RADIATION DAMPING INDUCED HALF-FREQUENCY-SPACED HARMONIC PEAKS IN 2-DIMENSIONAL J-RESOLVED NMR-SPECTRUM ON LIQUID WATER

In two-dimensional J-resolved NMR spectrum of radiation damped water w ith the simple pulse sequence of pi/2 - t(1)/2 - pi - t(1)/2-FID, harm onic peaks spaced by half frequency can either appear or disappear in the indirectly detected dimension, depending on the number of t(1) inc rements and on the sensitivity of probe head. When the experiments are performed with a less sensitive probe having a poor Q factor, the har monic peaks are hardly observable. The half frequency harmonic peaks o bserved with a tuned sensitive probe must be due to radiation damping. Theoretically, an inverted, strongly damped magnetization needs an in finitely long time to recover across the xy plane following the radiat ion damping pathway; while in experiments, it takes less than tens of milliseconds. Therefore, when t(1) is long enough compared to the radi ation damping time T-r, the ''echo'' is not symmetric about the pi pul se with respect to the pi/2 pulse, modulating the FID by a pair of ang les theta and phi that are complicated functions of both t(1) and the frequency offset Delta w, and producing the harmonic peaks. If t(1)(ma x) is short, the radiation damping effects during t(1) are wiped out b y the spin echo. Consequently, the harmonic peaks disappear. Experimen tal results have been exactly simulated based on the radiation damping line shape theory.