Free after single-pulse excitation

The principle of Fourier transform (FT) NMR spectroscopy is the observation of the so-called free induction decay (FID) after the application of radio frequency (rf) pulses to the resonating nuclei. The carrier frequency of the rf-pulses is the Larmor frequency. In many cases, the FID is observed after single-pulse (SP) excitation, e.g., after application of a so-called 7r/2-pulse which rotates the magnetization by 90° from the direction of the external magnetic field (z-direction) into the x,y-plane. The characteristic time constant for the free induction decay is the transverse relaxation time, T2, which is given by T2=(2/M2) =0.53 (A Vi/2)" for Gaussian lines. Fourier transformation of the FID yields the common absorption spectrum. 

Fig. 3.4.1 Schematic description of the three-dimensional SPI technique. Gz, Cx and Gy are the phase encode magnetic field gradients and are amplitude cycled to locate each /(-space point. A single data point is acquired at a fixed encoding time tp after the rf excitation pulse from the free induction decay (FID). TR is the time between excitation (rf) pulses. Notice that the phase encode magnetic field gradients are turned on for the duration of the /(-space point acquisition.

The redistribution of free voliunes also influences the sub-glass transition temperatures Tp and T observed for photoisomerization reactions in polymer solids. T, Tp and T are frequency-dependent, and the response of any process to the transitions at these temperatures depends on the time scale. The time scale of photoprocesses may not be equal to those of DSC or dynamic mechanical methods, which are of the order of 10 to 1( Hz. However, for photodecoloration of the merocyanine form of spiro-bepzopyran in polycarbonate film under steady-state irradiation of 560 nm light after laser-single-pulse induced coloration, it was found that the Arrhenius plot of the apparent rate coefficient broke at T (150 °C), Tp (20 C), and T (—120 °Q of the matrix polycarbonate these temperatures are the ones determined by dynamic mechanical measurements. The excited state lifetime of the merocyanine form in polycarbonate was 1.8 ns . Hence, the decolorating isomerization during the lifetime proceeded only in a small fraction of the molecules surrounded by a sufficient amount of free volume. Thus, it is likely that the temperature dependence of the apparent rate coefficient reflecting the relative quantum yield is controlled by the frequency of redistribution of free volumes, which may be comparable with the frequency determined by dynamic mechanical measurements. 

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