Phosphoroscope

Thomas and Colbow (59) have described an elegant method by which one can change the angular position between the slits in a Becquerel phosphoroscope. Their system uses separate mechanical chopping of the... 

Another simple modification of the Becquerel phosphoroscope was described by Lewis and Kasha (60). Here one has a cylinder which can be rotated at various speeds with the sample inside. The cylinder contains a window and the exciting light source is placed on one side of the cylinder, with the detector on the other. When the window faces the source the sample is excited, and when it faces the detector the intensity is measured. The time between excitation and detection can be changed by altering the speed of rotation. In this manner the complete decay curve can be plotted. Again it... 

Figure 11. Improved version of the Becquerel phosphoroscope [from Ref. (59)].

A sophisticated type of Becquerel phosphoroscope, utilizing electronic flash circuitry and photomultiplier gating has been reported by Peterson and Bridenbaugh (61). This device appears to be quite useful and is representative of a large number of similar instruments. For these two reasons it is considered in some detail. Figure 13 is a schematic diagram of the system. 

The Phosphoroscope. This is a simple mechanical device which allows the separation of long-lived emissions (phosphorescence) from short-lived emissions which consist of scattered light and fluorescence. It is a disc or drum in which there are holes or slots placed in such a way that the excitation and emission beams reach the sample and the detector respectively at different times. With the fastest practicable rotation velocities of the phosphoroscope, the cut-off time is of the order of 1 ms. 

Long-lived emissions of very low intensities can be observed against very high intensities of short-lived emissions. It is also possible to observe the decay kinetics of long-lived emissions directly on an oscilloscope, by varying the speed of rotation of the phosphoroscope. 

These methods, used in conjunction with suitable phosphoroscopes, can also be used to measure quantum yields of phosphorescence (process 14) (32). Data are very scanty, due to experimental difficulties, so that estimates of the relative importance of processes 2, 3, 4, 14, and 15 remain very imperfect (48,64). Emission from fluid solutions is only by process 2, although with thorough de-oxygenation to eliminate process 11 process 14 might be detectable (34). Otherwise, process 14 is observed only in rigid glassy solvents, as with naphthalene, phenanthrene, or coronene in boric acid glass at room temperatures. 

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

Some degree of temporal resolution of emission may be obtained by incorporating a phosphoroscope attachment in the simple apparatus described above. A mechanical or electronic device is used to allow periodic and out-of-phase excitation and detection of luminescence. In the simplest case a mechanical shutter interrupts the excitation beam periodically and the detection system is gated so that emission is observed only after a fixed interval of time has elapsed after excitation. Under these conditions short-lived processes such as prompt fluorescence will have decayed to zero intensity and only longer-lived emission will be recorded. For mechanical devices the limit of measurable lifetime is of the order of 1 ms, thus allowing time resolved studies to be made of certain phosphorescence and delayed emission procesres (see ... 

Emission Spectra. With the phosphoroscope, etl l p-azidobenzo-ate in EPA at 77 K,gave an emission spectrum which is shown in Fig.6. 

Because of its weak intensity, phosphorescence is much less widely applicable than fluorescence. However, phosphorimetry has been used for the determination of a variety of organic and biochemical species including nucleic acids, amino acids, pyrine and pyrimidine, enzymes, polycyclic hydrocarbons, and pesticides. Many pharmaceutical compounds exhibit measurable phosphorescence signals. The instrumentation for phosphorescence is also somewhat more complex than that for fluorescence. Usually, the phosphorescence instrument allows discrimination of phosphorescence from fluorescence by delaying the phosphorescence measurement until the fluorescence has decayed to nearly zero. Many fluorescence instruments have attachments, called phosphoroscopes, that allow the same instrument to be used for phosphorescence measurements. 

Instrumentation. Steady state luminescence measurements were made using a Perkin-Elmer MPF-3L spectrofluorimeter. Samples were contained in an optical dewar and temperature variation achieved using a flow of pre—heated or cooled dry nitrogen gas. Phosphorescence measurements were made using a rotating-can phosphoroscope. 

Phosphorescence of metal ion complexes with organic ligands is used in inorganic analysis much less frequently than fluorescence even though phosphorimetric-analytical methods are promising because the use of phosphoroscopes results in significantly lower detection limits, due to the reduction of blank magnitude caused by fluorescence of the... 

Phosphorescence can be observed without interference from fluorescence by a process called time resolution. Instruments for measuring phosphorescence are very similar to those used for fluorescence but a mechanism that allows the sample to be irradiated and then, after a time delay, allows measurement of phosphorescent intensity (phosphoroscope) is required as an extra component. The instrument should also have the capability of keeping samples at very low temperatures. Another type of long-lived photoluminescence is time-delayed fluorescence, where the electrons in the molecule obtain enough energy to be excited from a special excited state to the normal excited state and then fluoresce. 

Edmond Becquerel is the father of Henri Becquerel, who discovered radioactivity. Edmond Becquerel invented the famous phosphoroscope that bears his name. He was Professor at the Museum National d Histoire Naturelle and at the Conservatoire National des Arts et Metiers in Paris. 

Uncorrected Spectrofluorometers for Research. This category of instrument is adaptable to all kinds of research, and is generally much more expensive than the medium-priced spectrofluorometers. The best known example is the Aminco-Bowman SPF instrument. The latter can be used as a spectrofluorometer, but with the attachment of an Aminco-Keirs phosphoroscope, it can also be used as a spectrophosphorimeter. When used as a spectrofluorometer, it consists of essentially the same components as those shown in Figure 9.7,... 

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