Laser power, fluorescence signal

At low laser powers, the fluorescence signal is Imearly proportional to the power. Flowever, the power available from most tunable laser systems is suflFicient to cause partial saturation of the transition, with the result that the fluorescence intensity is no longer linearly proportional to the probe laser power. While more... 

The fluorescence signal is linearly proportional to the fraction/of molecules excited. The absorption rate and the stimulated emission rate 1 2 are proportional to the laser power. In the limit of low laser power,/is proportional to the laser power, while this is no longer true at high powers 1 2 <42 j). Care must thus be taken in a laser fluorescence experiment to be sure that one is operating in the linear regime, or that proper account of saturation effects is taken, since transitions with different strengdis reach saturation at different laser powers. 

Fluorescence from pharmaceutical capsule shells and tablet coatings has hindered measurement of their composition by Raman spectroscopy. By switching from the conventional backscattering mode to a transmission mode, Matousek et al. demonstrated that fluorescence could be eliminated in many instances [8]. Backscattering- and transmission-mode Raman spectra of several samples are shown in Figure 7.5. Each spectrum was acquired in 10s with 80mW 830-mn laser power. Matousek et al. also speculate that signal acquisition times could be relatively easily shortened to well below 0.1 s when the transmission mode is combined with optimized optics [8]. 

The FRAP apparatus can also be used in a semi-quantitative manner to measure the surface concentration and subsequent competitive displacement of adsorbed labelled species, such as the fluorescent-labelled protein in the adsorbed layer of a/w or o/w thin films [10]. This can be achieved by focusing the low power 488 nm beam on the film and detection of the emitted fluorescence using the FRAP photon counting photomultiplier. The detected fluorescence signal is proportional to the amount of adsorbed protein at the interfaces of the thin film provided that the incident laser intensity is kept constant. Calculations have proved that the contributions from non-adsorbed protein molecules in the interlamellar region of the film are negligible [12],... 

Raman spectroscopy has been widely used to study the composition and molecular structure of polymers [100, 101, 102, 103, 104]. Assessment of conformation, tacticity, orientation, chain bonds and crystallinity bands are quite well established. However, some difficulties have been found when analysing Raman data since the band intensities depend upon several factors, such as laser power and sample and instrument alignment, which are not dependent on the sample chemical properties. Raman spectra may show a non-linear base line to fluorescence (or incandescence in near infrared excited Raman spectra). Fluorescence is a strong light emission, which interferes with or totally swaps the weak Raman signal. It is therefore necessary to remove the effects of these variables. Several methods and mathematical artefacts have been used in order to remove the effects of fluorescence on the spectra [105, 106, 107]. 

Under these conditions of complete saturation the fluorescence signal becomes independent of laser power and the species number density N-T can be theoretically evaluated with only the knowledge1of the spectroscopic and instrumental constants. 

Figure 24 shows a plot of fluorescence intensity vs. laser power for the Swan Band system of C which we showed in Figure 19. It is apparent that fluorescence response becomes nonlinear at laser powers on the order of 1 joule/itr. However, it is equally apparent that the signal never reaches completg saturation (independent of laser power) even at 15 joules /m. ...

Figure 3. Fluorescence signal vs. laser power. Data was obtained jrom the curves in Figure 2 points were taken near the peaks and dips of the pulse waveforms (-------------------), a curve fit through the data using Equation 8.

The dye laser irradiates an external cell which contains 40 Pa (0.3 torr) Io vapor. A 1P28 photomultiplier whose face is covered by a 610 nm long pass filter measures the 12 fluorescence, while a photodiode monitors a reflected spot of the dye laser. The ratio of fluorescence to dye laser power is displayed on a strip chart recorder. The si dearm temperature is gradually (1-2 hours) raised from about 210 K or 220 K, where fluorescence in the external cell is strong, to whatever temperature is required to reduce the signal by about 60%. 

Fig. 2. Set-up of the ILP laser system. Intracavity frequency-doubling is realized with a KTP crystal which, together with a Brewster plate, serves as a Lyot filter. This allows to frequency time the laser by more than 500 GHz by changing the temperature of the KTP crystal. The 532 nm laser radiation, after passing an acousto-optical modulator (AOM), is directed into an external I2 fluorescence cell. A photomultiplier (PM) detects the fluorescence signal over a solid angle of almost 0.2 n. The photodiode D is used to detect a fraction of the 532 nm laser beam to power stabilize the 532 nm light via the AOM...

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