在反转恢复测试中磁化矢量的演化特征:辐射阻尼效应

在强辐射阻尼存在下,水样(90% H₂O in D₂O)的反转恢复实验表明:当两脉冲的相位相差180°且反转脉冲角<180°时,或两脉冲相位一致但反转脉冲角>180°时,在检测期观测到的信号强度将不发生从负极大值到正极大值的突变;在同样的条件下,如果存在频率偏置,信号强度存在波动,即beating效应.只有当两脉冲的相位一致而反转脉冲角<180°时,或两脉冲的相位相差180°但反转脉冲角>180°时,在检测期信号强度才发生突变,即jumping效应.这些现象都可通过辐射阻尼理论予以合理地解释.另外,在检测期当磁化矢量运动到-z轴附近(对应于τ=T_rdln{tan[(π±δ)/2]}),信号强度与理论预计的偏差实际上与T₁弛豫效应有关.

INVERSION RECOVERY MEASUREMENTS:THE RADIATION DAMPING EFFECTS

In the presence of strong radiation damping, the conventional two-pulse inversion recovery measurements have been performed on water sample(90%H₂O/10%D₂O). It shows that if two pulses in IR sequence are 180° out of phase and the flip angle of the inversion pule is smaller than 180°, or if two pulses are in phase and the flip angle of the inversion pulse is larger than 180°, the signal intensity will continuously changes with the recovery time. Conversely, if two pulses are in phase (or 180° out of phase) and the flip angle of the inversion pulse is smaller than 180° (or larger than 180°), the signal intensity will jump from the negative maximum to the positive maximum around τ=T_rdln{tan[(π±δ)/2]}. Under the off-resonance condition, no matter what pulse phase, the beating effect can be always observed, but the phase of the signal explicitly depends on the relative phase of the pulses. As a result, the phase cycling will greatly suppress the beating effect. These phenomena can be well explained by the radiation damping theory. In addition, when the magnetization locates near the-zaxis during the detection, the deviation of theoretical expectation from the experimental data is closely related to the longitudinal relaxation effect.