Pulse light emission

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

The levitated laser dye droplet was optically pumped by a pulsed (pulse length 5 ns, repetition rate 10 Hz), frequency-doubled Nd YAG laser (2 = 532 nm) in free-space optical setup. Droplet light emission was collected by a multimode optical fiber placed at an angle of approximately 50° relative to pump laser beam. Collected light was analyzed in a fixed-grating spectrometer with a resolution of FWHM 0.15 nm. 

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... 

In general, as the variation of the temporal profile of the non-stationary light beam due to the SS effect or the second-order GVD effect is continuous, emission of radiation field from the guiding region is also continuous upon propagation of the pulse. This emission prevents formation of a spatiotemporal soliton in the step-index guiding structures. 

Light emission was recorded with an IP 21 R.C.A. photomultiplier tube in the range 3000-7000 A. Interference filters (spectral bandwidth A A = 100-140 A.) showed that the light emission of the pic darret arises from the same emitter as that from the slow reaction—probably excited formaldehyde (16). The pic darret appears as a clear pulse on the record of light intensity vs. time (Figure 1). 

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). 

Since the energies of ionizing radiations greatly exceed the ionization potential of atoms and molecules, ions and electrons are produced. They have been observed in some cases at low temperature and are usually very short-lived at room temperature. They have been studied mainly by electron spin resonance, absorption spectroscopy and pulse radiolysis. Recombination of opposite charges on warming usually produces light emission known as thermoluminescence. 

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