Luminescence instrumentation lasers

The immense growth in the luminescence literature during the period between these two reviews had little to do with developments in fundamental theory. It was mainly due to the availability of new instrumentation, such as the photomultiplier (around 1950), the laser (around 1960), transistor and microcircuit electronics (around 1970), and ready access to laboratory computers (around 1975). All aspects of luminescence theory now being used to interpret luminescence measurements have been known since the early 1900 s and nearly all of the types of measurements now being made had been initiated with cruder techniques by 1930. We discuss here many of the latest techniques in luminescence analysis with selected highlights from the historical development of luminescence and a look at several recent developments in luminescence applications that appear likely to be important to future research. 

Most optical centers show luminescence decay times in the nanoseconds-milliseconds range. However, many other physical processes involved in optical spectroscopy are produced in the picoseconds-femtoseconds range, and mnch more complicated instrumentation becomes necessary. For instance, interband Inminescence in solids, which is of particular interest in semiconductors, can involve decay times in the range of picoseconds. Pulses generated from solid state lasers have already reached this femtosecond domain. 

The book starts with a short introduction to the fundamentals of optical spectroscopy, (Chapter 1) describing the basic standard equipment needed to measure optical spectra and the main optical magnitudes (the absorption coefficient, transmittance, reflectance, and luminescence efficiency) that can be measured with this equipment. The next two chapters (Chapters 2 and 3) are devoted to the main characteristics and the basic working principles of the general instrumentation used in optical spectroscopy. These include the light sources (lamp and lasers) used to excite the crystals, as well as the instrumentation used to detect and analyze the reflected, transmitted, scattered, or emitted light. 

In recent years luminescence nomenclature has become confusing within the literature and in practice. Luminescence involves both phosphorescence and fluorescence phenomena. While luminescence is the appropriate term when the specific photochemical mechanism is unknown, fluorescence is far more prevalent in practice. Moreover, the acronym LIE has historically inferred laser -induced fluorescence however, in recent years it has evolved to the more general term light -induced fluorescence due to the various light sources found within laboratory and real-time instruments. Within this chapter fluorescence and LIE are interchangeable terms. 

However, luminescence lifetime, which is a measure of the transition prob-abihty from the emitting level, may be effectively used. It is a characteristic and unique property and it is highly improbable that two different luminescence emissions will have exactly the same decay time. The best way to determine a combination of the spectral and temporal nature of the emission is by using laser-induced time-resolved spectra. The time-resolved technique requires relatively complex and expensive instrumentation, but its scientific... 

There has been a considerable decline in the number of papers which deal with the details of techniques of measurement of fluorescence decay. This is no doubt due to the fact that the alternative methods are now essentially well established. Nevertheless a microcomputerized ultrahigh speed transient digitizer and luminescence lifeline instrument has been described . A very useful multiplexed array fluorometer allows simultaneous fluorescence decay at different emission wavelength using single photon timing array detection . Data collection rates could approach that for a repetitive laser pulse system and the technique could be usefully applied to HPLC or microscopy. The power of this equipment has been exemplified by studies on aminotetraphenylporphyrins at emission wavelengths up to 680 nm. The use and performance of the delta function convolution method for the estimation of fluorescence decay parameters has been... 

Luminescent standards have been established for use in calibrating fluorescence spectrometers and have been suggested for Raman spectroscopy in the past (18). The standard is a luminescent material, usually a solid or liquid, that emits a broad reproducible luminescence spectrum when excited by a laser. Once the standard is calibrated for a particular laser wavelength, its emission spectrum is known, and it can provide the real standard output , d)i(AF) depicted in Figure 10.8. In practice, a spectrum of the standard is acquired with the same conditions as an unknown then the unknown spectrum is corrected for instrument response function using the known standard... 

Laser excitation for fluorescence detection has received much research interest, but as of yet there is no commercially available instrument. Fluorescence intensity increases with excitation intensity, and it is generally assumed that laser excitation would then offer improved limits of detection. However, as Yeung and Synovec have shown, various types of light scattering, luminescence from the flow cell walls, and emission from impurities in the solvent all increase with source intensity as well, yielding no net improvement in signal-to-noise ratio (53). Where laser excited fluorescence may prove useful is for the design of fluorescence detectors for microbore packed and open tubular LC columns, where the laser source can be focused to a small illuminated volume for on-column detection. 

The project Carl gave me was to build a sensitive instrument to search for luminescence from the permanganate ion, which had been the subject of a series of experimental single crystal absorption spectral studies and theoretical studies in the laboratory [6]. The spectrometer was built, but after repeated attempts using a range of crystals, excitation conditions and temperatures, no luminescence was detected. All subsequent efforts by others have confirmed this failure [7], under laser irradiation in iodide lattices some emission has been detected, but this is derived from the manganese ion MnO, 2 produced by a photoredox process [8]. This left me without many results to show for my year s work. I made some measurements on the intensely luminescent alkali metal platinocyanides but this did not lead to any new insights. 

In the case of OF luminescent sensors, both traditional optical systems and all solid-state systems have been used. Instruments from this second group operate using the same principle as the conventional system, except that LEDs and laser diodes form the main light sources and the signal processing units are fully electronic systems. 

The frequency-domain methods are very suitable for measuring the lifetimes of most luminescent lanthanide compounds. The reason is the range of rate constants of decay processes, which fits well in the frequency range of the conventional inexpensive lock-in amplifiers. Together with the easily modulated light-emitting diodes or laser diodes as the excitation sources, frequency-domain instruments for the lifetime measurements can be constructed in any workshop with rather minimal resources and expertise. 

Multiphoton or two-photon laser scanning microscopy is an alternative to confocal and time-resolved microscopy for bioimaging applications. The principle has been discussed in Lanthanides Luminescence Applications and concerns a two-photon excitation from the simultaneous absorption of two photons in a single quantized event. A bioprobe that normally absorbs ultraviolet light (Xex = 350 nm) can also be excited by two photons of NIR light, at 700 nm (the wavelength is twice that required for one-photon excitation). These two photons must interact simultaneously, which means in a very small lapse time. The instrumentation requires pulse lasers to provide sufficient power, as the photon density must... 

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