Homodyne Quasi-Optical
The quasi-optical instrument, one of the two spectrometers based on the 15/17 Tesla superconducting magnet, employs a 95 GHz Gunn diode generator combined with a Schottky-diode multiplier to obtain microwave frequencies 95±3, 190±6, 285±9, 380±12 and 475±15 GHz, or a 110 GHz Gunn diode generator combined with a Schottky-diode multiplier to obtain microwave frequencies 110±3, 220±6, 330±9, 440±12 and 550±15 GHz. The microwaves are directed toward the sample by a system of mirrors, enter a corrugated waveguide, get reflected from the sample compartment and travel further through a system of mirrors to a liquid-helium cooled InSb mixer. The quasi-optical arrangement and the use of a corrugated waveguide minimizes microwave power losses. Another advantage is the separation of the microwaves reflected from the sample chamber from the microwaves incident on the sample (they are polarized in mutually perpendicular planes). In this way, only the reflected microwaves reach the detector, resulting in significant noise reduction. The instrument does not employ resonance cavity. The superconducting magnet has been factory-calibrated, but for accurate measurements usage of g standards is recommended (DPPH or atomic hydrogen, for example). The superconducting magnet must be swept if a wide magnetic field range is required for a spectrum. For samples requiring a sweep width of no more than 2000 Gauss, an additional sweep coil is used while the superconducting magnet is put in the persistent mode (i.e., drawing no current from the power supply).
The quasi-optical instrument is particularly suitable for polycrystalline samples and for frozen aqueous solutions (and liquid solutions). The sample size is about 50 uL. Absolute sensitivity is in general better than that of the transmission instrument. Specialized flat plastic vessels are typically used as sample containers, but use of sealed narrow quartz tubes (2 mm OD) is possible. The sample temperature can be controlled over the range 3 K to 300 K. Transition metal complexes, certain compounds of the f-electron metals and organic radicals have been studied on this spectrometer.
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For more information, contact Andrew Ozarowski.
