Pulse delay

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

The remarkable stability and eontrollability of NMR speetrometers penults not only the preeise aeeiimulation of FIDs over several hours, but also the aequisition of long series of speetra differing only in some stepped variable sueh as an interpulse delay. A peak at any one ehemieal shift will typieally vary in intensity as this series is traversed. All the sinusoidal eomponents of this variation with time ean then be extraeted, by Fourier transfomiation of the variations. For example, suppose that the nomial ID NMR aequisition sequenee (relaxation delay, 90° pulse, eolleet FID) is replaeed by the 2D sequenee (relaxation delay, 90° pulse, delay i -90° pulse, eolleet FID) and that x is inereased linearly from a low value to ereate the seeond dimension. The polarization transfer proeess outlined in die previous seetion will then eause the peaks of one multiplet to be modulated in intensity, at the frequeneies of any other multiplet with whieh it shares a eoupling. 

The secondary stmcture elements are then identified, and finally, the three-dimensional protein stmcture is obtained from the measured interproton distances and torsion angle parameters. This procedure requites a minimum of two days of nmr instmment time per sample, because two pulse delays are requited in the 3-D experiment. In addition, approximately 20 hours of computing time, using a supercomputer, is necessary for the calculations. Nevertheless, protein stmcture can be assigned using 3-D nmr and a resolution of 0.2 nanometers is achievable. The largest protein characterized by nmr at this writing contained 43 amino acid units (51). However, attempts ate underway to characterize the stmcture of interleukin 2 [85898-30-2] which has over 150 amino acid units. 

The chemical shifts are obtained from the spectra of the T1 measurements in relation to the signal for the methyl carbon (20.6 ppm). The T1 measurements were performed using the inversion recovery (IR) method (180 deg. (12.7 us)-tau — 90 deg. (6.1 us)) with MAS 2200 scans were collected and the pulse delay time was veryshort, + 10 sec. 

Fig. 7. A C-13 relaxation time measurement of solid state wetted cellulose acetate (6% by weight water) using the inversion recovery (IR) method at 50.1 MHz and spinning at 3.2 kHz at the magic angle (54.7 deg) with strong proton decoupling during the aquisition time (136.3 ms), (upper part of the Figure). Tau represents the intervals between the 180 deg (12.2 us) inverting and 90 deg (6.1 us) measuring pulse. 2200 scans were collected and the pulse delay time was 10 s, Cf. Table 3 and Ref.281...

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. 

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

The PVA sample was run at 55 degrees centigrade. A 90 degree (6.8us) pulse was used with a inter-pulse delay of 2.1s. Exactly 800 scans were acquired with 16k complex data points and a sweep width of +/-2000 Hz. 

The most popular, and also a very accurate, experimental method for measuring nonselective spin-lattice relaxation-rates is the inversion recovery (180°-r-90°-AT-PD)NT pulse sequence. Here, t is the variable parameter, the little t between pulses, AT is the acquisition time, PD is the pulse delay, set such that AT-I- PD s 5 x T, and NT is the total number of transients required for an acceptable signal-to-noise ratio. Sequential application of a series of two-pulse sequences, each using a different pulsespacing, t, gives a series of partially relaxed spectra. Values of Rj can... 

Preirradiation (selective 180°) of each signal was followed by a 90° observed pulse delayed by 0.7s. This spectrum (550-1000 transients) was acquired simultaneously with a spectrum in which one selective pulse was 3 ppm upfield of tetramethylsilane, and the two spectra were computer subtracted to observe the enhancements. 

A 2D NMR experiment involves a selection of pulses, delays, frequencies, RF phases and amplitudes,... 

Solid-state NMR spectroscopy was also used to examine the post reaction behavior of pTrMPTrA samples prepared in bulk as thin films, as described in the experimental. All of the spectra in this aging study required a minimum of 720 scans on approximately 50 mg of sample with a 100 s pulse delay to achieve adequate signal/noise. Under these conditions, reliable peak areas could be obtained from the curve fits of the carbonyl region. Figure 3 depicts the evolution of the solid state spectrum of the sample stored under N2 over time and upon heating. The area of the peak at 174 ppm for the carbonyl adjacent to the reacted double bond increases as the peak at 166 ppm for pendant unsaturation decreases. The results of the aging study are given in Table I. 

Spectra were determined using a pulse width of 4 yseconds, which corresponds to a flip angle of 18° and a 1 second pulse delay time. The 4000 Hz spectrum was described using 8192 data points. 

Fig. 18b. 12. (a) Voltage-time profile for anodic stripping voltammetry (ASV) and (b) ASV of an unknown solution with two aliquot additions of 100 ppb each of Cd and Pb in the final solution. The peak at —190 mV is that of Cu present in the unknown. Experimental conditions Initial deposition potential, Ed = —800 mV, final potential = 0, deposition time, td — 120 s, quite time, tq — 30 s, step potential = 5 mV, pulse height = 20 mV, pulse delay = 100 ms, sampling width — 17 ms, and sampling frequency — 6000 Hz. 

Fig. 9.1 Basic HNN COSY pulse sequence. Narrow and wide pulses correspond to flip angles of 90° and 180°, respectively, whereas low-power (water flip-back) 90° l-l pulses are illustrated as smaller narrow pulses. Delays 8 = 2.25 ms 7=15 ms (can be shorter or longer) (a = 2.5 ms fb = 0.25 ms fc= 2.25 ms fd = 0.5 ms. Unless indicated, the phase of all pulses are applied along...

The time resolution of the instrument is governed not only by the pulse width but also by the electronics and the detector. The linear time response of the TAC is most critical for obtaining accurate fluorescence decays. The response is more linear when the time during which the TAC is in operation and unable to respond to another signal (dead time) is minimized. For this reason, it is better to collect the data in the reverse configuration the fluorescence pulse acts as the start pulse and the corresponding excitation pulse (delayed by an appropriate delay line) as the stop pulse. In this way, only a small fraction of start pulses result in stop pulses and the collection statistics are better. 

FIGURE 28. 283-MHz 19F NMR spectra of isomers of 8-F-rhodopsin in CFIAPS before (lower) and after photoirradiation (upper) (a) 11-cis (pulse delay, D5 = 5.0 s, number of acquisitions, NA = 5200, line broadening, LB = 80 Hz) (b) 9-cis (D5 = 50 ms, NA = 160000, LB = 80 Hz). Disappearance of the excess 9-cis aldehyde was due to repeated formation and bleaching of pigment during the irradiation process. Reprinted with permission from Reference 48. Copyright (1996) American Chemical Society... 

Re-evaluation of pulse delay times used to record fullerene 13C NMR spectra revealed that a 16 s pulse delay, twice the value for a standard detection, allowed the observation of a weak resonance in the sp3 region at 90.4 ppm in the 13C NMR spectrum of the unlabeled heterofullerene 114. Attempts were made to optimize the NMR experimental parameters for a long 7 i, i.e. the variation of delay times and pulse angles. Various conditions were tried on the labeled material without success. This is probably due to the mixture of the labeled and unlabeled 114 which give too low S/N for signal detection. Table 49 summarizes the NMR results obtained and illustrates a distinct pattern of the azafullerenes. 

Among other examples, time-resolved luminescence has recently been applied to the detection of different trace elements (i.e., elements in very low concentrations) in minerals. Figure 1.13 shows two time-resolved emission spectra of anhydrite (CaS04). The emission spectrum just after the excitation pulse (delay 0 ms) shows an emission band peaking at 385 nm, characteristic of Eu + ions. When the emission spectmm is taken 4 ms after the pulse, the Eu + luminescence has completely disappeared, as this luminescence has a lifetime of about 10/rs. This allows us to observe the weak emission signals of the Eu + and Sm + ions present in this mineral, which in short time intervals are masked by the En + Inminescence. The trivalent ions have larger lifetimes and their luminescence still remains in the ms delay range. 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

Gated decoupling and a long pulse delay time of 10 seconds were employed to obtain the spectrum. Frcxn the monomer cuid polymer peak areas, the extent of polymerization at equilibrium Ccui be determined. Measurements of chain end and the polymer peaks provide information on number-average degree of polymerization. The data collection time required to obtain this spectrum was almost three hours. 

Gated Decoupling to Suppress NOE Pulse Delay (>10 seconds)... 

In the systems that I have examined, I can satisfy the dynamic requirements with a ten second pulse delay. The longest methyl T] may be 3 seconds. In general, the longer the side chain, the longer will be the methyl Tj. We will hear more about this subject later on. We need not be too concerned about NOE factors because they are usually full under the experimental conditions (T = 120-130°C) used for polymer quantitative measurements. The Tj problem can be handled, even under non-equilibrium conditions, by utilizing resonances from the same types of carbon atoms in a quantitative treatment. Such an approach can sometimes lead to more efficient quantitative NMR measurements. Adequate pulse spaclngs will have to be used whenever one wishes to utilize all of the observed resonances. Quantitative measurements in branched polyethylenes are very desirable because this is one of the best applications of analytical polymer C-13 NMR. 

The H-NMR spectra of FCC feeds were recorded on a Bruker DRX 400 MHz NMR spectrometer. The concentration of the samples of 5 wt% in CDCI3 was recommended by Molina, Navarro Uribe, and Murgich [2] to avoid concentration dependence of the chemical shift. A 30° pulse sequence was applied, with 4.089 s acquisition time, 2 s pulse delay [2], 8012.8 Hz spectral width, and 64 scans. Hexamethyldisiloxane (HMDSO) was used as a reference. NMR processing was realized using MestReNova software. The phase and baseline of the resulting spectra were manually adjusted and corrected. The spectra were integrated six times and average values were taken for the purpose of calculations. The spectra were divided... 

The interferometric structure around the quarter revival timing discussed above is generated spontaneously by the intrinsic anharmonicity of the potential. Next we will show similar interferometric structures generated by the double-pulse excitation [39]. The pump and control pulses are generated by the gas-pressure tuning interferometer. The double-pulse delay t was stabilized by the feedback loop control. 

---