Train of pulses

Fig. 4. Temporal pulse characteristics of lasers (a) millisecond laser pulse (b) relaxation oscillations (c) Q-switched pulse (d) mode-locked train of pulses, where Fis the distance between mirrors and i is the velocity of light for L = 37.5 cm, 2L j c = 2.5 ns (e) ultrafast (femtosecond or picosecond) pulse.

There are many other advantages with pulsed FT systems in that we can create trains of pulses to make the nuclei perform dances which allow them to reveal more information about their environment. Ray Freeman coined the rather nice term spin choreography to describe the design of pulse sequences. If you are interested in this area, you could do much worse than listen to Ray explain some of these concepts or read his book Spin Choreography Basic Steps in High Resolution NMR (Oxford University Press, ISBN 0-19-850481-0) ... 

In 2006 Wimperis et al. proposed a method called satellite transitions acquired in real time by MAS (STARTMAS) [142, 202], which allows for the real-time acquisition of high-resolution NMR spectra of spin-3/2 nuclei under MAS. This method combines a train of pulses, similar to CPMG [109, 110], with sample rotation at the magic angle to refocus the quadrupolar broadening in a series of echoes, while allowing the isotropic quadrupolar shift and chemical shift to evolve. 

Figure 9.9. Sinusoidal excitation can be regarded as a succession of sufficiently narrow rectangular pulses in lime Then, at any particular time r, the fluorescence response to the train of pulses sinusoidally modulated is given by the superposition of all single-pulse responses initiated at times l < t.

The phase delay A(nwE) and modulation ratio M(na>E) information of the high-frequency signals is transferred to low-frequency signals by amplitude modulation (cross-correlation) of r.(t) and Iff) with a periodic train of pulses C(f) given by Ref. 29. 

When the light to be detected consists of a train of pulses, very high peak currents are generated inside the photomultiplier tube and a different arrangement is required (see Figure 3.19). In this case some capacitors, C, are included between the last... 

The autput of a mode-locked ruby laser 729) producing a train of pulses of 5 psec duration with a maximum peak power of 5 GW was focused into a cell pressurized with the sample gas. Pulse-energy conversion efficiencies into the Raman lines of up to 70 % have been obtained. The induced rotational lines are broadened this could be due to a strong optical Stark effect 730)... 

To use the two-photon fluorescence (TPF), i. e., the fluorescence excited by two-photon absorption, for the measurement of the halfwidth of ultrashort pulses from mode-locked lasers, the laser beam containing the train of pulses is split into two beams by a beam splitter (Fig. 12) which are then redirected by two mirrors Mi and M2 so that they are collinear and completely overlap in the dye cell DC which is situated in the center between the two mirrors. In the selection of the dye, it need only be remembered that it should, as far as possible, be free from absorption at the laser wavelength, have a strong absorption band at half the wavelength, and have the highest possible fluorescence quantum yield 97. Thus a... 

SM-FRET experiments are typically performed by using a dual-channel detection scheme. More specifically, one photo-excites the donor with CW radiation or a train of pulses, while simultaneously detecting the fluorescence photons from the donor and acceptor in a selective manner. The fraction of photons detected in the acceptor channel, over a given time averaging window of length Tw, provides a direct measure of the time-averaged FRET efficiency, which we will denote by E(Tw)- One may then define a time-averaged and TV-dependent donor-acceptor distance, which will be denoted by R)tw, such that... 

An SPT experiment requires identification of at least one 29Si satellite in the H NMR spectrum non-selective experiments (INEPT, DEPT) and JCP require for optimum performance an estimate of the coupling constant, and the correct estimate of the number of coupled protons improves the performance of INEPT and DEPT experiments. While an SPT experiment achieves enhancement through one selective irradiation, the other methods use series of pulses. The group of experiments that are referred to here as non-selective use a series of properly timed pulses, and ICP, which is also non-selective, differs from them in that it uses a train of pulses known as a spin-lock. 

The Doppler-free excitation can best be understood in the frequency domain where the train of pulses appears as a comb of modes, all equally spaced in frequency and covering a spectral region which is roughly equal to the inverse of the pulse duration of a single pulse in the train. For a typical dye laser this might be 500 GHz. If one of the modes is in two-photon resonance with the sample... 

The output is thus a continuous train of pulses separated by the dye cavity round-trip time. Even though the pump laser pulses may be of the order of 200 ps long, dye laser pulses of less than 10 ps can be achieved by synchronous pumping. This is because the dye molecules have extremely large stimulated emission cross-sections and the dye laser pulse passing through the dye stream immediately de-excites the dye molecule, in a few picoseconds, by stimulated emission [13]. 

We have introduced a novel method to enhance the uptake of macromolecules via stimulating endocytic-like processes by exposing cells to a train of pulsed low electric field, LEF (14-16). The enhanced uptake is attributed both to the direct stimulation of different endocytic pathways as well as to the indirect effect mediated through the increase of the adsorption of the macromolecules onto the exposed cells. This method has found an application in the treatment of different metastatic tumor models (17-20). 

The application of a typical train of pulses, when using stainless steel electrodes, resulted in small polarization of the electrodes and the appearance of a residual low DC component (<2 V). To avoid polarization, it is recommended to switch electrode polarity after each treatment and to clean and polish electrodes after each experiment. When employing stainless steel electrodes one observes, in some culture media, the formation of precipitates. It is, therefore, recommended to employ platinum electrodes, which do not lead to precipitation. 

Figure 2 A schematic of the digital sampling process (a) A signal, x is multiplied by a train of pulses, p producing the signal Xj,, (b) The analytical signal, x, (c) The carrier signal, p, (d) The resultant sampled signal is a train of pulses with amplitudes limited by x,...

FAM-II pulses were initially shown to enhance the 3QC <- SQC conversion process in a spin-5/2 system.Later they were also used for excitation purposes by transferring 3QC <->5QC coherences in SQM AS experiments. Figure 12c shows a schematic of a FAM-II sequence, which consists of a train of pulses of progressively decreasing duration with alternating phases of 0° and 180° separated by an interpulse delay. The interpulse delay is normally kept to the minimum value required for stabilization of the phase jumps between the pulses, say 0.2-0.4 p,s. Such a FAM-II sequence is denoted as F (r), where n again corresponds to the number of pulses (it can be odd or even in contrast to the F (t) case) and F is a vector containing the durations of the pulses and interpulse delays. 

While vesicle deformation in AC fields concerns stationary shapes, DC pulses induce short-lived shape deformations. In different studies, the pulse duration has been typically varied from several microseconds to milliseconds, while studies on cells have investigated a much wider range of pulse durations-from tens of nanoseconds to milliseconds and even seconds [80], as discussed in other chapters of this book. Various pulse profiles, unipolar or bipolar, as well as trains of pulses have been also employed (e.g., [81, 82]). Because the application of both AC flelds and DC pulses creates a transmembrane potential, vesicle deformations of similar nature are to be expected in both cases. However, the working fleld strength for DC pulses is usually higher by several orders of magnitude. Thus, the degree of deformation can be different. 

Order selective excitation has been achieved by the use of phase-cycled sequences of Rf pulses19-24 and composite pulse trains.25 Warren and Pines19-24 established that it was possible to construct TV/c-quantum selective excitation sequences (k = 0, +1, +2,...) by systematically repeating an appropriate train of pulses with a phase increment 2n/N between repetitions (Fig. 5). 

---