Lifetime of the lamp

By inserting either an interference filter or a colored filter, it is possible to select a more or less extended region of the spectrum likewise, by adding an optical fiber it is possible to direct the beam where desired. This set-up best exploits the characteristics of these powerful lamps, and offers an excellent choice for the irradiation of small surfaces. Consequently, spectrophotometric cuvettes or cylindrical cuvettes are used for the irradiation, which involves small volumes. Such restrictions, as well as the high price and short lifetime of the lamp and its accessories, favors the use of these arcs for kinetics studies and quantum yield measurements, rather than for preparative photochemistry. 

Hollow cathode lamps of the type commonly used in atomic absorption do not have sufficient intensity to be useful for atomic fluorescence. Increasing the current to a hollow cathode lamp is not sufficient for atomic fluorescence since very high currents may actually result in decreased emission intensity due to a high degree of self-absorption. The lifetime of the lamp also is reduced when high currents are used. 

The radiance drops constantly over the lifetime of the lamp because of the darkening of the lamp bulb, nevertheless even after some lOOOh of operation it is still well above the 25 % level, which means that the worsening of the LOD, in the case of shot-noise limitation, is still within a factor of two. 

If the flash lamp is pulsed very rapidly, the emergent beam appears at a rate governed by the lifetime of the inverted population. The resulting laser beam becomes almost continuous because the pulses follow each other so rapidly. However, such a solid-state laser should not be pulsed too rapidly because, if it is, the rod heats to an unacceptable extent, causing distortion and even fracture. Generally, solid-state lasers are not used in continuous mode because of this heating aspect. Liquid or gas lasers do not suffer from this problem. 

Once the fluorescence quantum yield has been determined, all that is required to calculate the fluorescence rate constant kf is the fluorescence lifetime rf. Direct measurement of this quantity, like the measurement of the fluorescence quantum yield, is difficult, in this case because of the short lifetime of the fluorescent state (shorter than the normal flash from a flash lamp ). There are, however, several methods which have been developed to determine fluorescence lifetimes and these will be the subject of this section. 

Mercury arcs are employed in these lamps because of the spectral distribution they emit, and because mercury vapor is relatively inert. It does not attack either the glass or the electrode materials (Kirk 1982). This contributes to the long lifetimes of mercury lamps. Low pressure mercury lamps are commonly called fluorescent lamps. High pressure mercury lamps are used in industrial environments and for street lighting and floodlighting. Other applications for mercury vapor lamps include motion-picture projection, photography, and heat therapy. 

A 13-W fluorescent light that fits in a standard screw-in socket provides the same light as the 60-W bulb it replaces. The expected lifetime of the fluorescent lamp is 10 000 h, whereas that of the incandescent bulb is 750 h. The fluorescent light is more expensive than the incandescent bulb but saves a great deal of electricity and money over its lifetime. 

The critical property of the Mercat process [3-8] that gives it its selectivity is that only Hg atoms in the vapor phase undergo reaction, because their absorption line is narrow and matched with the sharp emission line of the lamp. Mercury dissolved in the liquid phase has a broadened and shifted absorption band and Hg in solution has a short excited-state lifetime, so the liquid phase undergoes no significant reaction. In the vapor, Eqs. (2)—(5) produce the dehydro dimer, which condenses. 

While the first hollow cathode tubes were constructed in such a way that they could be repeatedly flushed with the purified noble gas, the inconvenience connected with such equipment led to the development of permanently sealed tubes. In order to insure a reasonable lifetime of such tubes, they have to be of a certain minimum volume. One of the reasons for the lifetime limits is leakage of air into the tube, but more important seems to be the loss of the filler gas which is slowly absorbed by the metal and the glass surface. Since the lamp operates by the sputtering off of the cathode lining, gradual loss of the latter leads to eventual deterioration of the lamp. Lamps for metals that sputter abundantly, like the alkali metals, or zinc and cadmium, have short lifetimes, mostly well below a hundred hours. 

Apart for the use of lasers, which may be advantageous in a suitably built apparatus, irradiation is usually done by means of a mercury arc. A worry when considering a photoreaction is the high price of the lamp and the (monetary and environmental) cost incurred for the electrical power required. As one can see from the literature, high-pressure mercury arcs are often satisfactorily general-purpose sources and many laboratories use 400-500 W lamps of this type. These are in fact somewhat expensive and with a limited lifetime (hundreds of hours). However, 125-150 W mercury arcs are often... 

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