4. Acquisition

Time requirement: 10 min

Sample: 90% H₂O with 10% D₂O

The probe-head should be tuned to the sample. Load standard ¹H parameters, record a normal ¹H NMR spectrum and note the offset of H₂O. You have to set:

transmitter attenuation [3 dB]
td	4 k
sw	500 Hz
o1	on resonance of H₂O signal
p1	exp. a: 360° and exp. b: 180° ¹H transmitter pulse
d1	2 s
rg	receiver gain for correct ADC input
ns	1

Determine the 360° pulse as described in Experiment 2.1 and record the spectrum a. In addition, record the spectrum b with a 180° pulse and compare the residual signal strength in both experiments.

5. Processing

Use standard 1D processing (see Exp. 3.1) applying an exponential window with a line-broadening factor lb = 0.3 Hz.

6. Result

The figure shows the result obtained on an AMX-500 spectrometer with an inverse probe-head. In a the absence of radiation damping after a 360° pulse, in b the hyperbolic secant signal behavior produced by radiation damping after a 180° pulse is shown. Both traces are plotted on the same vertical scale! Note that this signal is often even stronger than the signal after a 90° pulse.

7. Comments

The nutational behavior of the signals produced by radiation damping can be calculated from the Bloch equations [5]. Radiation damping can be observed even after a perfect 180° pulse, since transverse components can be created by thermal noise or r.f. leaking effects. Thus, if one applies a pulsed field gradient (see Chapter 11) after the 180° pulse to destroy any residual transverse components, the effect may still be observed [5]. In pulse sequences which establish −z-magnetization, radiation damping may cause loss of signal during the following delays, since the magnetization is driven back by the r.f. field in the coil. Radiation damping may further cause line-broadening and distortion in the relative intensities of multiplets. Possible remedies are detuning