Microwave bridge

Forbes M D E, Peterson J and Breivogel C S 1991 Simple modification of Varian E-line microwave bridges for fast time-resolved EPR spectroscopy Rev. Sc/. Instrum. 66 2662-5... 

Turn on the cooling water (to magnet coils, magnet power supply, microwave bridge). 

Switch on the microwave bridge to tune mode (and allow two minutes warm-up time). 

When the microwave bridge is in tune mode, the microwave source is at high voltage, and its guaranteed lifetime is ticking away (therefore, switch to off for a lunch break). 

A commonly used EPR spectrometer, similar to that commercially sold by Varian Associates, Palo Alto, California, is shown diagrammatically in Fig. 21. The spectrometer consists of a four-arm microwave bridge generally known as a hybrid tee. The action of this microwave bridge is somewhat... 

A typical layout of an EPR spectrometer is shown in Fig. 15.5. While the general principles of operation and detection are the same for different spectrometers, specific steps may vary. The readers are encouraged to consult the manual and vendor regarding each individual spectrometer. Examples used in discussions below are based on a Bruker EMX X-band spectrometer that is equipped with an ER-041X microwave bridge and a high-sensitivity cavity (ER-4119HS, Bruker Biospin, Inc.). 

The simplest type of ESR spectrometer consists basically ot an operator s console, microwave bridge, magnet, magnet power supply, sample cell or cavity, and a detector/recorder. Figure 5.5.4 shows a commercial ESR spectrometer. 

The microwave bridge is stable for days making two-dimensional ESEEM experiments routine. 

Electron Spin Resonance. Trapped free radicals in irradiated starch were studied utilizing an electron paramagnetic resonance instrument (Varian Associates Type 4500) fitted with a 100-kc. field modulation, Hi-lo power microwave bridge, and a multipurpose specimen cavity. The instrument is stated to have an accuracy of 10% and a minimum resolution of about lO spins per cc. Variants 0.1% pitch mixed with potassium chloride calibration standard containing 10 " spins per cm. of length was used as the reference curve. Samples and standard were contained in quartz tubes, 4 mm. in i.d., in sufficient depth to fill the cavity. 

ESR measurements were made by employing a Varian E-Line Century Series console and an E 102 microwave bridge (X band). A Hewlett-Packard 5245 L electronic counter was used to measure frequency. Both radical density and g value measurements were made relative to a sample of diphenylpicrylhydrazyl (DPPH). Runs were made in the single cavity mode, and the g value of DPPH was taken as 2.0037. No detuning of the cavity due to sample conductivity was observed. All data were manipulated in digital form and accumulated through a Nicolet 1180 computer (12-bit, 333-kHz A/D converter 2048 20-bit words/ spectrum LAB 11 software). Radical density measurements were made by double integration of the derivative spectra. 

In the last section of the present paper we offer a selective development of some of the key aspects of ESR instrumentation. Emphasis is placed on the microwave bridge and microwave resonant structure. Most of this material has not been published previously. 

The noise, both AM and FM, of microwave oscillators used in ESR bridges is equal to or worse than in early years. There has been little improvement in tube oscillators and solid state oscillators have somewhat worse noise in our experience. It is important to recognize that both oscillator AM and FM noise are enhanced in a microwave bridge of the reference arm type. Wilmshurst [12] discusses AM noise enhancement, deriving the following equation for the noise voltage... 

Intensity measurements are not a forte of microwave spectroscopy. Typically, cited errors range from 3 to 8 percent, provided the intensity ratios of several rotational transitions have been measured. Esbitt and Wilson27 have discussed the problems of relative intensity measurements in some detail. Harrington28 has described a system using a microwave bridge in which errors of about 1 percent are claimed, and this system has been incorporated into a commercial spectrometer. Errors of 3 percent in intensity ratio measurement lead to an uncertainty of 6 cm-1 for a given vibrational level at room temperature. 

Fuchs M, Weber S, Mdbius K, Rohrer M and Prisner T 1998 A submillimeter high-field EPR spectrometer using quasi-optical microwave bridge devices Magnetic Resonance and Related Phenomena ed D Ziessow, W Lubitz and F Lendzian (Berlin Technische Universitat Berlin)... 

The basic components of an ESR spectrometer are shown in Fig. 4 [15]. The microwave bridge suppUes microwaves at a fixed frequency and chosen power, however, the microwave frequency is tuneable over a Hmited frequency range. The microwave source is a klystron or a gundiode. If one wishes to obtain ESR spectra at different frequencies, then a wide range of microwave sources need to be called in. The most commonly used and commercially available frequency is... 

The continuous wave (CW) spectrometers are still mostly employed. The cavity is placed in a microwave bridge, thus avoiding the microwaves to reach the detector except at resonance (Fig. 1.8). The magnetic field is modulated at high frequency, 100 kHz is commonly employed. 

As a result of the field modulation and the phase-sensitive detection, the spectrum is recorded as the first derivative of the absorption, with the x-axis synchronized to the magnetic field that is swept across the resonance. For accurate measurements of the resonance parameters it is customary to calibrate the field, either with a field meter based on proton NMR, or to use a standard sample with known g-factors and hyperfine splittings. The microwave frequency is measured with a frequency meter attached to the microwave bridge. Modem instruments are computer controlled spectra with instrument parameters are saved and stored digitally. 

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