Optimization of Flash-CIDNP Experiments

The time resolution of flash-CIDNP experiments (time-resolved measurements of photochemically induced dynamic nuclear polarizations) can be increased by using NMR observation pulses that are delayed from scan to scan by an interval shorter than the pulse duration, and which need not be short on the time scale of the chemical kinetics. Recording a kinetic curve with such pulses amounts to passing it through a low pass filter. The dependence of the transfer function of the latter on duration and shape of theB₁pulse, as well as the influence of the flip angle are analyzed. It is shown that, in order to achieve maximum sensitivity, flip angle and acquisition time must be optimized in a way different from that used in conventional NMR experiments. With regard to hardware-dependent parameters, the time resolution of the method is basically limited by the rise time of theB₁pulses; in contrast, in the regime where the transfer function is determined by the pulse width, there is a trade-off between sensitivity and time resolution, and the strength of theB₁field is the most important factor.