Spectrometer phase-sensitive detection

At this point, some mention of the intensity of the lines observed in epr spectra should be made. In the case of a radical having no magnetic nuclei the difference in the energy of the a and 0 levels is of the order of 0.3 cm . At room temperature, kT is c. 200cm and the Boltzmann equation thus gives the ratio of the number of radicals in the lower state to those in the upper, Np/Na, as 1.0015. The absorption intensity is proportional to this difference and as a consequence epr absorptions are very weak. The phase-sensitive detection employed in epr spectrometers is thus a necessity if these weak... 

In order to observe a short-lived species it may be necessary to employ a rapid-scanning spectrometer, such as a diode-array instrument (Sms for a 240nm-800nm spectrum). In addition, the absorbances of electrogenerated species can be very small and signal-averaging or phase-sensitive detection may be necessary to achieve the required signal-to-noise ratio (cf. EMIRS and FTIR). 

EPR spectra were recorded with a Varian E9 X-band spectrometer using field (100 kHz) and light (13 or 83 Hz) modulation with phase-sensitive detection at the modulation frequencies (19). Typically, the field modulation amplitude employed ranged from 20 to 40 gauss, the microwave power from 0.1 to 0.5 mW. Measurements were performed on frozen solutions of the porphyrins at about 100 K using the standard Varian variable temperature accessory or at about 10 R with an Oxford Instruments helium gas cryostat. Light sources used for photoexcitation were a 1000 W Xe arc source powered by a Photochemical Research Associates Supply with electronic modulation... 

Instrumentation. Two electron spin resonance spectrometers have been used in the course of the work. The Southampton instrument was built in the laboratory, using a Varian magnet, and operated at 3 cm. wavelength. It employed an Hon rectangular cavity and obtained high sensitivity by magnetic field modulation at 100 kc.p.s. with a crystal detector phase sensitive detection... 

The Penn State measurements were made in a spectrometer constructed by Dr. H. Atwater in the Department of Electrical Engineering, which operated at a frequency of 9.11 X 109 c.p.s., and employed a Varian Type V 4612A magnet. Phase sensitive detection was used, in conjunction with a bolometer detector. The cylindrical cavity operated in the Hon mode. 

Figure 29.11 Result of field modulation and phase-sensitive detection in EPR spectrometers.

The simple spectrometer system illustrated in block diagrammatic form in figure 10.2 would be rather insensitive, but there are many refinements which greatly improve the situation. Perhaps the most important of these is signal modulation and in this section we consider a number of different modulation schemes which have been used to great effect. Modulation has several objectives, but one of them is to convert the output detector signal from d.c. to a.c. a.c. amplification and detection techniques, including phase-sensitive detection, can then be used. 

The sum frequency is much larger than the total bandwidth of the spectrometer and so is lost, leaving the difference frequency, which is the frequency of the observed FID. This lower frequency is usually further amplified by audio-amplifiers. It can be seen that phase-sensitive detection is equivalent to examining the NMR signal in a reference... 

In an FM MMW spectrometer the spectral source frequency is modulated at a certain rate /, typically 1 kHz. This gives rise to sidebands of the spectral source frequency above and below the carrier frequency. The frequency modulated MMW carrier has in its modulation envelope phase and amplitude relationships to the carrier. Mixing in the non-linear junction of the detector yields the modulation signals altered by their interaction with the cavity and gas inside it, with their preserved amplitude and phase relationship to the original modulation signals. Those properties are measured by passing the heterodyne mixer output and the thermal noise contribution from the mixer, to a filtered phase-sensitive detection system, with the original modulation as reference. 

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