Experiment Repetition Rate

The delay time between successive scans in a 2D experiment is, of course, of major importance for the sensitivity that can be obtained per unit time. For the case of a one-dimensional experiment in which a single 90° pulse is applied to a spin system and time averaging is used, we saw in Section 3.11 that the optimum time A between consecutive pulses is 1.27T, or alternatively that the Ernst flip angle can be used with more rapid repetition. For most 2D experiments, A = 1.27Tj is also close to optimum for a 90° pulse. This repetition time 

In some studies the number of scans necessary for adequate phase cycling provides more than enough sensitivity, and the repetition time may be decreased below the optimum to save overall experimental time. However, in some experiments too short a repetition time can introduce false lines into the 2D spectrum because the state of the spin system achieved in the preparation period becomes a function of q. For example, residual HDO protons often have a long Tj value, and rapid repetition rate causes an array of spurious resonances in the cot dimension at the HDO o 2 frequency. 

We list other books in 2D NMR in later chapters, where they have particular applicability. 

1 Distinguish clearly between a one-dimensional NMR experiment that uses a time increment, such as the inversion-recovery technique (Section 2.9 and Fig. 8.8a), and a two-dimensional NMR experiment, such as NOESY. 

2 Use the data in Fig. 10.6, along with chemical shift correlations from Chapter 4, to assign the 13C chemical shifts to the seven carbon atoms. Are there ambiguities  

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Since polarisation transfer is employed, the experiment repetition rate is dictated by the recovery of the faster relaxing proton spins, and repetition times should be around 1.3 times the proton Tis. Resolution in the proton dimension can be quite low for routine applications since one does not usually wish to resolve the proton fine-structure and because the homonuclear coupling is in-phase there is no fear of signal cancellation fi digital resolution may therefore be as low as 10 Hz/pt or so, requiring rather few ti increments. The number of scans per increment should be set such that the carbon resonances of interest can just be observed in the spectrum of the first recorded FID (which is equivalent to the ID INEPT experiment). 

Experiment repetition rate determined by these times. 

The most connnon commercially prepared amplifier systems are pumped by frequency-doubled Nd-YAG or Nd-YLF lasers at a 1-5 kHz repetition rate a continuously pumped amplifier that operates typically in the 250 kHz regime has been described and implemented connnercially [40]. The average power of all of the connnonly used types of Ti-sapphire amplifier systems approaches 1 W, so the energy per pulse required for an experiment effectively detennines the repetition rate. 

Hz repetition rate of the lasers and is usually sampled with a gated integrator, whose output is reeorded with a laboratory eomputer. Analogue, rather than digital, eleetronies is usually employed beeause of pile-up of the deteeted photon eounts in an experiment with reasonable produet intensities. 

Heteronuclear chemical shift-correlated spectroscopy, commonly called H-X COSY or HETCOR has, as the name implies, different and F frequencies. The experiment uses polarization transfer from the nuclei to the C or X nuclei which increases the SNR. Additionally, the repetition rate can be set to 1—3 of the rather than the longer C. Using the standard C COSY, the ampHtude of the C signals are modulated by the... 

A commercial fs-laser (CPA-10 Clark-MXR, MI, USA) was used for ablation. The parameters used for the laser output pulses were central wavelength 775 nm pulse energy -0.5 mj pulse duration 170-200 fs and repetition rate from single pulse operation up to 10 Hz. In these experiments the laser with Gaussian beam profile was used because of the lack of commercial beam homogenizers for femtosecond lasers. 

Experiments on the sky. Two experiments have been carried out at the sky, using two laser installations built for the American and French programmes for Uranium isotope separation, respectively AVLIS at the Lawrence Livermore Nat l Lab (California) in 1996 and SILVA at CEA/Pierrelatte (Southern France) in 1999. The average power was high pa 2 x 175 W, with a pulse repetition rate of 12.9 and 4.3 kHz, a pulse width of 40 ns and a spectral width of 1 and 3 GHz. Polarization was linear. The return flux was < 5 10 photons/m /s (Foy et al., 2000). Thus incoherent two-photon resonant absorption works, with a behavior consistent with models. But we do need lower powers at observatories ... 

The experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

In addition to measuring TCH for the polymorphic system in question, the proton T value must be determined since the repetition rate of a CP experiment is dependent upon the recovery of the proton magnetization. Common convention states that a delay time between successive pulses of 1-5 X T, must be used. Figure 10B outlines the pulse sequence for measuring the proton Tx through the carbon intensity. One advantage to solids NMR work is that a common proton Tx value will be measured, since protons communicate through a spin-diffusion process. An example of spectral results obtained from this pulse sequence is displayed in Fig. 12. 

The pump pulse energy is controlled to minimize two-photon phenomena and to maximize the concentration of the desired excited-state or other reactive intermediate. The optimal average power of the probe pulse changes with a specific experiment but is often maintained at 10 mW peak powers in the range of 0.1-10 MW with repetition rates of 1 kHz-1 MHZ are best for picosecond spontaneous Raman spectroscopy. 

To observe the transient spatiotemporal structure of the WP interference, we have performed the fs pump-fs probe experiment [37], The sample gas was prepared by molecular jet expansion of the mixture of iodine vapor and Ar buffer gas into a vacuum chamber. A continuous gas jet is preferable when we use a high-repetition-rate laser system. The estimated vibrational temperature was 170K[37]. 

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