Pulse typical emissions from

Figure 14.13 Typical emissions from pulse combustor. (Courtesy of Novadyne Ltd., Hastings, Ontario.)...

The smallest spatial scale at which outdoor air pollution is of concern corresponds to the air volume affected by pollutant chemical emissions from a single point source, such as a smokestack (Fig. 4-24). Chemicals are carried downwind by advection, while turbulent transport (typically modeled as Fick-ian transport) causes the chemical concentrations to become more diluted. Typically, smokestacks produce continuous pollutant emissions, instead of single pulses of pollutants thus, steady-state analysis is often appropriate. At some distance downwind, the plume of chemical pollutants disperses sufficiently to reach the ground the point at which this occurs, and the concentrations of the chemicals at this point and elsewhere, can be estimated from solutions to the advection-dispersion-reaction equation (Section 1.5), given a knowledge of the air (wind) velocity and the magnitude of Fickian transport. 

Comparing thin film to solution state, we find that there is very little change to the absorption spectrum apart from a small red shift of the whole absorption band, although in solution the band position can be both solvent and time dependent due to conformational changes of the PF backbone in solution [6]. Typical emission spectra of PF2/6 100 nm film at both 295 K and 20 K, excitation at 3.5 eV (10 uj, 200 ps pulse excitation) are shown in Fig. 2. At low... 

There are several significant advantages to the laser source over the more conventional flash lamp. The laser source is more intense. Its pulse width is near 5 psec, as compared for 2000 psec for a flash lamp. And finally, the repetition rate can be much higher, typically 800 kHz to 4 MHz. Because of all these factors it is possible to rapidly acquire data to a much higher level of statistical accuracy than with a flash lamp. For example, a recent paper by Small and co-workers describes a multi-component resolution of a histone, which contains a single tyrosine residue [31]. Because of the substantial increases in resolution, the laser sources are becoming more widely used in the biochemical applications of fluorescence, as illustrated by recent studies of the tryptophan emission from phospholipase Aj [44] and hemoglobin [45]. 

The LIBS technique is essentially aimed at the elemental analysis of solid and liquid materials. A pulsed laser beam is focused onto the surface of the material to be analysed and the emission from the resulting micro-plasma is collected and focused on to the slit of a monochromator equipped with an array detector capable of recording the entire spectrum from a single laser shot. The emission is initially dominated by Bremsstrahlung (white light), but this is short lived and essentially follows the laser intensity profile. This is followed by emission from atomic ions (typically 1 ps in duration) and finally by emission from neutral atomic species (very weak emission from metastable species, which decay more slowly, may... 

In a typical pump-probe experiment, a sample is excited with a pulse with frequency < i and wavevector ki, and is probed by a second pulse with frequency C02 and wavevector 2- The optical path of (Mie of the pulses is varied to change the delay between the two pulses. The measured signal is the difference between the intensities of the transmitted probe pulses in the presence and absence of the excitation pulses, and usually is averaged over many pulses (Fig. 1.9). In a system with only two electronic states, the difference can reflect either stimulated emission from the excited state or bleaching of the absorption band of the ground state. The probe frequency often is selected by dispersing a spectrally broad probe beam after... 

Pump-probe techniques using picosecond and sub-picosecond laser pulses have made it possible to probe chemical processes on the time scale of nuclear motions [22]. Figure 11.7A shows a typical measurement of the early time course of stimulated emission from a dye molecule (1R132) in solution [23]. The dye was excited on the blue side of its absorption band (830 nm) and stimulated emission was measured at 900 nm. The signal includes a slow rise component with a time constant of several hundred femtoseconds that represents part of the Stokes shift of... 

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