Lock, external signal

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. 

When solubility dictates use of different deuteriated solvents for reference and measured samples, the chemical shift (5corr measured against the external reference and corrected for susceptibility difference must be also corrected for the difference in the chemical shifts of the lock signals, A. With a deuterium lock this difference is just the difference in 2H NMR chemical shifts of the two deuteriated solvents, which equals the difference in XH shifts of their pro tic isotopomers (equation 4) ... 

FIG. 9. Li chemical shifts in organolithium compounds, measured in three different solvents. The shieldings were originally determined relative to the proton frequency of tetramethylsilane which served as an internal lock signal, and subsequently referenced to external LiBr (0-7g/ml). (80)... 

The most convenient means of making time-resolved SH measurements on metallic surfaces is to use a cw laser as a continuous monitor of the surface during a transient event. Unfortunately, in the absence of optical enhancements, the signal levels are so low for most electrochemical systems that this route is unattractive. A more viable alternative is to use a cw mode-locked laser which offers the necessary high peak powers and the high repetition rate. The experimental time resolution is typically 12 nsec, which is the time between pulses. A Q-switched Nd YAG provides 30 to 100 msec resolution unless the repetition rate is externally controlled. The electrochemical experiments done to date have involved the application of a fast potential step with the surface response to this perturbation followed by SHG [54, 55,116, 117]. Since the optical technique is instantaneous in nature, one has the potential to obtain a clearer picture than that obtained by the current transient. The experiments have also been applied to multistep processes which are difficult to understand by simple current analysis [54, 117]. 

As shown in previous sections of this chapter, when an external perturbation is applied to the polymer film (such as irradiation), the ATR guided modes shift their angular positions and the reflectivity is modulated (Fig. 31b). These angular shifts are very small in the case of electrooptic experiments they correspond to refractive index variations of the order of 10 . One has then to modulate the measuring electric field at a low frequency Q( = cos fit) and to detect the modulated signal with lock-in amplifiers. The lock-in signals detected at the modulation frequency and its second harmonic give, respectively, the linear (or Pockels) and the quadratic (or Kerr) electrooptic effects. The amplitude of the modulation of the thickness and the refractive indices is evaluated by a computer fit, and allows the determination of Pockels (r) and Kerr (s) coefficients (Eqs. 28) ... 

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