Very short pulse techniques

While for some purposes it may be necessary to have accurate frequency definition, for others good time discrimination is useful. These are opposite requirements. Because of the Fourier relationship between frequency and time, the more precisely the time of a signal is known, the greater bandwidth of frequencies is necessary (there is a close analogy here with Heisenberg s uncertainty principle). Approximately, the time resolution t is the reciprocal of the bandwidth Bw, so that their product Bwr 1. 

The design of a system for working with short pulses follows the same principles as other pulsed ultrasonic systems such as ultrasonic flaw detectors, but in this case it is necessary to achieve very much greater stability and higher bandwidth. A schematic circuit is shown in Fig. 5.4 A very short impulse is generated by a step recovery diode. The pulse has a width of half the period of the centre frequency of the lens if it is shorter than that the energy in the pulse is reduced without any improvement in the signal bandwidth. Thus the lens acts as a sort of matched filter with poor time resolution but optimal 

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The first experimental measurements of the time dependence of the hydrated electron yield were due to Wolff et al. (1973) and Hunt et al. (1973). They used the stroboscopic pulse radiolysis (SPR) technique, which allowed them to interpret the yield during the interval (30-350 ps) between fine structures of the microwave pulse envelope (1-10 ns). These observations were quickly supported by the work of Jonah et al. (1973), who used the subharmonic pre-buncher technique to generate very short pulses of 50-ps duration. Allowing... 

That the hydrated electron is a separate chemical entity has been demonstrated by the technique of pulse radi l sis This consists of subjecting a sample of pure water to a very short pulse of accelerated electrons. The energetic electrons have the same effect upon water as a beam of y-ray photons. Shortly after the pulse of electrons has interacted with the water, a short flash of radiation (ultraviolet and visible radiation from a discharge tube) is passed through the irradiated water sample at an angle of 90° to the direction of the pulse to detect the absorption spectra... 

Time Scales. The time scales measurable by the two techniques are illustrated in Figure 3. The time resolution of pulsed lasers is far superior, reaching to as short as 20 fs, with 200-fs measurements becoming routine in many laboratories. Pulse radiolysis measurements achieve rise times of 20-30 ps in only a few laboratories, while 1-10 ns is more common. Pulse radiolysis is slower because the accelerated particles, usually electrons, repel each other, making it difficult to bunch many of them into a very short pulse. Pulses as short as 5 ps have been reported and new accelerators may achieve 1 ps, but it is likely that pulsed lasers will remain the leader in very high time resolution. 

The broad gain medium of Ti S Au — 10 Hz) allows the build-up of very short pulses with a Fourier limitation of roughly 1 / Ai/ = 10 fe. Recently, pulse lengths of about 5.5 fs have been attained directly from a Ti S laser, cor-respondmg to about two optical cycles [9.152]. Using subsequent compression of pulses which have first been spectrally broadened by self-pheise modulation as already discussed in comiection with Fig. 8.50, a pulse length of 4.6 s has been attained [8.153]. Techniques for ultra-short laser-pulse generation are... 

Since 1875, thanks to Kerr s discovery [ 1], it is known that a static electric field can induce a modification of the optical properties of a liquid. Many years later researchers found out that also an optical electromagnetic field was capable of producing a measurable modification of the dielectric properties, inducing a hirefiringence effect the first experimental observation of the optical Ken-effect (OKE) was reported in 1963 [2]. After few years, with the introduction of the first pulsed lasers, spectroscopists discovered the chance to induce in a material a transient birefringence and to measure its relaxation toward the equiUhrium [3]. They also realized that this could be a relevant new spectroscopic tool able to collect new information on the dynamical processes present in the material. The spectroscopic research, worked out in the following years, confirmed this forecast beyond the expectations. Two important experimental improvements of this spectroscopic technique have been made. On one hand, the pulsed laser sources have become able to produce very short pulses of high... 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a... 

The potentiostatic technique has a number of variations and the potential may be increased or decreased incrementally, changed continuously at a predetermined rate (potential sweep) or applied as pulses of very short duration. The applications of the potentiostatic technique are considered in detail in Sections 1.4, 1.5 and 19.2, and will not be considered here. 

More common in the liquid phase is pulse radiolysis6. In this technique, electron accelerators which can deliver intense pulses of electrons lasting a very short time (ns up to /is) are used. Each single pulse can produce concentrations of intermediates which are high enough to be studied by methods such as light absorption spectroscopy or electrical conductivity. 

McLeman<7, ) used a method which was really a pre-cursor of the cross-correlation technique. He injected a pulse of air into the conveying line as a result of which there was a very short period during which the walls at any particular point were not subject to... 

In the 1960s, Oppenheim et al. [10,19,20] succeeded in obtaining photographs with better resolution by means of schlieren technique with microsecond flash and then with the very short (less than 10 s) laser light pulses. This facilitated the attainment of a stroboscopic set of essentially still photographs that revealed many details of DDT. At the same time, Soloukhin [21] published a series of streak photographs taken with schlieren system and Denisov and Troshin [22] discovered that detonation leaves a record of its passage in the form of imprint on a wall coated with the thin layer of soot. 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

A very interesting technique for radical generation is flash photolysis, which employs a very intense pulse of radiation (visible or u.v.) of very short duration. This produces a very high immediate concentration of radicals, which may be detected—and whose fate may be followed—by spectroscopy through one or more subsequent pulses of lower intensity radiation of suitable wavelength. This is, of course, primarily a technique for the study of radicals rather than for their use in preparative procedures. Radicals may also be generated, in suitable cases, by irradiation of neutral molecules with X-rays or with y-rays radiolysis. 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

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