A new fast scan submillimeter spectroscopic technique ͑FASSST͒ has been developed which uses a voltage tunable backward wave oscillator ͑BWO͒ as a primary source of radiation, but which uses fast scan (ϳ10 5 Doppler limited resolution elements/s͒ and optical calibration methods rather than the more traditional phase or frequency lock techniques. Among its attributes are ͑1͒ absolute frequency calibration to ϳ1/10 of a Doppler limited gaseous absorption linewidth (Ͻ0.1 MHz, 0.000 003 cm Ϫ1 ), ͑2͒ high sensitivity, and ͑3͒ the ability to measure many thousands of lines/s. Key elements which make this system possible include the excellent short term spectral purity of the broadly (ϳ100 GHz) tunable BWO; a very low noise, rapidly scannable high voltage power supply; fast data acquisition; and software capable of automated calibration and spectral line measurement. In addition to the unique spectroscopic power of the FASSST system, its implementation is simple enough that it has the prospect of impacting a wide range of scientific problems.

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Pressure broadening and line shift parameters for the 1←0 and 2←1 rotational transitions of carbon monoxide in collision with helium have been measured between 1 and 600 K. Measurements below 30 K were made using the collisional cooling technique, while measurements at higher temperatures were made in an equilibrium cell. The experimental apparatus and techniques employed in the study are described. Pressure broadening and line shift cross sections are compared with theoretical predictions based on the best available potential energy surface, and differences between theoretical predictions and the experimental results are discussed. Potential sources of systematic experimental error are examined as a possible origin of these differences.

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