MDR-23). Light detection was performed by a germanium photodiode DPD 2000 (Dilas Co.). A silicon filter was used to additionally cut off the excitation radiation. 

Luminescence of Probe Molecules. These studies permit evaluation of polymer properties. In particular, measurement of the relative Intensities of fluorescence of a probe molecule polarized parallel to and perpendicular to the plane of linearly polarized exciting radiation as a function of orientation of a solid sample yields Information concerning the ordering of polymer chains. In solution, similar polarization studies yield Information on the rotational relaxation of chains and the viscosity of the microenvironment of the probe molecule. More recently, the study of luminescence Intensity of probe molecules as a function of temperature has been used as a method of studying transition temperatures and freeing of subgroup motion in polymers. 

The luminc.sccncc proces.ses in such a system arc as follows. The exciting radiation is absorbed by the activator, raising it to an excited state (Fig. 1.2). The excited state returns to the ground state by emission of radiation. This suggests that every ion anil every material shows luminescence. This is not the case. The reason for this is that the radiative emission process has a competitor, viz. the nonradiative return to the ground state. In that process the energy of the excited state is used to excite the vibrations of the host lattice, i.e. to heat the host lattice. In order to create efficient luminescent materials it is necessary to suppress this nonradiative process. 

In many luminescent materials the situation is more complicated than depicted in Fig. I.l, because the exciting radiation is not absorbed by the activator, but elsewhere. For example, we can add another ion to the host lattice. This ion may absorb... 

One of the molecules which plays a role in this field is depicted in Fig. 1.9. The luminescent species is the F.u ion which we met already above. It is surrounded by a cage containing molecules of bipyridine. The whole complex is called a cryptate and its formula is written as [Eu C bpy.bpy.bpy) ". The cage protects the Eu " ion against the (aqueous) surroundings which tries to quench the luminescence. If this cryptate is excited with ultraviolet radiation, the bipyridine molecules absorb the exciting radiation and transfer their excitation energy subsequently to the Eu ion which then shows its red luminescence. 

The radiant efficiency rf is defined as the ratio of the emitted luminescent power and the power absorbed by the material from the exciting radiation. The luminous efficiency L is the ratio of the luminous flux emitted by the material and the absorbed... 

In thi.s. section we will consider those types of exciting radiation which create many electrons and holes in the luminescent material under consideration. This type of excitation is usually indicated as high energy ionizing radiation. The most well-known examples are cathode rays and X rays, but one can also think of y rays and a particles. It is usually assumed that the excitation process proceeds as follows ... 

Finnic and coworkers identified the source of the green luminescence to be arsenic oxide microcrystals formed during porous-etching [211, 212] and that of the infrared band to the scattered excitation radiation, exciting luminescence from the relatively unperturbed outer regions of the etchpit [211]. 

The exciting radiation is not only absorbed by the activator because defects and other ions exist in many luminescent materials. For example, the sensitizer can absorb the exciting radiation and subsequently transfer it to the activator. In Fig. 1.8, the S S transition is the excitation or absorption, and the A2 A transition is the emission. The Ai A2 is a nonradiative process and prevents back transfer. In particular, UV radiation is absorbed by Sb " but not by Mn ... 

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