Construction of adiabatic rapid-passage ESR spectrometer

为了在100微秒内检测低温固体中10^-15摩尔的顺磁中心，研制了一台快速灵敏的ESR波谱仪。该光谱仪探测到了在绝热快速通过顺磁中心的开共振条件下产生的瞬时ESR信号。由于压控微波振荡器扫频产生的相位噪声会引起信号幅度的不规则振荡，因此磁场快速扫频的绝热通道比扫频的绝热通道要好得多。绝热通道是通过施加微波脉冲，同时重复扫描磁场来实现的。对瞬时ESR信号的数值分析表明，绝热快速通道适合于横向弛豫时间大于100微秒的顺磁中心的探测。对于这样的样品，光谱仪的灵敏度是传统连续波ESR光谱仪的100倍。结果表明，绝热快通道ESR光谱仪对于探测低温固体中具有很长弛豫时间的极少量的顺磁中心，如有机自由基，具有很强的探测能力。

A fast and sensitive ESR spectrometer was developed for detecting 10^-15 mole of paramagnetic centers in cryogenic solids within 100 microseconds. The spectrometer detected transient ESR signals generated during adiabatic rapid-passage through the on-resonant condition of paramagnetic centers. The adiabatic passage by the rapid sweep of the magnetic field was much better than the frequency sweep, since a voltage-controlled microwave oscillator for the frequency sweep generated phase noises which induced irregular oscillation of the signal amplitude. The adiabatic passage was attained by applying microwave pulses while sweeping the magnetic field repeatedly. The numerical analysis of the transient ESR signals revealed that the adiabatic rapid passage was suitable for detecting paramagnetic centers with the transverse relaxation time longer than 100 microseconds. The sensitivity of the spectrometer for such a sample was 100 times more than a conventional CW ESR spectrometer. It was concluded that the adiabatic rapid-passage ESR spectrometer was quite powerful for detecting a very small amount of paramagnetic centers with very long relaxation times such as organic free radicals in cryogenic solids.