Pulse train

Figure Bl.14.6. J -maps of a sandstone reservoir eore whieh was soaked in brine, (a), (b) and (e), (d) represent two different positions in the eore. For J -eontrast a saturation pulse train was applied before a standard spin-eeho imaging pulse sequenee. A full -relaxation reeovery eiirve for eaeh voxel was obtained by inerementing the delay between pulse train and imaging sequenee. M - ((a) and (e)) and r -maps ((b) and (d)) were ealeulated from stretehed exponentials whieh are fitted to the magnetization reeovery eurves. The maps show the layered stnieture of the sample. Presumably -relaxation varies spatially due to inliomogeneous size distribution as well as surfaee relaxivity of the pores. (From [21].)...

Optical detectors can routinely measure only intensities (proportional to the square of the electric field), whether of optical pulses, CW beams or quasi-CW beams the latter signifying conditions where the pulse train has an interval between pulses which is much shorter than the response time of the detector. It is clear that experiments must be designed in such a way that pump-induced changes in the sample cause changes in the intensify of the probe pulse or beam. It may happen, for example, that the absorjDtion coefficient of the sample is affected by the pump pulse. In other words, due to the pump pulse the transparency of the sample becomes larger or smaller compared with the unperturbed sample. Let us stress that even when the optical density (OD) of the sample is large, let us say OD 1, and the pump-induced change is relatively weak, say 10 , it is the latter that carries positive infonnation. 

Figure 4-247 shows a sketch of principle of the system and of the phase-shiftkeying technique. Frames of data are transmitted in a sequence. Each frame contains 16 words, and each word has 10 bits. Some important parameters may be repeated in the same frame, for example, in Figure 2-248, the torque Tp, the resistivity R and the gamma ray GR, are repeated four times. The weight on bit WOB is repeated twice, and the alternator voltage one time. Note that a synchronization pulse train starts the frame. 

The basis of all control Icd-potcntial techniques is the measurement of the current response to an applied potential. There exist a multihide of potential excitations, including a ramp, potential steps, pulse trains, a sine wave, and various combinations thereof. The present chapter reviews those techniques that are widely used. 

Normal-pulse voltammetry consists of a series of pulses of increasing amplitude applied to successive drops at a preselected time near the end of each drop lifetime (4). Such a normal-pulse train is shown in Figure 3-4. Between the pidses, the electrode is kept at a constant (base) potential at which no reaction of the analyte occurs. The amplitude of the pulse increases linearly with each drop. The current is measured about 40 ms after the pulse is applied, at which time the contribution of the charging current is nearly zero. In addition, because of the short pulse duration, the diffusion layer is thinner than that in DC polarography (i.e., there is larger flux of... 

Linearly polarized, near-diffraction-hmited, mode-locked 1319 and 1064 nm pulse trains are generated in separate dual-head, diode-pumped resonators. Each 2-rod resonator incorporates fiber-coupled diode lasers to end-pump the rods, and features intracavity birefringence compensation. The pulses are stabilized to a 1 GHz bandwidth. Timing jitter is actively controlled to < 150 ps. Models indicate that for the mode-locked pulses, relative timing jitter of 200 ps between the lasers causes <5% reduction in SFG conversion efficiency. 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

A number of solid compounds have been examined with this time-domain method since the first report of coherent phonons in GaAs [10]. Coherent phonons were created at the metal/semiconductor interface of a GaP photodiode [29] and stacked GaInP/GaAs/GalnP layers [30]. Cesium-deposited [31-33] and potassium-deposited [34] Pt surfaces were extensively studied. Manipulation of vibrational coherence was further demonstrated on Cs/Pt using pump pulse trains [35-37]. Magnetic properties were studied on Gd films [38, 39]. 

The idea of exploration of relaxation correlation was first reported in 1981 by Peemoeller et al. [23] and later by English et al. [24] using an inversion-recovery experiment detected by a CPMG pulse train. This pulse sequence is shown in Figure 2.7.1. 

Fig. 2.7.2 Diffusion-relaxation correlation se- The detection (2nd) segment for both is a quences using pulsed field gradients, (a) The CPMG pulse train that is similar to that in first segment is a spin-echo with the echo Figure 2.7.1. The amplitude or the duration of appearing at a time 2tcpi after the first pulse, the gradient pairs in both sequences is (b) The first segment is a stimulated echo incremented to vary the diffusion effects, appearing at a time tcpi after the third pulse.

Fig. 2.9.4 Typical gradient pulse trains with cases except for the pulse train (e), where the...

Phase encoding of higher order terms (locally time dependent accelerations) is feasible in principle with pulse trains such as the example shown in Figure 2.9.4(d). However, the relatively long acquisition times of NMR parameter maps conflict with snapshot records that would be needed for such maps. In any case, in Ref. [27] fast imaging techniques are described and discussed that could be employed for moderate versions of snapshot experiments. 

Madhu et al. proposed the use of two consecutive RAPT pulse trains to obtain signal enhancement in spin-5/2 nuclei, such as 27Al and 170 [72]. In their scheme, the saturation of population achieved for the outermost ( 5/2) — 3/2)) transitions was followed by saturation of the 3/2) —> l/2) populations before detection of the CT NMR signal. Kwak et al. used similar RAPT pulse trains to obtain significantly improved signal enhancement on 27Al and 93Nb [73],... 

Fig. 3.13. (a) Transient SHG signal (solid curve) obtained from 0.33 ML Cs on Pt(lll) for excitation with 2.9-THz repetition rate pulse-train (dashed curve). (b) FT spectra of the SHG signal from the same surface obtained with a single pump pulse (top trace) and with pulse trains at different repetition rates (center and bottom traces). From [35,36]... 

Fig. 3.15. Left reflectivity change of GeTe showing the coherent /l i, phonon under repetitive excitation with a pulse train. The intervals between the pump pulses, Af 12, At23, and Af,34 are 290, 320, and 345 fs, respectively. Right pump power dependence of the frequency of the Aig mode for excitation with a single pulse and the pulse train. From [43]...

Removing Physiological Noise. One method of removing the low frequency artifact is to convolve the response signal with a model of stimulus signal. Such methods have been used to increase the SNR in fMRI [54], The stimulus signal is usually modeled as a pulse train with evenly spaced interstimulus interval as in Equation (11)... 

Total correlation spectroscopy (TOCSY) is similar to the COSY sequence in that it allows observation of contiguous spin systems [35]. However, the TOCSY experiment additionally will allow observation of up to about six coupled spins simultaneously (contiguous spin system). The basic sequence is similar to the COSY sequence with the exception of the last pulse, which is a spin-lock pulse train. The spin lock can be thought of as a number of homonuclear spin echoes placed very close to one another. The number of spin echoes is dependent on the amount of time one wants to apply the spin lock (typically 60 msec for small molecules). This sequence is extremely useful in the identification of spin systems. The TOCSY sequence can also be coupled to a hetero-nuclear correlation experiment as described later in this chapter. 

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