Low Deadtime

One of the major aims of the HIPER project  was to reduce the deadime to the order of a nanosecond.    The figure below illustrates the problem.

 

deadtimeproblem

The blue line represents the signal from the sample after illumination with a pulse. Most commercial systems have deadtimes of the order of 80-100ns, usually dominated by cavity ring-down or the length of the Pi/2 pulse. Thus any information from broad lines (i.e. short lifetimes) will be lost. If one achieves a low deadtime, one can detect the whole spectrum. A low dead-time is achieved in the HIPER system by using low Q sample cavities,  very high performance quasi-optical isolators to reduce stray reflections in the system to the -100dB(!) level and using high quality optics that allow induction isolation between source and detector to approach 100dB in favourable circumstances.  To reduce deadtime to 1 ns for a kW system requires improvements over current systems of over 15 orders of magnitude!   The current HIPER system can provide 12 orders of magnitude improvement.  This is insufficient at high powers to make FID a general technique.  However, at low powers (with small angle excitation and favourable samples) it is possible to measure the response of the system within the pulse itself!!  The diagram below shows the detected signal from a small sample of BDPA on resonance and off resonance.