Pulsed light sources

Chan HH, Kono T (2004) The use of lasers and intense pulsed light sources for the treatment of pigmentary lesions. Skin Therapy Lett 9(8) 5-7... 

In phase-modulation fluorometry, the pulsed light source typical of time-domain measurements is replaced with an intensity-modulated source (Figure 10.5). Because of the time lag between absorption and emission, the emission is delayed in time relative to the modulated excitation. At each modulation frequency (to = 2nf) this delay is described as the phase shift (0, ), which increases from 0 to 90° with increasing modulation frequency. The finite time response of the sample also results in demodulation to the emission by a factor m which decreases from 1.0 to 0.0 with increasing modulation frequency. The phase angle (Ow) and the modulation (m, ) are separate... 

A Gd + doped crystal is illuminated with a pulsed light source, so that the l7/2 excited state of this ion is populated by absorbing 1 mJ of energy per incident pulse. Determine the heat delivered to the crystal per excitation pulse if the nonradiative rate from this state is 10 s The fluorescence lifetime of the l7/2 state is 30 /xs. 

Figure 23. Outline of a new kind of pulsed light source with the optical ring condenser.

Rather than using short-duration pulsed-light sources, which usually yield information concerning nonequilibrium->equilibrium relaxations, it is often of great interest to study equilibrium- equilibrium relaxations and excitations. This requires the use of step-function light sources. The temporal resolution in this case is determined by the rise time of the step. There are, of course, two types of steps that one need consider (a) a source that is initially off and then is rapidly turned on and maintained on for some time, and (b) a source that is initially on and then is rapidly turned off and then maintained off for some time. 

Figure 7.34 Principle of kinetic single-photon counting. L, pulsed light source S, sample P, photodiode F, interference filter or monochromator D, photomultiplier R, voltage ramp generator (1 to start the ramp, 0 to stop it). The voltage V is fed into a multichannel analyser M. Inset voltage ramp V(t)...

Finally, the use of CCD cameras as detectors in combination with short-pulsed light sources enables not only spatially resolved imaging, but also a time-resolved fluorescence detection. Time resolution offers two different new alternatives ... 

The apparatus used for the experiments is shown in Figure 3. The pulsed light source was a 80 MHz mode-locked Ti sapphire laser (Spectra-Physics Tsunami) (1) pumped by a continuous wave (cw) diode laser (Spectra-Physics Millennia) (2), that produced 90 fs pulses at a centre photon energy of o>=l.575 0.005 eV (787 nm). The apertures (3) were used to reset the beam direction following any drift in the laser alignment. 

In the time-correlated single-photon counting (TCSPC) technique, the sample is excited with a pulsed light source. The light source, optics, and detector are adjusted so that, for a given sample, no more than one photon is detected. When the source is pulsed, a timer is started. When a photon reaches the detector, the time is measured. Over the course of the... 

In the strobe or pulse sampling technique, the sample is excited with a pulsed light source. The intensity of the fluorescence emission is measured in a very narrow time window on each pulse and saved on the computer. The time window is moved after each pulse. When data have been sampled over the appropriate range of time, a decay curve of emission intensity vs. time can be constructed. 

The name strobe technique comes about because the photomultiplier PMT is gated -or strobed - by a voltage pulse that is synchronized with the pulsed light source. The strobe has the effect of turning on the PMT and measuring the emission intensity over a very short time window (Figure 7.6) (Bennett 1960 James et al. 1992). 

Fig. 14 Principle ofTRF. A pulsed light source is used for excitation of a label with a long excited-state lifetime. The detection is switched on after a time delay, which allows for suppression of the short-lifetime fluorescence background caused by reagents and the microtiter plate.

Recent developments of pulsed light sources, optical components, fast and sensitive detectors and electronic equipment for data collection and analysis have permitted the construction of numerous instruments, often commercially available, for the collection of luminescence data with excellent resolution in time, spectral distribution and space. The sensitivity has reached the ultimate level that allows the characterization of such properties for single molecules (see Section 3.13). Only an overview of some of these techniques is given here. 

The sharply increased acidity of phenols in the excited state can be used to lower the pH of aqueous solutions by a pulsed light source within nanoseconds (photoacid). However, the equilibrium is rapidly re-established in the ground state by diffusion-controlled recombination of the released protons with the basic phenolates. 

Fluorescence can be resolved overtime. The use of very short pulsed light sources (picosecond lasers and laser diodes) has rendered accessible graphs of fluorescence decay as a function of the time. New applications based upon a greater knowledge of the lifetime are under development, although they are still not used much in chemical analysis. 

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