Luminescence instrumentation spectra

When a luminescence spectrum is obtained on an instrument such as that used to produce the spectra in Figure 7.23, it will depend on the characteristics of the emission monochromator and the detector. The transmission of the monochromator and the quantum efficiency of the detector are both wavelength dependent and these would yield only an instrumental spectrum. Correction is made by reference to some absolute spectra. Comparison of the absolute and instrumental spectra then yields the correction function which is stored in a computer memory and can be used to multiply automatically new instrumental spectra to obtain the corrected spectra. The calibration must of course be repeated if the monochromator or the detector is changed. 

Some luminescence instruments allow simultane-ously scanning both the excitation and the emission wavelengths with a sntall w avelength difference between them. The spectrum that results is known as a synchronous spectrum. A luminescence signal Is ob-... 

It is worth noting some historical aspects in relation to the instrumentation for observing phosphorescence. Harvey describes in his book that pinhole and the prism setup from Newton were used by Zanotti (1748) and Dessaignes (1811) to study inorganic phosphors, and by Priestley (1767) for the observation of electroluminescence [3], None of them were capable of obtaining a spectrum utilizing Newton s apparatus that is, improved instrumentation was required for further spectroscopic developments. Of practical use for the observation of luminescence were the spectroscopes from Willaston (1802) and Frauenhofer (1814) [13]. 

Fig. 22. 10 K absorption spectrum of 5% Mo + Cs2NaYbCl6. The absorption features are labeled according to the chromophore, Yb + or Mo +. 10 K luminescence of this sample with 10,826 cm Yb + excitation. This spectrum has been corrected for the instrument response. Note that the Yb + luminescence is magnified hy a factor of 1000, showing nearly quantitative Yb3+ Mo + ET...

Luminescence decay curves may be observed by displaying the output of the photomultiplier on an oscilloscope. Precautions must be taken to correct for instrumental distortion of fast decay curves (D13). In multicomponent systems with differing decay times, electronic gating may be used to isolate the signal due to one component (time resolved phosphorimetry) (SI). A complete emission spectrum can be observed using a spectrograph with a photographic plate or television camera tube, but these systems are as yet only of specialist interest. 

Energies in the infrared spectrum are conventionally expressed in wave numbers, which are defined as the number of waves per centimeter, i.e., the reciprocal of the wavelength measured in centimeters. The infrared spectrum extends from 12,500 to 50 cm (i.e., a wavelength of 0.8-200 fjLia.) and the far infrared from 40-10 cm (260 p.m-1 mm), but the upper limit of most commercial instruments is about 200 cm (50 ixm). Spectra are most frequently obtained by absorption and reflection techniques, but polarization, emission, and luminescence are also used (C26). Similar components are used in all types of instrument. Reflection measurements of samples with low transmission are made in the near infrared with a conventional spectrophotometer fitted with a reflec-... 

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... 

Become familiar with the operation of the fluorescence spectrophotometer in your laboratory. In particular, you should understand how the following instrumental parameters affect the intensity and signal-to-noise ratio (S/N) of a luminescence spectrum ... 

Another type of luminescence spectrum is shown in Figure I5 h. The total luminescence spectrum is cither a three-dimensional representation or a contour plot. Both simultaneously show the luminescence signal as a function of excitation and emission wavelengths. Such data aie often called an excitation-emission matrix. Although the total luniinescence spectrum can be obtained on u normal coni])U(cri/ed instrument, it can be acquired more rapidly with array-detector-based systems (see next seel ion). 

The excitation wavelength selector can be either a filter or a monochromator. Filters offer better detection Hmits, but do not provide spectral scanning capabilities. Often, a filter is used in the excitation beam along with a monochromator in the emission beam to allow emission spectra to be acquired. FuU emission and excitation spectral information can be acquired only if monochromators are used in both the excitation and emission beams. In modern instruments with array detectors, a polychromator is used in the emission beam instead of a monochromator. Recent research instraments are able to scan both wavelengths automatically and combine all data into a 2D excitation—emission spectrum. In lifetime spectrometers, a pulsed light source and a gated detector are synchronized in order to measure the time dependence of the luminescence emission. 

However, luminescence lifetime, which is a measure of the transition probability from the emitting level, may be effectively used. It is a characteristic and an unique property and it is highly improbable that two different luminescence emissions will have exactly the same decay time. The best way for a combination of the spectral and temporal nature of the emission can be determined by laser-induced time-resolved spectra. Time-resolved technique requires relatively complex and expensive instrumentation, but its scientific value overweights such deficiencies. It is important to note that there is simple relationship between steady-state and time-resolved measurements. The steady-state spectrum is an integral of the time-resolved phenomena over intensity decay of the sample, namely ... 

---