Pulse shape radiation

For studies in molecular physics, several characteristics of ultrafast laser pulses are of crucial importance. A fundamental consequence of the short duration of femtosecond laser pulses is that they are not truly monochromatic. This is usually considered one of the defining characteristics of laser radiation, but it is only true for laser radiation with pulse durations of a nanosecond (0.000 000 001s, or a million femtoseconds) or longer. Because the duration of a femtosecond pulse is so precisely known, the time-energy uncertainty principle of quantum mechanics imposes an inherent imprecision in its frequency, or colour. Femtosecond pulses must also be coherent, that is the peaks of the waves at different frequencies must come into periodic alignment to construct the overall pulse shape and intensity. The result is that femtosecond laser pulses are built from a range of frequencies the shorter the pulse, the greater the number of frequencies that it supports, and vice versa. 

We do not consider here the dependence of the group velocity on the beam divergence and the related spatiotemporal effeets in the nonlinear medium leading to additional changes in the pulse shape. In the region of the core, these effects are small, and the radiation field power for the levels of the input pulse power considered here is low. 

The pulse produced at the output of a radiation detector has to be modified or shaped for better performance of the counting system. There are three reasons that necessitate pulse shaping ... 

To set up the lifetime experiment, fluorescence excitation spectra were recorded using pulses of 90 ns duration corresponding to a spectral bandwidth of 15 MHz at a repetition rate of 1 MHz. The bandwidth was limited by the pulses rise and fall times, the pulse shapes and the frequency jitter of the laser. Fig. 8(a) shows a typical spectrum with four individual molecular resonances A, B, C and D. On average, the lines are about 25 MHz wide as compared to a homogeneous linewidth of about 8 MHz measured by earlier experiments using cw radiation and lower excitation energies. The best fit of the absorption profile of molecule C was obtained using a Lorentzian profile with a FWHM of 27 MHz. 

MHz frequency as shown. A proton spectrum occurs over a chemical shift range of 10 ppm, which corresponds to 2.5 kHz at 500 MHz. As seen in Figure 3.22, all of the protons in the sample would see 98%-100% of the power of the 500 MHz radiation delivered and all would be excited simultaneously. A pulse programmer is used to control the timing and shape of the RF pulses used to excite the sample. Square wave pulses are commonly used, but multipulse experiments and 2D NMR experiments with other pulse shapes are performed. There are hundreds of pulse sequences and 2D experiments that have been developed, with curious names like attached proton test (APT), DEPT, INEPT, INADEQUATE, COSY, and many more, some of which will be discussed later in the chapter. Each pulse sequence provides specific and unique NMR responses that enable the analyst to sort out the NMR spectrum and deduce the chemical structure of a molecule. 

An important general point to emerge from these experiments is that tailored excitation-pulse shapes can significantly alter the mass spectrum produced by laser radiation. This approach could, therefore, provide a method for the multidimensional analysis of complex molecules, as varying ion distributions, each with different information content, can be obtained as a function of pulse shape. The use of tailored femtosecond laser pulses may, therefore, open new avenues for mass spectromettic analysis of large and biologically relevant molecules. 

PULSE SHAPE DISCRIMINATION (PSD) Electronic methods for separating pulses of differing shape, thus enabling pulses from one type of radiation to be separated from those of another. For example in neutron detectors, neutrons may be separated from gammas by PSD. Same as pulse shape analysis, PSA. 

Interference of ionizing radiation with infrared detectors is a potential source of problems. However, use can he made of the large difference in the charge deposit rate and therefore the rise time of the pulse caused hy an ionizing particle as compared to the response to an infrared source. Using electronic pulse shape discrimination techniques it will he possible to limit the disturbance caused hy particle hits to a small fraction of the total observing time. 

The time-scales of some of the various changes that occur between absorption of a pulse of radiation in (for example) an aqueous solution and the stage when uniform reaction conditions have been attained are as follows. Ionisation or excitation of solvent molecules by the fast dry irradiation electrons takes 10 to 10 s. The time required for these initiating electrons to be thermalised (i.e., brought to thermal equilibrium with the solvent) depends on the structure and shape of the solvent molecules for quasi-spherical molecules such as C(CH3)4, it is (at room temperature) a few picoseconds or less, but for atomic molecules with no vibrational degrees of freedom (Ar, Kr, Xe) it is longer than a nanosecond. In water, electrons are completely solvated within 0.3 ps at this time the species e. ... 

In Chapter 3 we showed that on a uniform fiber, pulse spreading is proportional to distance z along the fiber. Aceordingly, the only influenee slowly varying nonuniformities can have on this result is to modify the coefficient multiplying z, in addition to changing the pulse shape and redueing pulse power by radiation. The latter is discussed in Section 5-13. However, as we show below, these effects tend to be small when the fiber variations are of small amplitude. 

The Champ-Sons model is a most effieient tool allowing quantitative predictions of the field radiated by arbitrary transducers and possibly complex interfaces. It allows one to easily define the complete set of transducer characteristics (shape of the piezoelectric element, planar or focused lens, contact or immersion, single or multi-element), the excitation pulse (possibly an experimentally measured signal), to define the characteristics of the testing configuration (geometry of the piece, transducer position relatively to the piece, characteristics of both the coupling medium and the piece), and finally to define the calculation to run (field-points position, acoustical quantity considered). 

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