Electron paramagnetic resonance (EPR) spectroscopy and electron spin resonance (ESR) spectroscopy are generally used to study molecular structure in chemistry, physics, biology, and medicine. EPR is also used to determine electron wave functions, lifetimes, and impurities in dielectrics used in solid state sciences. Prior EPR spectrometers comprise four main components: 1) a magnet to provide a steady DC magnetic field; 2) a high Q factor microwave resonator in which a sample is placed; 3) a microwave bridge capable of producing an oscillating electromagnetic field which is coupled via a waveguide, coaxial cable, or the like to the resonator; and 4) a signal detector with field modulation, signal amplification and display systems.
In EPR, a steady magnetic field is applied to the chemical sample in the microwave resonator. The steady magnetic field causes the electrons in the sample to precess at a frequency defined by the sample composition. The precession frequency is called the paramagnetic resonant frequency and is proportional to the intensity of the applied steady magnetic field. The precession is altered by application of high frequency energy when the frequency of the energy is near the paramagnetic resonance frequency. If the sample contains unpaired electrons, the precession change can be detected. A comparison of change in electron precession as a function of magnetic field or frequency provides valuable information relating to the chemical characteristics of the sample.
A typical EPR spectrometer uses a reflection type measurement on the electrical resonator that contains the sample. In a reflection type spectrometer a single resonator is used. The sample is placed in the resonator and microwave energy is injected via a waveguide, coaxial cable, or the like into the resonator while the sample and resonator are positioned in the steady magnetic field. A microwave device called a circulator is usually used to separate the desired EPR signal from the microwave source power. A disadvantage of the reflection mode of operation is that any portion of the microwave source power that is reflected from the resonator will interfere with the EPR signal generated in the resonator. In one type of measurement, also called “continuous wave” because the input microwave energy is applied as a continuous AC signal, the resonator is tuned to provide minimal reflection of the input energy.
Analysis is accomplished by sweeping the magnetic field or the microwave frequency source until the precession frequency matches the frequency of the input microwave power. When the two frequencies are the same, microwave energy is absorbed by the sample resulting in reflected energy that can be detected by the detector electronics.
In conventional CW, reflection type spectrometers; phase-sensitive detection at the magnetic field modulation frequency is used to improve the signal-to-noise. One disadvantage of this for continuous wave (CW) measurements is that the EPR signal is minute compared to the magnitude of the injected microwave energy. The signal detector must detect the EPR signal while separating out the injected microwave energy. It has proved difficult to completely separate the EPR signal from the input power.
Another difficulty arises in that any parasitic reflection of the microwave source caused by improper coupling of the input power to the resonator may create significant noise in the EPR signal. In addition, the source input waveguide and the detector waveguide can be considered to be critically coupled to the resonator to prevent a large reflection of the input power that would add to the EPR signal and saturate the detector electronics.
Phase noise or noise frequency modulation of the microwave source is converted to noise amplitude modulation in the reflected signal by the resonator, creating further noise in the EPR signal. Phase noise cannot be eliminated from microwave sources. It can be reduced but this can be expensive. Since the phase noise intensity is proportional to the source intensity, it becomes more serious at higher powers. Hence, current EPR tools must be operated at low power which in turn requires larger samples. A phase or dispersion component of the reflected EPR signal is difficult or impossible to study in reflection-type spectrometers because of this phase noise.
EPR tools can also be used for pulse-type measurements such as electron spin echo (ESE). In pulse type measurements, the input energy is provided by a high power pulse rather than a continuous wave microwave source. The pulse causes a near instantaneous change in the precession and a gradual decay as the sample returns to the baseline state created by the DC magnetic field. In this type of measurement the difficulty in separating input power from the EPR signal requires a delay after the application of the input pulse before a measurement can be made. Because the energy stored in the resonator by the input pulse must “ring-down” or dissipate before a measurement can be taken, much of the ESE signal can be lost in a reflection-type spectrometer.
Two types of resonators may be used in EPR spectrometry. Cavity resonators were used in early spectrometers due to their easily modeled performance, availability, and high Q factor. Cavity resonators are called distributed element circuits because the microwave, magnetic, and electric field are continuously distributed and mix throughout the cavity. Characteristic dimensions of cavity resonators are of the same order of magnitude as the wavelength of the electromagnetic fields used. More recently, lumped element resonators have been suggested because their dimensions can be much smaller than the wavelengths of interest.
Lumped element resonators have much less mixing of the microwave, magnetic and electric fields; each is confined largely to separate physical areas of the resonator. The area where the magnetic field is concentrated can be identified primarily as an inductor. The area where the electric field is concentrated is identified primarily as a capacitor. A term used to describe some of these lumped element resonators is “loop-gap resonators”. In this case, the loop is primarily inductive and the gap is primarily capacitive. In loop-gap resonators used for spectrometers, the sample is positioned in the loop so as to interact with the magnetic field.
Most loop-gap resonators are used in the reflection type spectrometers discussed above. Conventional loop-gap resonators have low Q factor compared to cavity resonators, however, because the magnetic field is concentrated in the vicinity of the sample, good EPR signals can be obtained.
One means that has been tried in order to reduce the problems associated with reflection type resonators is a bimodal resonator. A bimodal resonator structure would take advantage of the fact that the EPR signal is circularly polarized. In other words, the EPR signal is a rotating vector field and is equivalent to two signals that are 90 degrees out of phase in space and time. One of these EPR signals is coupled to the input source and can be detected in the conventional reflection type spectrometers discussed above. A bimodal resonator is a structure that detects the other EPR signal that is isolated from the input source. In theory, if two uncoupled modes with microwave magnetic fields oriented 90 degrees in space could be excited in a resonator, one mode might be used to couple microwave energy into the sample and the other to detect the EPR signal. Hence, the bimodal resonator design promises to offer superior separation of input power from the EPR signal and make the EPR spectrometer immune to noise caused by reflected input power or phase noise.
Rapid scan is an electron paramagnetic resonance (EPR) method in which the magnetic field is scanned through resonance in a time that is short relative to T2, and the absorption and dispersion signals are recorded by direct detection. Phase-sensitive detection at the magnetic field modulation frequency is not used. Oscillations are observed on the trailing edge of the signal. Rapid scan signals obtained with either triangular or sinusoidal scans can be deconvolved to give the conventional spectra.
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