The Gravity Probe B mission provided two new quantitative tests of Einstein’s theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earth’s orbit. Data from four gyroscopes gave a geodetic drift-rate of −6601.8 ± 18.3 marc-s yr−1 and a frame-dragging of −37.2 ± 7.2 marc-s yr−1, to be compared with GR predictions of −6606.1 and −39.2 marc-s yr−1 (1 marc-s = 4.848 × 10−9 radians). The present paper introduces the science, engineering, data analysis, and heritage of Gravity Probe B, detailed in the accompanying 20 CQG papers.

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A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10-100 cm-i and exhibits a maximum absorption sensitivity near one part in 106. Each laser line can be tuned by f 110 GHz with first-order sidebands. Applications of this instrument are detailed in the preceding paper. THE BERKELEY TUNABLE FAR-INFRARED LASER SPECTROMETERA. General description Tunable far-infrared (FIR) lasers have become powerful tools for investigating the structures of ions, radicals, and clusters, and for probing intermolecular forces through measurement of FIR spectra of van der Waals complexes. In the preceding paper we have described the rapid evolution of FIR laser spectroscopy and some recent applications. In this article we present a detailed description of the tunable FIR laser spectrometers currently used at Berkeley. We begin with a relatively general overview of the design, and then proceed to the details of construction and operation. It is our hope that this article will serve as a useful guide to those who seek to construct similar systems.The design of the tunable FIR laser systems used at Berkeley is similar to that of Farhoomand et al. ' In the following, we first present a general description of this design, in sufficient detail to afford all readers a reasonable understanding of the underlying principles and function. We then proceed to describe each component of the system in sufficient detail to effectively guide those actually seeking to construct a similar apparatus.The overall experimental design is diagrammed in Fig. 1. The complete spectrometer is built on a 5 ft. x 12 ft. vibration isolation honeycomb table. A COz. laser provides an intense mid-infrared beam (maximum power > 150 W) that is used to pump a molecular gas FIR laser. The CO2 laser is line tunable over some 100 different vibrationrotation transitions between 9.1 and 11 .O pm using a precision grating in first-order autocollimation. The output frequency is fine-tuned over the 65 MHz free spectral range of the cavity (limited by its 2.3 m length) using a piezoelectric transducer (PZT), and the zeroth-order beam reflected from the grating is focused into a CO2 spectrum analyzer to identify the laser line.The FIR laser is pumped coaxially by the COz laser beam, which circulates between the FIR laser end mirrors after expanding through a 4 mm hole in the input coupler. The 2.5 m cavity of the FIR laser is of the dielectric waveguide design, with planar gold-coated copper end mirrors. FIR power is coupled out through a lo-mm-diam hole in the end mirror, which is backed by a hybrid quartz/ dielectric mirror to reflect the pump beam, while transmitting the FIR output. The output beam of the FIR laser then enters a Martin-Puplett polarizing diplexer, which couples the laser radiation onto the (Schottky diode) comer cube mixer, while s...

The state of the art in far infrared (FIR) spectroscopy is reviewed. The development of tunable, coherent FIR radiation sources is discussed. Applications of tunable FIR laser spectrometers for measurement of rotational spectra and dipole moments of molecular ions and free radicals, vibration-rotation-tunneling (VRT) spectra of weakly bound complexes, and vibration-rotation spectra of linear carbon clusters are presented. A detailed description of the Berkeley tunable FIR laser spectrometers is presented in the following article.

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