Pulse lengths

The source of radiation is a linear accelerator with selectable primary energies of 6, 9 or 11 MeV ( VARIAN Linatron 3000 A). The output of the LINAC at 9 MV is 3000 rad ( 30 Gy) per minute. The pulse length is 3.8 microseconds with repetition frequencies between 50 and 250 Hertz. 

The absolute measurement of areas is not usually usefiil, because tlie sensitivity of the spectrometer depends on factors such as temperature, pulse length, amplifier settings and the exact tuning of the coil used to detect resonance. Peak intensities are also less usefiil, because linewidths vary, and because the resonance from a given chemical type of atom will often be split into a pattern called a multiplet. However, the relative overall areas of the peaks or multiplets still obey the simple rule given above, if appropriate conditions are met. Most samples have several chemically distinct types of (for example) hydrogen atoms within the molecules under study, so that a simple inspection of the number of peaks/multiplets and of their relative areas can help to identify the molecules, even in cases where no usefid infonnation is available from shifts or couplings. 

Pulsed, or time-domain, EPR spectrometers have also been developed at higher frequencies up to 140 GHz [55. 56]. They are generally low-power units with characteristically long pulse lengths (typically 50 ns for a n/2-pulse) due to tire limited MW powers available at millimetre wavelengths and the lack of fast-switching... 

MW em and GW em Speeial lasers used in nuelear frision experiments may even reaeh 10 W em [78, 79]. Ideally the monoehromatieity, Av, is related to the pulse length. At, tlirough... 

Lasing substances Physical state Laser wavelength (nm) Pulse length or continuous wave Typical maximum power output (watts)... 

From the Heisenberg uncertainty principle as stated in Equation (1.16) estimate, in cm and Hz, the wavenumber and frequency spread of pulsed radiation with a pulse length of 30 fs, typical of a very short pulse from a visible laser, and of 6 ps, typical of pulsed radiofrequency radiation used in a pulsed Fourier transform NMR experiment. 

Pulse lengths of <100 fs (1 femtosecond = 10 s) have been achieved by mode locking. 

In Section 9.2.2 we saw that a pulsed Tp sapphire laser can produce pulses less then 10 fs in length. There are also other laser techniques which can be used to produce pulse lengths... 

Pulsed ECM (PECM) may be a promising way to improve dimensional accuracy control and also to simplify tool design. Accuracies as fine as 0.002 mm have been quoted using current pulse lengths of ca 0.2 to 2.0 ms, at current densities of 55 A/cm. Pulse offtimes are from 1 to 2 ms (7). 

H-NMR studies were performed on a Bruker MSL-400 spectrometer operating in the Fourier transform mode, using a static multinuclei probehead operating at 400.13 MEtz. A pulse length of 1 iis is used for the 90° flip angle and the repetition time used (1 second) is longer than five times Tjz ( H) of the analyzed samples. 

Figure 13-9. Timc-intcgralcd light output versus pulse length at various applied voltages. The lines arc least-square fils and their extrapolation yields the lime delay t,. Inset shows the liming between the application of a voltage pulse and the observed eleclroluniineseencc. Reproduced with permission from I22. Copyright 1998 by the American Physical Society.

FIGURE 4. Transient absorption spectrum immediately after pulse radiolysis of dimethyl sulphoxide alone. Pulse length 50 ns, dose 1000-2000rad , path length 2.5m, time resolution 3ns +, path length 5 cm, time resolution 10 ns. The dashed line represents the spectrum of the short-lived component (t1/2,14 ns), subtracted from the overlapping longer-lived component which is unaffected by N20. Reproduced by permission of the authors from Reference 29. 

Mt. Wilson Observatory. The UnISIS excimer laser system is deployed on the 2.5 m telescope at Mt. Wilson Observatory (Thompson and Castle, 1992). A schematic of the system layout is shown in Fig. 11. The30W, 351 nm excimer laser is located in the coude room. The laser has a 20 ns pulse length, with a repetition rate of 167 or 333 Hz. The laser light is projected from the 2.5 m mirror and focused at 18 km. A fast gating scheme isolates the focused waist. A NGS is needed to guide a tip-tilt mirror. Even with relatively poor seeing, UnISIS has been able to correct a star to the diffraction limit. 

LLNL AVLIS Laser. The first WFS measurements using a Na LGS were performed at LLNL (Max et al., 1994 Avicola et al., 1994). These experiments utilized an 1100 W dye laser, developed for atomic vapor laser isotope separation (AVLIS). The wavefront was better than 0.03 wave rms. The dye laser was pumped by 1500 W copper vapor lasers. They are not well suited as a pump for LGSs because of their 26 kHz pulse rate and 32 ns pulse length. The peak intensity at the Na layer, with an atmospheric transmission of 0.6 and a spot diameter of 2.0 m, is 25 W/cm, 4x the saturation. The laser linewidth and shape were tailored to match the D2 line. The power was varied from 7 to 1100 W on Na layer to study saturation. The spot size was measured to be 7 arcsec FWHM at 1100 W. It reduced to 4.6 arcsec after accounting for satura-... 

Pulsed current experiments of aqueous acetate solutions indicate that at least in aqueous solution a platinum oxide layer seems to be prerequisite for the da arboxy-lation to occur. Only at longer pulse durations (> 10 s) is ethane produced [73,74]. These are times known to be necessary for the formation of an oxide film. At a shorter pulse length (<10"" s) acetate is completely oxidized to carbon dioxide and water possibly at a bare platinum surface [75]. The potent dynamic response in the electrolysis of potassium acetate in aqueous solution also points to an oxide layer, whose... 

In studies of molecular dynamics, lasers of very short pulse lengths allow investigation by laser-induced fluorescence of chemical processes that occur in a picosecond time frame. This time period is much less than the lifetimes of any transient species that could last long enough to yield a measurable vibrational spectrum. Such measurements go beyond simple detection and characterization of transient species. They yield details never before available of the time behavior of species in fast reactions, such as temporal and spatial redistribution of initially localized energy in excited molecules. Laser-induced fluorescence characterizes the molecular species that have formed, their internal energy distributions, and their lifetimes. 

A solid-state nuclear magnetic resonance (NMR) experiment was carried out in 4 mm double bearing rotor made from Zr02 on a Bruker DSX 200 MHz spectrometer with resonance frequency at 75.468 MHz. The pulse length was 3.5 ps and the contact time of IH-13C CP was 2-5 ms. 

A number of parameters have to be chosen when recording 2D NMR spectra (a) the pulse sequence to be used, which depends on the experiment required to be conducted, (b) the pulse lengths and the delays in the pulse sequence, (c) the spectral widths SW, and SW2 to be used for Fj and Fi, (d) the number of data points or time increments that define t, and t-i, (e) the number of transients for each value of t, (f) the relaxation delay between each set of pulses that allows an equilibrium state to be reached, and (g) the number of preparatory dummy transients (DS) per FID required for the establishment of the steady state for each FID. Table 3.1 summarizes some important acquisition parameters for 2D NMR experiments. 

Accurate calibration of pulse lengths is essential for the success of most 2D NMR experiments. Wide variations (>20%) in the setting of pulse lengths may significantly reduce sensitivity and may lead to the appearance of artifact signals. In some experiments, such as inverse NMR experiments, accurately set pulse lengths are even more critical for successful outcomes. 

In routine analysis, often a one-dimensional so-called end-point titration can be automatically carried out up to a pre-set pH or potential value and with a previously chosen overall titration velocity in order to avoid overshoot, the inflection point should be sufficiently sharp and the titrant delivery must automatically diminish on the approach to that point in order to maintain equilibrium, and stop in time at the pre-set value. For instance, the Metrohm 526 end-point titrator changes both the dosing pulse length and its velocity by means of a pulse regulator in accordance with the course of the titration curve in fact, the instrument follows the titration two-dimensionally, but finally reports only a one-dimensional result. The Radiometer ETS 822 end-point titration system offers similar possibilities. However, automated titrations mostly represent examples of a two-dimensional so-called eqilibrium titration, where the titration velocity is inversely proportional to the steepness of the potentiometric titration curve hence the first derivative of the curve can usually also be recorded as a more accurate means of determining the inflection... 

We carried out two sets of experiments in which we set the pulse angle first at 90°, then at 30°. Using these two values we then varied the relaxation delay. Since the greatest difference in the relaxation times is that between the OH proton and the aromatic protons, we show in Fig. 11 the comparison between the integration values of the aromatic protons (set equal to 2.0) and of the OH proton for 90° pulses and for 30° pulses. The values approach each other with a relaxation delay of 10 sec and are virtually equal for a delay of 25 sec, but the 90° pulses give values which are completely wrong if a conventional delay of 1-2 sec is used On the other hand, the error is quite low if the delay is set at 2 sec and the pulse length is 30°. 

Figure 22 Pulse sequence of the HMBC-RELAY experiment. Filled and open bars represent 90° and 180° pulses, respectively. All other phases are set as x, excepted otherwise stated. A two-phase cycle x, —x is used for the pulse phases (j>, and

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